Overview
This chapter studies the two motions of the earth and everything that follows from them. The first motion is rotation, the spinning of the earth on its axis from west to east once in about 24 hours. The chapter explains the axis, the direction and speed of spin, the proofs that the earth really rotates, from the Foucault pendulum to the deflection of winds, and the effects of rotation: day and night, the circle of illumination, twilight, the difference of time between places, the deflection of winds and currents by the Coriolis force, the tides and the equatorial bulge. The second motion is revolution, the journey of the earth round the sun in an elliptical orbit once in 365¼ days, with perihelion and aphelion, the leap year and the proofs of revolution. Because the axis is tilted at 23½° and always points the same way, revolution produces the seasons, the apparent north-south migration of the sun between the tropics, the solstices and equinoxes, the changing length of day and night, the midnight sun of the polar regions and the heat zones of the earth. The chapter closes by comparing the seasons of the two hemispheres and showing how India's monsoon calendar fits into this planetary rhythm. Everything in climate and time-keeping rests on these two motions.
Learning Objectives
- Describe the rotation of the earth, its axis, direction, period and speed at different latitudes.
- State the evidence that proves the earth rotates and revolves.
- Explain how rotation causes day and night, the circle of illumination and twilight.
- Explain the relationship between rotation, longitude and time, and the Coriolis deflection of winds and currents.
- Describe the revolution of the earth, its elliptical orbit, perihelion, aphelion and the leap year.
- Explain how the tilt of the axis and revolution together cause the seasons.
- Describe the positions of the earth at the solstices and equinoxes and the resulting length of day and night.
- Explain the apparent movement of the sun between the tropics and the heat zones of the earth.
- Compare the seasons of the northern and southern hemispheres.
Topics in this chapter
12 topics · tap a topic title to jump straight to it.
Rotation of the earth: axis, direction, period and speed
The earth has two motions at once, like a spinning top carried round a room: it spins on its own axis, which is rotation, and it travels round the sun, which is revolution. Both are ancient knowledge in India — Aryabhata in 499 CE described the earth turning on its axis — but they were established in Europe only after Copernicus (1543) and Galileo.
The axis. The earth rotates about an imaginary line passing through its centre and through the two poles, called the axis of rotation. The point where the axis meets the surface in the north is the North Pole and in the south the South Pole. The axis is not upright to the plane of the earth's orbit; it is tilted at 23½° from the perpendicular, or 66½° to the orbital plane, and it always points towards the same spot in the sky near the Pole Star. This inclination is called the obliquity of the ecliptic and it is the key to the seasons discussed later.
Direction. The earth rotates from west to east, that is anticlockwise when viewed from above the North Pole. This is why the sun, moon and stars appear to rise in the east and set in the west: they are not moving; we are turning towards them and then away from them. The direction of rotation also decides that places to the east see the sun earlier than places to the west.
Period. The earth takes 23 hours 56 minutes 4 seconds to make one complete turn relative to the distant stars; this is the sidereal day. But because the earth has meanwhile moved about 1° along its orbit, it must turn a little more, about 4 minutes' worth, before the sun returns to the same position overhead; this gives the solar day of 24 hours, the day of clocks and calendars. The rotation is so regular that it was the standard of time for all history until atomic clocks showed it is slowing by about 2 milliseconds a century because of tidal friction.
Speed of rotation. Every point on the earth turns once in 24 hours, but points on the equator have farther to go, so they move faster. At the equator the circumference of 40,075 km is covered in 24 hours, a speed of about 1,670 km per hour. The speed decreases with latitude in proportion to the cosine of the latitude: at 30° it is about 1,446 km/h, at Kolkata (22½° N) about 1,540 km/h, at 60° about 835 km/h, and at the poles zero — a person standing exactly at the pole simply turns round once a day. The angular speed, 15° per hour or 360° per day, is the same everywhere. We do not feel this motion because the atmosphere, the oceans and everything on the surface move with the earth at the same steady speed, just as a passenger in a smooth aircraft feels no motion.
Rotation and the earth's shape. The centrifugal effect of this spin, greatest at the fast-moving equator, is what has bulged the earth into an oblate spheroid 42 km wider across the equator than through the poles.
- Seen from above the North Pole the earth turns anticlockwise, so New York sees the sunrise about 10½ hours after Kolkata, and Tokyo about 3½ hours before it.
- Speed of rotation = 1,670 × cos(latitude) km/h: at the equator 1,670, at Kolkata (22.5° N) about 1,540, at London (51.5° N) about 1,040, at the poles zero.
- The sidereal day is 23 h 56 min 4 s; the solar day is 24 h; the difference of 3 min 56 s is the extra turn the earth needs because it has moved 1° round the sun.
- Rotation: west to east (anticlockwise from above the North Pole); sidereal day 23 h 56 min 4 s; solar day 24 h; angular speed 15° per hour.
- Linear speed at latitude φ = 1,670 × cos φ km/h; at the equator 1,670 km/h, at the poles 0.
- Axis tilted 23½° from the perpendicular to the orbit (66½° to the orbital plane), pointing towards the Pole Star.
Proofs of the rotation of the earth
Because we move with the earth, rotation cannot be felt, and for two thousand years most people believed instead that the sky turned round a fixed earth. The following evidence proves that it is the earth that turns.
1. The Foucault pendulum (1851). The French physicist Léon Foucault hung a 28 kg iron ball on a 67 m wire from the dome of the Panthéon in Paris and set it swinging. A freely swinging pendulum keeps its plane of swing fixed in space; yet the plane appeared to turn slowly clockwise, completing a circle in about 32 hours at the latitude of Paris. Since nothing pushed the pendulum, it was the floor — the earth — that had turned beneath it. At the pole the plane would turn once in 24 hours; at the equator not at all. Foucault pendulums now swing in science museums, including Kolkata's Birla Industrial and Technological Museum, and are the most direct proof of rotation.
2. Deflection of winds and currents (the Coriolis effect). Winds, ocean currents, rivers and even long-range shells are deflected to the right of their path in the northern hemisphere and to the left in the southern. The trade winds blow from north-east rather than from the north, cyclones spin anticlockwise north of the equator and clockwise south of it. This deflection, explained by Gaspard Coriolis in 1835, can only happen on a rotating earth.
3. Apparent movement of the heavenly bodies. The sun, moon and all the stars rise in the east and set in the west, moving across the sky at 15° an hour — exactly the rate and direction expected if the earth turns west to east. It would be absurd for stars at distances of thousands of light years to circle the earth daily at speeds far beyond that of light; the simple explanation is that we turn.
4. The oblate shape of the earth. The equatorial bulge of 42 km is exactly what rotation would produce by centrifugal effect, and the bulge has been measured; a stationary earth would be a perfect sphere.
5. The eastward deflection of falling bodies. An object dropped from a great height lands slightly to the east of the point directly below, because the top of a tower moves faster eastward than its base. In an experiment in a mine shaft in Germany in 1831 a ball dropped 158 m landed 28 mm to the east. A ball fired straight up also returns a little to the west.
6. Time differences. When it is noon in Kolkata it is early morning in London and evening in Japan; clocks must be changed on every long journey east or west. This follows from a turning earth that presents different meridians to the sun in succession.
7. Satellites and space photographs. A satellite in polar orbit passes over a different strip of the earth on each circuit because the earth has turned beneath it; geostationary satellites must orbit at the earth's own rate to stay above one place. Time-lapse photographs from space show the continents wheeling past.
8. Day and night and the daily rhythm of temperature. The regular alternation of day and night, and the regular daily warming and cooling of the air, follow the sun's apparent passage caused by rotation.
- Foucault's pendulum in the Panthéon (1851): the plane of swing turned clockwise about 11° an hour, completing a circle in 32 hours at 49° N — the earth turning beneath the pendulum.
- A ball dropped down a 158 m mine shaft at Freiberg in 1831 struck the bottom 28 mm east of the vertical, because the top of the shaft moves eastward faster than the bottom.
- Cyclones in the Bay of Bengal always spin anticlockwise and those off Australia clockwise — the Coriolis deflection that exists only on a rotating body.
- Foucault pendulum: the plane of swing turns through 360° × sin(latitude) per day — 360° at the pole, about 272° at Paris (49° N), 0° at the equator.
- Coriolis rule: moving objects deflect to the right in the northern hemisphere and to the left in the southern; the deflection is zero at the equator and greatest at the poles.
Day and night, the circle of illumination and twilight
The most obvious effect of rotation is the alternation of day and night. The sun is a source of light and the earth is an opaque sphere, so at any moment the sun lights exactly one half of the earth; the other half is in its own shadow. As the earth turns from west to east, every place is carried into the lit half — it experiences sunrise, morning, noon, afternoon — and then out of it into the dark half — sunset, evening, midnight, and back to dawn. One complete cycle takes one solar day of 24 hours.
The circle of illumination. The boundary between the lit half and the dark half is a great circle round the earth called the circle of illumination or the terminator. It is always at right angles to the sun's rays, so it always divides the earth into equal halves of day and night. A person on this circle sees the sun on the horizon — rising on the western edge of the circle (the morning side) and setting on the eastern edge. The circle of illumination does not coincide with the axis of rotation except at the equinoxes; because the axis is tilted, the circle usually cuts the parallels of latitude unequally, so that one hemisphere has more of each parallel in daylight than the other. This is why day and night are equal only at the equator throughout the year and everywhere on the two equinox days, and why they are unequal elsewhere at other times — a matter taken up under revolution.
Noon and midnight. At any moment there is one meridian, the noon meridian, where the sun is at its highest and it is exactly 12 o'clock, and on the opposite side of the earth the midnight meridian. As the earth turns, the noon meridian sweeps westward round the globe at 15° an hour, or 1° every four minutes.
Twilight. Night does not fall the moment the sun sets, nor does day begin sharply at sunrise. For a time before sunrise (dawn) and after sunset (dusk) the sky is faintly lit even though the sun is below the horizon. This is twilight, and it is caused by the atmosphere: the sun's rays, still striking the upper air above the observer, are scattered and reflected by dust and air molecules down to the ground. Astronomers divide it into civil twilight (sun up to 6° below the horizon, enough light for outdoor work), nautical twilight (6°–12°, the horizon still visible for sailors) and astronomical twilight (12°–18°, after which the sky is fully dark). The duration of twilight depends on latitude and season: at the equator the sun sets almost vertically and twilight lasts barely 20 minutes, while at high latitudes the sun sets at a slant, lingers just below the horizon and twilight lasts an hour or more — in Scotland in June it lasts all night. Twilight is long in the Darjeeling hills in summer compared with the plains partly for this reason, and partly because the height of the observer lets the sun's rays reach the upper air longer.
If the earth did not rotate. One side would face the sun permanently, scorched to the temperature of Venus, and the other would freeze in endless night; winds would blow only from cold to hot and there would be no daily rhythm of life. Rotation spreads the sun's heat round the whole earth every day.
- At the equinox the circle of illumination passes through both poles and every parallel is exactly half in the light, so every place has a 12-hour day; at the June solstice it swings past the North Pole, putting the whole Arctic in daylight.
- At Kolkata twilight after sunset lasts about 25 minutes; at Edinburgh (56° N) in June it lasts more than two hours and never fully ends.
- When it is noon on the 88° E meridian near Kolkata, the noon meridian moves on to 87° E four minutes later and reaches Delhi (77° E) 44 minutes later.
- Circle of illumination: the great circle separating day from night, always at right angles to the sun's rays, always dividing the earth into two equal halves.
- Twilight: civil (sun 0°–6° below the horizon), nautical (6°–12°), astronomical (12°–18°).
Rotation, longitude and time
Because the earth turns through 360° of longitude in 24 hours, it turns through 15° in one hour and 1° in four minutes. This fixed rate links longitude to time and is the basis of all time-keeping on the earth.
Local time. Every place has its own local time, reckoned from the moment the sun is highest over its meridian, which is 12 noon. Places to the east of an observer have already had their noon and are ahead in time; places to the west are behind. The difference is four minutes for every degree of longitude. Thus if it is noon at Kolkata (about 88° E), it is 12:44 p.m. at a place at 99° E and 11:16 a.m. at Delhi (77° E). In the days of sundials every town kept its own local time, and a railway timetable across a country was chaos.
Standard time. To end the confusion, each country chooses the local time of one central meridian as the standard time for the whole country. Indian Standard Time (IST) is the local time of the meridian of 82½° E, which passes near Mirzapur in Uttar Pradesh and is close to the centre of the country. India spans about 30° of longitude from 68° E to 97° E, or two hours of sun-time, so the sun rises about an hour earlier in Arunachal Pradesh than in Gujarat, though the clocks are the same. Large countries use several time zones: Russia has eleven, the USA six.
Greenwich Mean Time and time zones. The world's time is reckoned from the Prime Meridian (0°) through the Royal Observatory at Greenwich, London, chosen in 1884. The earth is divided into 24 time zones of 15° each; each zone is one hour ahead of the one to its west. Places east of Greenwich are ahead of GMT (India is GMT + 5½ hours) and places west are behind (New York is GMT − 5 hours). Today GMT is formally called Coordinated Universal Time (UTC).
The International Date Line. If one goes eastward round the earth adding an hour for every 15°, one arrives home 24 hours ahead of the calendar; going westward one arrives a day behind. To correct this the International Date Line is drawn along the 180° meridian, on the opposite side of the earth from Greenwich, through the empty Pacific, with bends to avoid dividing island groups. Crossing it eastward (towards America) one goes back a day; crossing it westward (towards Asia) one goes forward a day. A traveller flying from Tokyo to Los Angeles arrives on the same date at an earlier hour than the departure.
Worked examples. (1) It is 8 a.m. at Greenwich; what is IST? India is 82½° east, so 82½ × 4 = 330 minutes = 5½ hours ahead: 1:30 p.m. (2) When it is 10 a.m. at a place on 30° E, what is the local time at 90° E? Difference 60° = 240 minutes = 4 hours; 90° E is east and ahead: 2 p.m. (3) A ship's chronometer shows 3 p.m. GMT when local noon is observed; the ship is 3 hours = 45° west of Greenwich, at 45° W longitude. This last is how sailors found longitude at sea.
- Indian Standard Time = local time of 82½° E (near Mirzapur) = GMT + 5½ hours; when it is noon at Greenwich it is 5:30 p.m. in India.
- When it is 10 a.m. at 30° E it is 2 p.m. at 90° E (60° × 4 min = 4 h ahead) and 8 a.m. at 0° (30° × 4 min = 2 h behind).
- A ship observes local noon when its chronometer reads 3 p.m. GMT: it is 3 h × 15° = 45° west of Greenwich.
- 360° of longitude = 24 hours; 15° = 1 hour; 1° = 4 minutes.
- Time difference = difference in longitude × 4 minutes; east is ahead, west is behind.
- IST = GMT + 5 h 30 min (meridian 82½° E); the International Date Line follows 180° with local bends.
The Coriolis effect, tides and other effects of rotation
The Coriolis effect. On a rotating earth every freely moving thing — wind, ocean current, river, aircraft, rocket — appears to be deflected from a straight path. In the northern hemisphere the deflection is to the right of the direction of motion; in the southern hemisphere to the left. This is called the Coriolis effect or Coriolis force, after the French mathematician who described it in 1835, and it is the direct consequence of rotation: the surface beneath a moving body turns while the body moves, so the body seems to curve. The effect is zero at the equator, where the surface moves in a straight line eastward, and greatest at the poles; it is proportional to the speed of the moving body and it acts only on things moving over the surface, not on things at rest. This is why the trade winds blowing towards the equator from the north come as north-east trades, and from the south as south-east trades; why the westerlies come from the south-west in the northern hemisphere; why cyclones spin anticlockwise north of the equator and clockwise south of it; why ocean currents circle clockwise in the North Atlantic and North Pacific and anticlockwise in the southern oceans; and why the right bank of rivers in the northern hemisphere tends to be eroded more (Ferrel's law). Aircraft and long-range artillery must correct for it. The Coriolis effect also keeps cyclones from forming within about 5° of the equator, where it is too weak to start the spin.
Tides. The regular rise and fall of the sea twice a day is caused by the gravitational pull of the moon and the sun on the water of the oceans, combined with the rotation of the earth. The moon's pull raises a bulge of water on the side of the earth facing it, and inertia raises another on the opposite side; as the earth rotates, every coast passes through both bulges in a day, giving two high tides and two low tides about 12 hours 25 minutes apart (the extra 25 minutes because the moon has moved on). At new moon and full moon the sun and moon pull in line and the tides are highest — spring tides; at the quarter moons they pull at right angles and the tides are smallest — neap tides. The tidal range in the Hooghly is over 5 m and the tidal bore races up past Kolkata; the Sundarbans are flooded and drained twice daily; and the tides are being harnessed for electricity at the Gulf of Kutch.
Other effects of rotation.
- The equatorial bulge: the earth is 42 km wider across the equator because of the centrifugal effect of spin.
- The apparent daily movement of the sun, moon and stars from east to west, and the direction of sunrise.
- The daily rhythm of temperature, of plant and animal life and of human work and sleep.
- The magnetic field: the rotation of the liquid iron outer core generates the earth's magnetism, which gives the compass and shields the atmosphere.
- Time reckoning: the day, the hour and all of standard time are derived from rotation.
- Wind systems and ocean gyres: the whole pattern of the planetary winds and the great circular currents is shaped by the Coriolis deflection.
- A wind blowing from the north pole towards the equator is bent to the right and arrives as a north-east wind — the north-east trade wind of the northern hemisphere.
- Cyclone Amphan in May 2020 spun anticlockwise as it crossed the Sundarbans; a cyclone off Madagascar in the same month spun clockwise.
- At Kolkata's Garden Reach the tide rises over 5 m twice a day; spring tides at new and full moon send a bore up the Hooghly that can reach 2 m in height.
- Coriolis deflection: to the right in the northern hemisphere, to the left in the southern; zero at the equator, maximum at the poles; increases with the speed of the moving body.
- Tides: two high and two low tides every 24 h 50 min; spring tides at new and full moon, neap tides at the quarters.
Revolution of the earth: orbit, period and proofs
While it spins, the earth also travels round the sun. This movement along a fixed path is called revolution, and the path is the orbit.
The orbit. The earth's orbit is not a circle but an ellipse, a slightly stretched circle, with the sun at one of its two foci, as Johannes Kepler discovered in 1609. The orbit is very nearly circular — the eccentricity is only 0.017 — and it is about 94 crore km long. Because it is an ellipse, the earth's distance from the sun changes through the year. The nearest point, perihelion (from the Greek for near the sun), is reached about 3 January, when the distance is 14.71 crore km; the farthest point, aphelion, about 4 July, at 15.21 crore km. The difference of 50 lakh km is only about 3 per cent of the distance and has little effect on climate — indeed the northern hemisphere has its winter when the earth is nearest the sun. The mean distance, 14.96 crore km, is the astronomical unit.
The plane of the orbit and the ecliptic. The orbit lies in a flat plane called the plane of the ecliptic; the sun's apparent yearly path among the stars, as seen from the earth, is also called the ecliptic. The earth's axis is inclined to this plane at 66½°, that is 23½° from the perpendicular, and, because of the earth's spin, the axis keeps this inclination and this direction throughout the orbit, like a gyroscope; this is the parallelism of the axis, and it is what makes revolution produce seasons.
Period and speed. One revolution takes 365 days 5 hours 48 minutes 46 seconds, the tropical year, on which the calendar is based. The earth moves at about 30 km per second, or 1,07,000 km per hour, a little faster near perihelion and slower near aphelion, in accordance with Kepler's second law that the line from the sun to the earth sweeps out equal areas in equal times. This is why the northern summer half of the year (equinox to equinox, March to September) is about 186 days and the winter half about 179 days.
The leap year. Since the year is nearly a quarter of a day longer than 365 days, the calendar would drift by a day every four years. Julius Caesar's calendar (46 BCE) added a day, 29 February, every fourth year. But the year is 11 minutes 14 seconds short of 365¼ days, so by 1582 the drift had reached 10 days, and Pope Gregory XIII reformed the rule: century years are leap years only if divisible by 400. Thus 1900 and 2100 are not leap years, 2000 and 2400 are. The Gregorian calendar is now used everywhere for civil purposes; the Indian national calendar (Saka) and the Bengali calendar (Bangabda) are also solar, while the Islamic calendar is lunar.
Proofs of revolution. (1) The seasons and the changing altitude of the noon sun through the year can be explained only by the earth's changing position on its orbit with a tilted axis. (2) The stellar parallax: a nearby star seen from opposite ends of the orbit (six months apart) shifts very slightly against the far background, first measured by Bessel in 1838; the shift can occur only if the observer has moved. (3) The aberration of starlight, discovered by Bradley in 1728: the apparent position of stars shifts by up to 20 seconds of arc because of the earth's velocity through space. (4) The annual change in the constellations seen at night: Orion is a winter constellation and Scorpius a summer one because the night side of the earth faces different parts of the sky as it goes round the sun. (5) The regular shift in the spectrum of starlight (Doppler shift) towards blue in the half of the year when the earth is moving towards a star and towards red when moving away.
- Perihelion on about 3 January at 14.71 crore km, aphelion on about 4 July at 15.21 crore km — the earth is closest to the sun in the northern winter, so distance does not cause the seasons.
- Leap years: 2024 and 2028 are leap; 2100 is not (divisible by 100 but not 400); 2000 was, being divisible by 400.
- Orion is seen in the evening sky in December and January but not in June, because in June the night side of the earth faces the opposite part of the sky — a sign that the earth has moved round the sun.
- Revolution: elliptical orbit with the sun at one focus; period 365 d 5 h 48 min 46 s; speed ≈ 30 km/s (1,07,000 km/h); orbit length ≈ 94 crore km.
- Perihelion ≈ 3 January (14.71 crore km); aphelion ≈ 4 July (15.21 crore km); mean distance 14.96 crore km = 1 AU.
- Leap year rule: a year divisible by 4 is leap, except a century year, which is leap only if divisible by 400.
The tilt of the axis and the cause of seasons
If the earth's axis were upright to the plane of its orbit, the sun would be overhead at the equator every day of the year, every place would have twelve hours of day and twelve of night always, and there would be no seasons anywhere. Seasons exist because the axis is tilted at 23½° and because it keeps pointing in the same direction — towards the Pole Star — all the way round the orbit (the parallelism of the axis). As a result, during one half of the year the northern hemisphere is leaned towards the sun and during the other half away from it, and the southern hemisphere the reverse.
The tilt changes two things for each hemisphere, and these two things together cause the seasons.
1. The angle of the sun's rays. When a hemisphere is tilted towards the sun, the sun climbs high in its sky and its rays fall steeply, nearly vertically. Vertical rays concentrate their heat on a small area and pass through a thin layer of atmosphere, so they heat the ground strongly. When the hemisphere is tilted away, the sun stays low, its rays fall at a slant, spread the same heat over a larger area and travel through more atmosphere, and the ground heats little. A torch shone straight down at a table makes a small bright circle; shone at a slant it makes a large dim oval — the same energy spread thinner.
2. The length of the day. When a hemisphere is tilted towards the sun, the circle of illumination cuts its parallels so that more than half of each is in daylight: days are longer than nights, and the longer the sun shines the more the ground warms and the less time it has to cool. When the hemisphere is tilted away, nights are longer than days. At the equator, which the circle of illumination always halves, day and night remain equal.
So in the months when the northern hemisphere leans towards the sun (around June) it receives steep rays for long days and has summer; at the same time the southern hemisphere receives slanting rays for short days and has winter. Six months later (around December) the positions are reversed. In between (around March and September) neither hemisphere leans towards the sun, the rays are vertical at the equator, days and nights are equal everywhere, and both hemispheres have the mild seasons of spring and autumn.
What does not cause the seasons. It is a common mistake to think that summer comes when the earth is nearest the sun. In fact the earth is nearest (perihelion) on 3 January, in the northern winter, and farthest on 4 July, in the northern summer; the 3 per cent change in distance is trivial compared with the effect of tilt. If distance mattered, both hemispheres would have summer together, which they never do.
The seasons of India. India lies mostly north of the Tropic of Cancer and so has the northern-hemisphere pattern: the sun is highest and days longest in June, lowest and days shortest in December. But because it is in the tropics and has the monsoon, India recognises not four but six traditional seasons — grishma (summer), varsha (rains), sharad (autumn), hemanta (late autumn), shita (winter), vasanta (spring) — and the meteorological four: the hot season (March–May), the rainy season (June–September), the retreating monsoon (October–November) and the cool season (December–February).
- A torch held vertically over a table lights a small bright circle; tilted, the same beam spreads into a large dim oval — the slanting winter sun spreads its heat the same way.
- In June, Kolkata (22½° N) gets the sun almost overhead at noon and about 13½ hours of daylight; in December the noon sun is only 44° high and the day is about 10¾ hours — the difference between a temperature of 35 °C and 20 °C.
- Christmas in Sydney falls in midsummer and in Kolkata in winter, because the two hemispheres are tilted opposite ways at the same moment.
- Causes of seasons: (1) tilt of the axis at 23½° + (2) parallelism of the axis + (3) revolution → changing angle of the sun's rays and changing length of day.
- Heating by the sun increases as the rays become more vertical and as the day becomes longer; the distance from the sun is NOT the cause.
Solstices and equinoxes: the four key positions
As the earth goes round the sun with its tilted axis, four positions on the orbit mark the turning points of the year.
1. The summer solstice — 21 June. The North Pole is leaned towards the sun by the full 23½°. The sun's vertical rays fall on the Tropic of Cancer (23½° N) — this is the farthest north the overhead sun ever reaches, and the word solstice means sun-standing, because the sun seems to stop and turn back. The circle of illumination swings 23½° beyond the North Pole, so the whole Arctic region within the Arctic Circle (66½° N) has 24 hours of daylight and the whole Antarctic region 24 hours of night. In the northern hemisphere this is the longest day and shortest night of the year, and the height of summer; in the southern hemisphere the shortest day and midwinter. At Kolkata the day is about 13 h 30 min; at London 16 h 40 min; at the Arctic Circle exactly 24 h.
2. The autumnal equinox — 23 September. Neither pole leans towards the sun; the axis is sideways to the sun's rays. The vertical rays fall on the equator, the circle of illumination passes through both poles, and every place on earth has 12 hours of day and 12 of night — equinox means equal night. It is autumn in the northern hemisphere and spring in the southern. The sun rises exactly in the east and sets exactly in the west everywhere.
3. The winter solstice — 22 December. The South Pole is leaned towards the sun. The vertical rays fall on the Tropic of Capricorn (23½° S), the farthest south the overhead sun reaches. The Antarctic region has 24 hours of daylight and the Arctic 24 hours of night. In the northern hemisphere it is the shortest day and longest night — about 10 h 45 min at Kolkata, 7 h 50 min at London — and midwinter; in the southern hemisphere the longest day and midsummer.
4. The vernal (spring) equinox — 21 March. Again the axis is sideways to the sun, the vertical rays are on the equator, and day and night are equal everywhere. It is spring in the northern hemisphere and autumn in the southern.
The apparent movement of the sun. Putting the four positions together, the overhead noon sun appears to travel from the Tropic of Cancer (21 June) south to the equator (23 September), on to the Tropic of Capricorn (22 December), back north to the equator (21 March) and up to the Tropic of Cancer again — a yearly swing of 47° of latitude. Every place between the tropics has the sun overhead twice a year; at the tropics once; beyond them never. This apparent movement is the Indian Uttarayana (the sun moving north, 22 December to 21 June) and Dakshinayana (moving south, 21 June to 22 December), marked by Makar Sankranti in January.
Between the positions. Between the solstice and the equinox the overhead sun moves gradually and the length of day changes gradually; the change is fastest near the equinoxes and slowest near the solstices, when the sun seems to stand still. The dates vary by a day or so from year to year because of the leap-year cycle: the June solstice may fall on 20, 21 or 22 June.
- 21 June: sun overhead at the Tropic of Cancer; Kolkata's longest day (about 13 h 30 min); 24-hour daylight north of 66½° N; midwinter in Australia.
- 23 September and 21 March: sun overhead at the equator; 12-hour day everywhere on earth; sunrise due east and sunset due west.
- 22 December: sun overhead at the Tropic of Capricorn; Kolkata's shortest day (about 10 h 45 min); the Arctic in 24-hour night; midsummer in Australia.
- Summer solstice 21 June: vertical rays on 23½° N; longest day in the northern hemisphere. Winter solstice 22 December: vertical rays on 23½° S; shortest day in the north.
- Equinoxes 21 March and 23 September: vertical rays on the equator; 12-hour day and night everywhere.
- The overhead sun migrates 47° between the tropics each year; Uttarayana = 22 December to 21 June, Dakshinayana = 21 June to 22 December.
Variation in the length of day and night
Because the circle of illumination is tilted to the parallels of latitude for most of the year, the proportion of each parallel that lies in daylight changes with latitude and with the season. This gives the changing length of day and night, which is one of the two engines of the seasons.
At the equator. The circle of illumination, being a great circle, always cuts the equator (another great circle) in half. So at the equator day and night are each 12 hours long throughout the year; there is no long summer evening and no long winter night in Singapore or Nairobi, and the sun rises and sets at almost the same time every day.
Between the equator and the polar circles. Away from the equator the inequality grows with latitude. In the summer half of the year (when that hemisphere leans towards the sun) the day is longer than 12 hours and grows longer the farther one goes from the equator; in the winter half the day is shorter than 12 hours and grows shorter with latitude. The table shows the length of the longest day (at the June solstice in the northern hemisphere):
| Latitude | Longest day | Shortest day |
| 0° (equator) | 12 h 00 min | 12 h 00 min |
| 10° N | 12 h 35 min | 11 h 25 min |
| 22½° N (Kolkata) | 13 h 30 min | 10 h 45 min |
| 30° N | 13 h 56 min | 10 h 04 min |
| 40° N | 14 h 51 min | 9 h 09 min |
| 50° N | 16 h 09 min | 7 h 51 min |
| 60° N | 18 h 30 min | 5 h 30 min |
| 66½° N (Arctic Circle) | 24 h 00 min | 0 h 00 min |
| 90° N (Pole) | 6 months | 6 months |
The longest day at any latitude plus the shortest day at the same latitude always equals 24 hours, because the two are mirror images six months apart.
Beyond the polar circles. At the Arctic Circle (66½° N) the sun does not set on 21 June and does not rise on 22 December. Farther north the period of continuous daylight in summer and continuous darkness in winter lengthens: at 70° N about two months of each, at 80° N about four months. At the North Pole the sun rises once a year, on 21 March, circles the sky without setting, climbing to 23½° on 21 June, and sets on 23 September for six months of night; the South Pole has the opposite. The land of the midnight sun — northern Norway, Sweden, Finland, Alaska, Siberia — attracts tourists to see the sun at midnight in June.
Sunrise and sunset directions. The changing day-length goes with a changing sunrise point: only at the equinoxes does the sun rise due east; in the northern summer it rises north of east and sets north of west, in winter south of east and south of west. The farther from the equator, the greater the swing.
Significance. The long summer days of high latitudes allow crops to ripen in a short growing season — wheat is grown in Canada at 55° N and barley in Norway at 70° N — while the long winter nights bring the intense cold. In India the difference is modest, about 2¾ hours between the longest and shortest day at Kolkata, which is why the seasons of the plains are governed more by the monsoon than by day-length.
- At Kolkata (22½° N) the longest day, 21 June, is about 13 h 30 min and the shortest, 22 December, about 10 h 45 min; the two add up to 24 hours.
- At Tromsø in Norway (69½° N) the sun does not set from about 20 May to 22 July and does not rise from about 27 November to 15 January.
- At the North Pole the sun rises on 21 March, circles higher each day to 23½° on 21 June, sinks and sets on 23 September, and the six-month night begins.
- At the equator: 12-hour day all year. At the polar circles (66½°): one 24-hour day and one 24-hour night a year. At the poles: six months of day and six of night.
- Longest day + shortest day at any latitude = 24 hours.
Heat zones of the earth
Because the angle of the sun's rays and the length of the day vary with latitude, the amount of solar heat received varies with latitude too, and the earth can be divided into belts of temperature called heat zones or temperature zones. The boundaries are the four key parallels defined by the 23½° tilt: the two tropics and the two polar circles.
1. The Torrid Zone (tropical zone) lies between the Tropic of Cancer (23½° N) and the Tropic of Capricorn (23½° S). Here the noon sun is overhead at least once a year — twice everywhere between the tropics, once at the tropics themselves — and it is never lower than 43° above the horizon at noon. The rays are nearly vertical all year, days and nights are nearly equal, and the zone is hot throughout the year; there is no winter, only wet and dry seasons. It covers about 40 per cent of the earth's surface and includes the Amazon and Congo basins, the Sahara, Arabia, peninsular India south of the Tropic of Cancer, South-East Asia, northern Australia and Central America. Most of India, from the Tropic of Cancer southward, lies in it, and the Tropic passes through Purulia and Nadia in West Bengal.
2. The Temperate Zones lie between the tropics and the polar circles: the North Temperate Zone from 23½° N to the Arctic Circle (66½° N) and the South Temperate Zone from 23½° S to the Antarctic Circle (66½° S). The sun is never overhead here, but it is above the horizon every day of the year; the rays fall at a slant that changes strongly with the season, and the length of day varies from 10¾ hours to 24 at the edges. The result is the four distinct seasons — warm summer, cold winter, spring and autumn — and moderate temperatures on average, hence the name temperate. Most of the world's people, most of Europe, the United States, China, Japan, northern India, Argentina and southern Australia live in these zones. The parts near the tropics are sub-tropical (warm temperate), the parts near the polar circles are cool temperate.
3. The Frigid Zones (polar zones) lie within the Arctic Circle round the North Pole and the Antarctic Circle round the South Pole. Here the sun never rises high — at best 47° at the circles and 23½° at the poles — and for part of the year it does not rise at all; the rays are always very slanting and the surface is snow and ice that reflects most of what little heat arrives. The zones are bitterly cold all year, with a short cool summer and a long dark winter. They include Greenland, the Arctic Ocean, northern Canada, Siberia, Alaska, Scandinavia's far north and the whole of Antarctica, which has the lowest recorded temperature on earth, −89 °C at Vostok.
Why the zones are only approximate. The boundaries are drawn by latitude alone, but real temperatures are also shaped by altitude (Darjeeling at 27° N is cool, Kolkata at 22½° N is hot), by distance from the sea, by ocean currents (the North Atlantic Drift makes Norway habitable at 70° N), by winds and by cloud. So the heat zones are a first sketch of world climate, refined in later chapters by the actual pattern of temperature and rainfall.
- Torrid Zone: 23½° N to 23½° S, sun overhead at least once a year, hot all year — peninsular India, Brazil, the Congo, Indonesia.
- Temperate Zones: 23½° to 66½° in each hemisphere, sun never overhead but never absent, four seasons — northern India, Europe, the USA, China, Argentina.
- Frigid Zones: within 66½° of each pole, sun absent for part of the year, ice-bound — Greenland, Siberia, Antarctica; the Antarctic Circle has one 24-hour day and one 24-hour night.
- Heat zones: Torrid (23½° N – 23½° S); North and South Temperate (23½° – 66½°); North and South Frigid (66½° – 90°).
- Tropics = 23½° (the tilt of the axis); polar circles = 90° − 23½° = 66½°.
Seasons of the two hemispheres compared
Because the axis leans one hemisphere towards the sun while the other leans away, the two hemispheres always have opposite seasons. When it is summer in the north it is winter in the south, and when it is spring in the north it is autumn in the south. The table sets the four seasons side by side.
| Period | Position of the earth | Northern hemisphere | Southern hemisphere |
| 21 March – 21 June | Sun moves from the equator to the Tropic of Cancer | Spring (days lengthening) | Autumn (days shortening) |
| 21 June – 23 September | Sun moves from the Tropic of Cancer back to the equator | Summer (longest days) | Winter (shortest days) |
| 23 September – 22 December | Sun moves from the equator to the Tropic of Capricorn | Autumn (days shortening) | Spring (days lengthening) |
| 22 December – 21 March | Sun moves from the Tropic of Capricorn back to the equator | Winter (shortest days) | Summer (longest days) |
Consequences. Australia and South Africa harvest wheat in December and celebrate Christmas on the beach; the cricket season in England is June to September and in Australia November to March, which is why international tours alternate. Fruit and vegetables from Chile and South Africa fill northern markets in the northern winter. Migratory birds — the Siberian cranes and ducks that winter in the wetlands of Bharatpur and the Sundarbans — fly south in October and north in March, following the sun. Ships took the southern route round Cape Horn in the southern summer to avoid the storms of the southern winter.
Why the southern hemisphere's seasons are slightly milder. The southern hemisphere is mostly ocean, and water heats and cools slowly, so its summers are less hot and its winters less cold than those of the land-dominated north at the same latitude. The southern summer also falls at perihelion, when the earth is 3 per cent nearer the sun and receives about 7 per cent more radiation, but the ocean absorbs the difference; the southern winter falls at aphelion and, by Kepler's law, lasts about a week longer than the northern winter.
The tropics and the poles. Within the Torrid Zone the word season changes its meaning. At the equator the sun is overhead twice a year (at the equinoxes) and the temperature hardly changes; the seasons are wet and dry, governed by rainfall. In India, which straddles the Tropic of Cancer, the sun's northward passage in April–May makes the hot season, the monsoon that follows makes the rainy season, and the sun's retreat brings the cool, dry winter; the northern-hemisphere pattern is present but the monsoon rewrites it. At the poles there are only two seasons, six months of day and six of night.
If the tilt were different. With no tilt there would be no seasons and no polar day or night; the tropics and polar circles would vanish. With a tilt of 45° the tropics would lie at 45° and the polar circles also at 45°, so Kolkata would have the sun overhead in June and a 24-hour night in December; seasons would be violent. The 23½° tilt gives the earth moderate seasons spread over a wide habitable band, one more reason it is the living planet. The tilt itself slowly varies between 22° and 24½° over a cycle of 41,000 years, and this, with other slow changes in the orbit, is thought to pace the ice ages.
- When Kolkata has its longest day on 21 June, Sydney (34° S) has its shortest, about 9 h 54 min; on 22 December the positions are reversed.
- Test cricket is played in England from June to September and in Australia from November to March — the summers of the two hemispheres.
- Siberian ducks and waders arrive in the wetlands of Bengal in October as the northern winter closes in and leave in March as the sun moves north.
- Northern summer (21 June – 23 September) = Southern winter; northern winter (22 December – 21 March) = Southern summer; spring and autumn likewise opposite.
- Summer half-year in the north (equinox to equinox via June) ≈ 186 days; winter half ≈ 179 days, because the earth moves faster near perihelion in January.
Effects of rotation and revolution summarised
The two motions of the earth together explain a very large part of physical geography. This topic gathers their effects so that the student can see them whole and answer the examination's favourite question, to distinguish between rotation and revolution.
Effects of rotation (the daily motion).
- Day and night, as each place is carried through the lit and dark halves of the earth.
- The apparent daily movement of the sun, moon and stars from east to west.
- Differences in time between places: 4 minutes per degree of longitude, the basis of local time, standard time, time zones and the International Date Line.
- The Coriolis deflection of winds, currents and all moving bodies — right in the northern hemisphere, left in the southern — shaping the trade winds, westerlies, cyclones and ocean gyres.
- Tides twice a day as the earth turns through the two tidal bulges raised by the moon and sun.
- The oblate shape of the earth — the equatorial bulge from centrifugal effect.
- Twilight and the daily rhythm of temperature and of life.
- The magnetic field, generated by the rotating liquid core.
Effects of revolution (the yearly motion) together with the tilt of the axis.
- The year and the calendar, including the leap year.
- The seasons — summer, autumn, winter, spring — opposite in the two hemispheres.
- The changing length of day and night through the year, from equal at the equator to six-month days at the poles.
- The apparent annual migration of the sun between the Tropic of Cancer and the Tropic of Capricorn, with the solstices and equinoxes.
- The heat zones — torrid, temperate and frigid — bounded by the tropics and polar circles.
- Perihelion and aphelion, the changing distance from the sun.
- The changing night sky of constellations through the year.
- Through the seasons, the migration of birds and animals, the cycle of crops and the calendar of festivals.
Comparison at a glance.
| Point | Rotation | Revolution |
| Meaning | Spinning on its axis | Travelling round the sun |
| Path | About the axis through the poles | Elliptical orbit, sun at one focus |
| Direction | West to east | Anticlockwise seen from above the North Pole |
| Period | 24 hours (23 h 56 min 4 s sidereal) | 365 d 5 h 48 min 46 s |
| Speed | 1,670 km/h at the equator, 0 at the poles | About 30 km/s (1,07,000 km/h) |
| Chief effects | Day and night, time, Coriolis deflection, tides | Year, seasons, varying day length, heat zones |
| Proof | Foucault pendulum, Coriolis effect | Seasons, stellar parallax, changing constellations |
The two motions are not independent in their effects: the seasons need rotation to spread the sun's heat round each parallel daily, and the length of day needs the tilt that only matters because of revolution. Together, with the tilt of 23½°, they make the earth a planet of days and years, of winds and seasons, of monsoons and harvests — the rhythm on which the rest of geography, and of life, is built.
- Rotation explains why Kolkata's clocks are 5½ hours ahead of London's; revolution explains why Kolkata's June is hot and its December cool.
- The north-east monsoon wind of winter is deflected by rotation (Coriolis) and is set in motion by the seasonal shift of pressure belts that follows the sun's migration — an effect of revolution.
- A student asked for three effects of rotation may answer: day and night, difference of time, deflection of winds; for three effects of revolution: the year, the seasons, the varying length of day.
- Rotation → daily effects: day and night, time, Coriolis, tides, bulge. Revolution + tilt → yearly effects: year, seasons, day-length, solstices/equinoxes, heat zones.
Key Concepts
- Rotation
- The spinning of the earth on its axis from west to east once in about 24 hours, causing day and night.
- Revolution
- The movement of the earth round the sun in an elliptical orbit once in 365¼ days, causing the year and, with the tilt, the seasons.
- Axis of the earth
- The imaginary line through the centre and the two poles about which the earth rotates, tilted at 23½° from the perpendicular to the orbit.
- Sidereal day
- The time for one rotation relative to the stars, 23 hours 56 minutes 4 seconds, shorter than the 24-hour solar day.
- Circle of illumination
- The great circle dividing the lit half of the earth from the dark half, always at right angles to the sun's rays.
- Twilight
- The faint light before sunrise and after sunset caused by the scattering of sunlight by the upper atmosphere.
- Foucault pendulum
- A long freely swinging pendulum whose plane of swing appears to turn slowly, proving that the earth rotates beneath it.
- Coriolis effect
- The apparent deflection of moving bodies to the right in the northern hemisphere and to the left in the southern, caused by the earth's rotation.
- Indian Standard Time
- The local time of the 82½° E meridian adopted for all India, 5½ hours ahead of Greenwich Mean Time.
- International Date Line
- The line along the 180° meridian where the calendar date changes by one day when crossed.
- Perihelion and aphelion
- The points of the orbit nearest the sun (about 3 January, 14.71 crore km) and farthest from it (about 4 July, 15.21 crore km).
- Leap year
- A year of 366 days, every fourth year except century years not divisible by 400, which keeps the calendar in step with the 365¼-day year.
- Parallelism of the axis
- The property of the earth's axis of pointing in the same direction, towards the Pole Star, throughout the year.
- Summer solstice
- 21 June, when the sun is overhead at the Tropic of Cancer and the northern hemisphere has its longest day.
- Winter solstice
- 22 December, when the sun is overhead at the Tropic of Capricorn and the northern hemisphere has its shortest day.
- Equinox
- 21 March and 23 September, when the sun is overhead at the equator and day and night are equal everywhere.
- Tropic of Cancer
- The parallel of 23½° N, the northern limit of the overhead sun, passing through India including Purulia and Nadia in West Bengal.
- Arctic Circle
- The parallel of 66½° N, north of which the sun does not set on 21 June and does not rise on 22 December.
- Heat zones
- The five temperature belts of the earth — torrid, two temperate and two frigid — bounded by the tropics and polar circles.
- Uttarayana and Dakshinayana
- The six-month northward (22 December to 21 June) and southward (21 June to 22 December) apparent movement of the sun.
End-of-Chapter Trial Paper & Test Questions
Topic-wise questions to test your understanding of every concept in this chapter.
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What is rotation of the earth? State its direction, period and speed at different latitudes. / पृथ्वी का घूर्णन क्या है? इसकी दिशा, अवधि और विभिन्न अक्षांशों पर गति बताइए।
Show answer
Rotation is the spinning of the earth on its own axis, the imaginary line passing through its centre and the two poles, which is tilted at 23½° from the perpendicular to the orbit. The earth rotates from west to east, that is anticlockwise when seen from above the North Pole, which is why the sun and stars appear to rise in the east and set in the west. One complete rotation relative to the stars takes 23 hours 56 minutes 4 seconds, the sidereal day, and relative to the sun 24 hours, the solar day. All places turn through 360° in this time, but the linear speed depends on latitude because a point on the equator travels the full circumference of 40,075 km: the speed is about 1,670 km per hour at the equator, about 1,540 km per hour at Kolkata (22½° N), about 835 km per hour at 60° and zero at the poles, decreasing with the cosine of the latitude. / घूर्णन पृथ्वी का अपनी धुरी पर घूमना है, वह काल्पनिक रेखा जो इसके केंद्र और दोनों ध्रुवों से होकर गुज़रती है और कक्षा के लंब से 23½° झुकी है। पृथ्वी पश्चिम से पूर्व की ओर घूमती है, अर्थात उत्तरी ध्रुव के ऊपर से देखने पर वामावर्त, इसीलिए सूर्य और तारे पूर्व में उगते और पश्चिम में डूबते दिखते हैं। तारों के सापेक्ष एक पूरा घूर्णन 23 घंटे 56 मिनट 4 सेकंड लेता है, नाक्षत्र दिन, और सूर्य के सापेक्ष 24 घंटे, सौर दिन। सभी स्थान इस समय में 360° घूमते हैं, पर रैखिक गति अक्षांश पर निर्भर करती है क्योंकि भूमध्य रेखा का बिंदु 40,075 किमी की पूरी परिधि तय करता है: गति भूमध्य रेखा पर लगभग 1,670 किमी प्रति घंटा, कोलकाता (22½° उ.) में लगभग 1,540 किमी प्रति घंटा, 60° पर लगभग 835 किमी प्रति घंटा और ध्रुवों पर शून्य है, जो अक्षांश की कोज्या के साथ घटती है।
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Describe two proofs of the rotation of the earth. / पृथ्वी के घूर्णन के दो प्रमाणों का वर्णन कीजिए।
Show answer
The first proof is the Foucault pendulum. In 1851 Léon Foucault hung a heavy iron ball on a 67 m wire from the dome of the Panthéon in Paris and set it swinging. A free pendulum keeps its plane of swing fixed in space, yet the plane appeared to turn slowly clockwise, completing a full circle in about 32 hours at that latitude. Since nothing pushed the pendulum, it was the earth that had turned beneath it; at the pole the plane would turn once in 24 hours and at the equator not at all, exactly as rotation predicts. The second proof is the Coriolis deflection: winds, ocean currents and other freely moving bodies are deflected to the right of their path in the northern hemisphere and to the left in the southern, so that the trade winds blow from the north-east and south-east instead of from the north and south, and cyclones spin anticlockwise north of the equator and clockwise south of it. Such deflection can occur only on a rotating earth whose surface turns beneath the moving body. Other proofs are the eastward drift of objects dropped from a height, the equatorial bulge and the time difference between places. / पहला प्रमाण फूको का लोलक है। 1851 में लियोन फूको ने पेरिस के पैंथियन के गुंबद से 67 मीटर के तार पर लोहे का भारी गोला लटकाकर झुलाया। मुक्त लोलक अंतरिक्ष में अपने दोलन का तल स्थिर रखता है, फिर भी वह तल धीरे-धीरे दक्षिणावर्त घूमता दिखा और उस अक्षांश पर लगभग 32 घंटे में पूरा चक्कर लगा गया। चूँकि लोलक को किसी ने धकेला नहीं, यह पृथ्वी थी जो उसके नीचे घूम गई; ध्रुव पर तल 24 घंटे में एक बार घूमता और भूमध्य रेखा पर बिल्कुल नहीं, ठीक जैसा घूर्णन बताता है। दूसरा प्रमाण कोरिओलिस विक्षेपण है: पवनें, समुद्री धाराएँ और अन्य मुक्त रूप से चलते पिंड उत्तरी गोलार्ध में अपने मार्ग के दाईं ओर और दक्षिणी में बाईं ओर मुड़ जाते हैं, जिससे व्यापारिक पवनें उत्तर और दक्षिण के बजाय उत्तर-पूर्व और दक्षिण-पूर्व से बहती हैं, और चक्रवात भूमध्य रेखा के उत्तर में वामावर्त और दक्षिण में दक्षिणावर्त घूमते हैं। ऐसा विक्षेपण केवल घूमती पृथ्वी पर ही हो सकता है जिसकी सतह चलते पिंड के नीचे घूमती है। अन्य प्रमाण हैं ऊँचाई से गिराई वस्तुओं का पूर्व की ओर खिसकना, भूमध्यरेखीय उभार और स्थानों के बीच समय का अंतर।
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What is the circle of illumination? Why are day and night of equal length at the equator throughout the year? / प्रदीप्ति वृत्त क्या है? भूमध्य रेखा पर वर्ष भर दिन और रात बराबर क्यों होते हैं?
Show answer
The circle of illumination is the great circle on the earth's surface that separates the half lit by the sun from the half in darkness; it is always at right angles to the sun's rays and therefore always divides the earth into two equal halves, one having day and the other night. Because the earth's axis is tilted, this circle usually does not pass through the poles and it cuts most parallels of latitude unequally, so that one hemisphere has longer days than the other. The equator, however, is itself a great circle, and any two great circles on a sphere bisect each other; so the circle of illumination always cuts the equator exactly in half, whatever the season. Half of the equator is thus always in daylight and half in darkness, and every point on it spends twelve hours in each as the earth rotates. That is why at the equator day and night are each twelve hours long on every day of the year, while elsewhere they are equal only on the two equinoxes. / प्रदीप्ति वृत्त पृथ्वी की सतह पर वह महावृत्त है जो सूर्य से प्रकाशित आधे भाग को अंधकार वाले आधे भाग से अलग करता है; यह सदा सूर्य की किरणों के लंबवत होता है और इसलिए पृथ्वी को सदा दो बराबर भागों में बाँटता है, एक में दिन और दूसरे में रात। चूँकि पृथ्वी की धुरी झुकी है, यह वृत्त प्रायः ध्रुवों से होकर नहीं गुज़रता और अधिकांश अक्षांश वृत्तों को असमान भागों में काटता है, जिससे एक गोलार्ध में दिन दूसरे से लंबे होते हैं। किंतु भूमध्य रेखा स्वयं एक महावृत्त है, और गोले पर कोई भी दो महावृत्त एक-दूसरे को समद्विभाजित करते हैं; अतः प्रदीप्ति वृत्त भूमध्य रेखा को हर ऋतु में ठीक आधा काटता है। इस प्रकार भूमध्य रेखा का आधा भाग सदा प्रकाश में और आधा अंधकार में रहता है, और पृथ्वी के घूमने पर उस पर हर बिंदु प्रत्येक में बारह घंटे बिताता है। इसीलिए भूमध्य रेखा पर वर्ष के हर दिन दिन और रात बारह-बारह घंटे के होते हैं, जबकि अन्यत्र वे केवल दो विषुवों पर बराबर होते हैं।
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Explain the relationship between longitude and time. What is Indian Standard Time? / देशांतर और समय के संबंध की व्याख्या कीजिए। भारतीय मानक समय क्या है?
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The earth rotates through 360° of longitude in 24 hours, so it turns through 15° in one hour and 1° in four minutes. Every meridian has its own local time, reckoned as 12 noon when the sun is highest over it; places to the east have had their noon earlier and are ahead, and places to the west are behind, by four minutes for every degree of longitude. Thus if it is noon at 88° E it is 11:16 a.m. at 77° E and 12:44 p.m. at 99° E. World time is measured from the Prime Meridian at Greenwich, and the earth is divided into 24 time zones of 15° each. Because a country using many local times would be in confusion, each country adopts the local time of one central meridian as its standard time. Indian Standard Time is the local time of the meridian of 82½° E, which passes near Mirzapur in Uttar Pradesh and lies close to the middle of India's 68° E to 97° E span; it is 5½ hours ahead of Greenwich Mean Time, since 82½ × 4 minutes = 330 minutes. So when it is noon at Greenwich it is 5:30 p.m. throughout India, though the sun actually rises about an hour earlier in Arunachal Pradesh than in Gujarat. / पृथ्वी 24 घंटे में 360° देशांतर घूमती है, अतः यह एक घंटे में 15° और चार मिनट में 1° घूमती है। हर याम्योत्तर का अपना स्थानीय समय होता है, जिसमें दोपहर 12 बजे तब माने जाते हैं जब सूर्य उसके ऊपर सबसे ऊँचा हो; पूर्व के स्थानों की दोपहर पहले हो चुकी होती है और वे आगे होते हैं, और पश्चिम के स्थान पीछे, हर अंश देशांतर पर चार मिनट। इस प्रकार यदि 88° पू. पर दोपहर है तो 77° पू. पर 11:16 पूर्वाह्न और 99° पू. पर 12:44 अपराह्न। विश्व समय ग्रीनविच की प्रधान याम्योत्तर से मापा जाता है, और पृथ्वी 15°-15° के 24 समय क्षेत्रों में बँटी है। चूँकि अनेक स्थानीय समय उपयोग करने वाला देश भ्रम में पड़ जाएगा, हर देश एक केंद्रीय याम्योत्तर के स्थानीय समय को अपना मानक समय अपनाता है। भारतीय मानक समय 82½° पू. याम्योत्तर का स्थानीय समय है, जो उत्तर प्रदेश के मिर्ज़ापुर के पास से गुज़रती है और भारत के 68° पू. से 97° पू. के विस्तार के लगभग मध्य में है; यह ग्रीनविच माध्य समय से 5½ घंटे आगे है, क्योंकि 82½ × 4 मिनट = 330 मिनट। अतः जब ग्रीनविच में दोपहर होती है तो पूरे भारत में शाम 5:30 होते हैं, यद्यपि सूर्य वास्तव में अरुणाचल प्रदेश में गुजरात से लगभग एक घंटा पहले उगता है।
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What is the Coriolis effect? Give two of its consequences. / कोरिओलिस प्रभाव क्या है? इसके दो परिणाम बताइए।
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The Coriolis effect is the apparent deflection of any body moving freely over the earth's surface — wind, ocean current, river, aircraft or projectile — from a straight path, caused by the rotation of the earth beneath it. In the northern hemisphere the deflection is to the right of the direction of motion and in the southern hemisphere to the left; it is zero at the equator, greatest at the poles, and increases with the speed of the moving body. It was explained by the French mathematician Gaspard Coriolis in 1835. One consequence is the direction of the planetary winds: the trade winds blowing from the sub-tropical high pressure towards the equator are turned into north-east trades in the northern hemisphere and south-east trades in the southern, and the westerlies blow from the south-west and north-west respectively. A second consequence is the spin of cyclones and ocean currents: tropical cyclones rotate anticlockwise in the Bay of Bengal and the North Atlantic and clockwise south of the equator, and the great ocean gyres circle clockwise in the northern oceans and anticlockwise in the southern. It also explains why cyclones cannot form within about 5° of the equator and, by Ferrel's law, why the right banks of northern-hemisphere rivers are more eroded. / कोरिओलिस प्रभाव पृथ्वी की सतह पर मुक्त रूप से चलते किसी भी पिंड — पवन, समुद्री धारा, नदी, विमान या प्रक्षेप्य — का सीधे मार्ग से आभासी विक्षेपण है, जो उसके नीचे पृथ्वी के घूर्णन के कारण होता है। उत्तरी गोलार्ध में विक्षेपण गति की दिशा के दाईं ओर और दक्षिणी गोलार्ध में बाईं ओर होता है; यह भूमध्य रेखा पर शून्य, ध्रुवों पर सबसे अधिक होता है और चलते पिंड की गति के साथ बढ़ता है। इसे 1835 में फ्रांसीसी गणितज्ञ गैस्पार्ड कोरिओलिस ने समझाया था। एक परिणाम ग्रहीय पवनों की दिशा है: उपोष्ण उच्च दाब से भूमध्य रेखा की ओर बहने वाली व्यापारिक पवनें उत्तरी गोलार्ध में उत्तर-पूर्वी व्यापारिक पवनों और दक्षिणी में दक्षिण-पूर्वी व्यापारिक पवनों में बदल जाती हैं, और पछुआ पवनें क्रमशः दक्षिण-पश्चिम और उत्तर-पश्चिम से बहती हैं। दूसरा परिणाम चक्रवातों और समुद्री धाराओं का घूमना है: उष्णकटिबंधीय चक्रवात बंगाल की खाड़ी और उत्तरी अटलांटिक में वामावर्त और भूमध्य रेखा के दक्षिण में दक्षिणावर्त घूमते हैं, और बड़े महासागरीय चक्र उत्तरी महासागरों में दक्षिणावर्त और दक्षिणी में वामावर्त घूमते हैं। यह यह भी समझाता है कि भूमध्य रेखा के लगभग 5° के भीतर चक्रवात क्यों नहीं बनते और, फेरेल के नियम के अनुसार, उत्तरी गोलार्ध की नदियों के दाएँ तट अधिक क्यों कटते हैं।
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What is revolution of the earth? Explain perihelion, aphelion and the leap year. / पृथ्वी का परिक्रमण क्या है? उपसौर, अपसौर और अधिवर्ष (लीप वर्ष) की व्याख्या कीजिए।
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Revolution is the movement of the earth round the sun along a fixed elliptical path, the orbit, with the sun at one focus, completed once in 365 days 5 hours 48 minutes 46 seconds at a speed of about 30 km per second, in an anticlockwise direction as seen from above the North Pole. Because the orbit is an ellipse, the distance from the sun changes: at perihelion, reached about 3 January, the earth is nearest the sun at 14.71 crore km, and at aphelion, about 4 July, it is farthest at 15.21 crore km; the difference is only about 3 per cent and does not cause the seasons, since the northern hemisphere has winter at perihelion. The leap year arises because the year is nearly a quarter of a day longer than 365 days; to stop the calendar drifting, an extra day, 29 February, is added every fourth year, making 366 days. Since the true year is about 11 minutes short of 365¼ days, the Gregorian calendar of 1582 omits the extra day in century years not divisible by 400: 1900 and 2100 are not leap years but 2000 and 2400 are. / परिक्रमण पृथ्वी का सूर्य के चारों ओर एक निश्चित दीर्घवृत्ताकार मार्ग, कक्षा, पर चलना है, जिसके एक नाभि पर सूर्य है, जो 365 दिन 5 घंटे 48 मिनट 46 सेकंड में लगभग 30 किमी प्रति सेकंड की गति से, उत्तरी ध्रुव के ऊपर से देखने पर वामावर्त दिशा में, एक बार पूरा होता है। चूँकि कक्षा दीर्घवृत्त है, सूर्य से दूरी बदलती रहती है: उपसौर पर, जो लगभग 3 जनवरी को आता है, पृथ्वी सूर्य के सबसे निकट 14.71 करोड़ किमी पर होती है, और अपसौर पर, लगभग 4 जुलाई को, सबसे दूर 15.21 करोड़ किमी पर; अंतर केवल लगभग 3 प्रतिशत है और ऋतुओं का कारण नहीं है, क्योंकि उत्तरी गोलार्ध में उपसौर पर शीत ऋतु होती है। अधिवर्ष इसलिए बनता है क्योंकि वर्ष 365 दिनों से लगभग एक चौथाई दिन लंबा है; कैलेंडर को खिसकने से रोकने के लिए हर चौथे वर्ष एक अतिरिक्त दिन, 29 फरवरी, जोड़ा जाता है, जिससे 366 दिन होते हैं। चूँकि वास्तविक वर्ष 365¼ दिनों से लगभग 11 मिनट छोटा है, 1582 का ग्रेगोरियन कैलेंडर 400 से विभाज्य न होने वाले शताब्दी वर्षों में अतिरिक्त दिन छोड़ देता है: 1900 और 2100 अधिवर्ष नहीं हैं पर 2000 और 2400 हैं।
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Why do seasons occur on the earth? Explain with a diagram. / पृथ्वी पर ऋतुएँ क्यों होती हैं? चित्र सहित समझाइए।
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Seasons occur because the earth's axis is tilted at 23½° to the perpendicular of its orbit, because the axis always points the same way towards the Pole Star as the earth revolves, and because of the revolution itself. As a result, for half the year the northern hemisphere is leaned towards the sun and for the other half away from it, the southern hemisphere being the opposite. When a hemisphere leans towards the sun two things happen: the sun climbs high in the sky and its rays fall nearly vertically, concentrating their heat on a small area and passing through little atmosphere; and the circle of illumination cuts its parallels so that more than half of each is in daylight, giving long days and short nights, so the ground heats for longer and cools for less time. Together these give summer. When the hemisphere leans away, the rays fall at a slant and spread their heat thinly, the days are short and the nights long, and it is winter. At the equinoxes neither hemisphere leans towards the sun and both have the mild seasons of spring and autumn. The diagram shows the earth at four points of its orbit with the axis parallel in each: on 21 June the North Pole tilted towards the sun (northern summer), on 22 December away from it (northern winter), and on 21 March and 23 September sideways (equal day and night). The changing distance from the sun is not the cause, since the earth is nearest the sun in January. / ऋतुएँ इसलिए होती हैं क्योंकि पृथ्वी की धुरी उसकी कक्षा के लंब से 23½° झुकी है, क्योंकि परिक्रमण करते समय धुरी सदा ध्रुव तारे की ओर एक ही दिशा में रहती है, और परिक्रमण के कारण भी। फलस्वरूप आधे वर्ष उत्तरी गोलार्ध सूर्य की ओर झुका रहता है और शेष आधे वर्ष उससे दूर, दक्षिणी गोलार्ध इसके विपरीत। जब कोई गोलार्ध सूर्य की ओर झुकता है तो दो बातें होती हैं: सूर्य आकाश में ऊँचा चढ़ता है और उसकी किरणें लगभग लंबवत गिरती हैं, अपनी ऊष्मा को छोटे क्षेत्र पर केंद्रित करते हुए और कम वायुमंडल से गुज़रते हुए; और प्रदीप्ति वृत्त उसके अक्षांश वृत्तों को ऐसे काटता है कि हर एक का आधे से अधिक भाग प्रकाश में रहता है, जिससे दिन लंबे और रातें छोटी होती हैं, अतः ज़मीन अधिक देर गर्म होती है और कम देर ठंडी। ये मिलकर ग्रीष्म देते हैं। जब गोलार्ध दूर झुकता है तो किरणें तिरछी गिरती हैं और अपनी ऊष्मा पतली फैलाती हैं, दिन छोटे और रातें लंबी होती हैं, और शीत ऋतु होती है। विषुवों पर कोई गोलार्ध सूर्य की ओर नहीं झुकता और दोनों में वसंत और शरद की मृदु ऋतुएँ होती हैं। चित्र में पृथ्वी अपनी कक्षा के चार बिंदुओं पर हर एक में समांतर धुरी के साथ दिखाई गई है: 21 जून को उत्तरी ध्रुव सूर्य की ओर झुका (उत्तरी ग्रीष्म), 22 दिसंबर को उससे दूर (उत्तरी शीत), और 21 मार्च तथा 23 सितंबर को बगल की ओर (बराबर दिन-रात)। सूर्य से बदलती दूरी कारण नहीं है, क्योंकि पृथ्वी जनवरी में सूर्य के सबसे निकट होती है।
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Describe the position of the earth on 21 June and 22 December and their effects. / 21 जून और 22 दिसंबर को पृथ्वी की स्थिति और उनके प्रभावों का वर्णन कीजिए।
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On 21 June, the summer solstice, the North Pole is tilted towards the sun by the full 23½°, so the sun's vertical rays fall on the Tropic of Cancer at 23½° N, the farthest north the overhead sun ever reaches. The circle of illumination extends 23½° beyond the North Pole, so the whole area within the Arctic Circle has 24 hours of daylight while the area within the Antarctic Circle has 24 hours of night. The northern hemisphere has its longest day and shortest night — about 13½ hours of daylight at Kolkata and 16½ at London — and its summer, while the southern hemisphere has its shortest day and winter. On 22 December, the winter solstice, the South Pole is tilted towards the sun and the vertical rays fall on the Tropic of Capricorn at 23½° S. The Antarctic Circle now has 24 hours of daylight and the Arctic Circle 24 hours of darkness. The northern hemisphere has its shortest day and longest night — about 10¾ hours of daylight at Kolkata — and its winter, while the southern hemisphere has its longest day and its summer. The two dates mark the turning points of the sun's apparent annual journey between the tropics. / 21 जून, ग्रीष्म संक्रांति, को उत्तरी ध्रुव पूरे 23½° सूर्य की ओर झुका होता है, अतः सूर्य की लंबवत किरणें 23½° उ. पर कर्क रेखा पर पड़ती हैं, जो सिर के ऊपर के सूर्य की सबसे उत्तरी सीमा है। प्रदीप्ति वृत्त उत्तरी ध्रुव से 23½° आगे तक फैल जाता है, अतः आर्कटिक वृत्त के भीतर पूरे क्षेत्र में 24 घंटे दिन रहता है जबकि अंटार्कटिक वृत्त के भीतर 24 घंटे रात। उत्तरी गोलार्ध में सबसे लंबा दिन और सबसे छोटी रात होती है — कोलकाता में लगभग 13½ घंटे और लंदन में 16½ घंटे दिन — और ग्रीष्म ऋतु, जबकि दक्षिणी गोलार्ध में सबसे छोटा दिन और शीत ऋतु। 22 दिसंबर, शीत संक्रांति, को दक्षिणी ध्रुव सूर्य की ओर झुका होता है और लंबवत किरणें 23½° द. पर मकर रेखा पर पड़ती हैं। अब अंटार्कटिक वृत्त में 24 घंटे दिन और आर्कटिक वृत्त में 24 घंटे अंधकार रहता है। उत्तरी गोलार्ध में सबसे छोटा दिन और सबसे लंबी रात होती है — कोलकाता में लगभग 10¾ घंटे दिन — और शीत ऋतु, जबकि दक्षिणी गोलार्ध में सबसे लंबा दिन और ग्रीष्म। ये दो तिथियाँ कटिबंधों के बीच सूर्य की आभासी वार्षिक यात्रा के मोड़ बिंदु हैं।
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What is an equinox? Why are day and night equal all over the earth on 21 March and 23 September? / विषुव क्या है? 21 मार्च और 23 सितंबर को पूरी पृथ्वी पर दिन और रात बराबर क्यों होते हैं?
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An equinox, meaning equal night, is either of the two days in the year, 21 March (the vernal or spring equinox) and 23 September (the autumnal equinox), when the sun's vertical rays fall on the equator and day and night are of equal length, twelve hours each, everywhere on the earth. On these two dates the earth is at the points of its orbit where its tilted axis is neither leaning towards the sun nor away from it but sideways to the sun's rays. The circle of illumination therefore passes exactly through both poles and coincides with a meridian circle, so it divides every parallel of latitude, from the equator to the polar circles, into two equal halves, one in daylight and one in darkness. As the earth rotates, each place spends twelve hours in each half. The sun rises due east and sets due west everywhere, and both poles see the sun on the horizon at the same time, one at its six-month sunrise and the other at its sunset. On all other days the tilt makes the circle of illumination cut the parallels unequally, so day and night differ except at the equator. / विषुव, जिसका अर्थ है समान रात, वर्ष के दो दिनों में से कोई एक है, 21 मार्च (वसंत विषुव) और 23 सितंबर (शरद विषुव), जब सूर्य की लंबवत किरणें भूमध्य रेखा पर पड़ती हैं और पृथ्वी पर सर्वत्र दिन और रात बराबर, बारह-बारह घंटे के, होते हैं। इन दो तिथियों पर पृथ्वी अपनी कक्षा के उन बिंदुओं पर होती है जहाँ उसकी झुकी धुरी न सूर्य की ओर झुकी होती है न उससे दूर, बल्कि सूर्य की किरणों की बगल में होती है। अतः प्रदीप्ति वृत्त ठीक दोनों ध्रुवों से होकर गुज़रता है और एक याम्योत्तर वृत्त से मेल खाता है, जिससे यह भूमध्य रेखा से ध्रुवीय वृत्तों तक हर अक्षांश वृत्त को दो बराबर भागों में बाँटता है, एक प्रकाश में और एक अंधकार में। पृथ्वी के घूमने पर हर स्थान प्रत्येक भाग में बारह घंटे बिताता है। सूर्य सर्वत्र ठीक पूर्व में उगता और ठीक पश्चिम में डूबता है, और दोनों ध्रुव एक ही समय सूर्य को क्षितिज पर देखते हैं, एक अपने छह महीने के सूर्योदय पर और दूसरा अपने सूर्यास्त पर। अन्य सभी दिनों में झुकाव के कारण प्रदीप्ति वृत्त अक्षांश वृत्तों को असमान काटता है, अतः भूमध्य रेखा को छोड़कर दिन और रात भिन्न होते हैं।
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Explain why the length of day and night varies with latitude and season. What happens at the poles? / दिन और रात की लंबाई अक्षांश और ऋतु के साथ क्यों बदलती है, समझाइए। ध्रुवों पर क्या होता है?
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The length of day at a place is the fraction of its parallel of latitude that lies within the lit half of the earth. Because the axis is tilted, the circle of illumination is tilted to the parallels for most of the year, and so it divides them unequally: in the hemisphere leaning towards the sun more than half of each parallel is in daylight, and the fraction grows with latitude, so days lengthen from the equator towards the pole; in the hemisphere leaning away, less than half is lit and days shorten towards the pole. The equator, being a great circle, is always halved and always has a 12-hour day. As the earth revolves, the lean of each hemisphere changes from towards to away from the sun, so the same parallel has its longest day at one solstice and its shortest at the other, with equal day and night at the equinoxes; at Kolkata the day varies from about 13½ hours in June to 10¾ in December, at 60° N from 18½ to 5½. Beyond the polar circles the circle of illumination fails to reach the parallels at the solstices, giving 24-hour days in summer and 24-hour nights in winter that lengthen towards the pole. At the poles themselves the sun rises once a year at the spring equinox, circles the sky for six months, climbing to 23½° at the solstice, and sets at the autumn equinox for six months of night; the two poles have their six-month days at opposite times. / किसी स्थान पर दिन की लंबाई उसके अक्षांश वृत्त का वह अंश है जो पृथ्वी के प्रकाशित आधे भाग में पड़ता है। चूँकि धुरी झुकी है, प्रदीप्ति वृत्त वर्ष के अधिकांश समय अक्षांश वृत्तों के प्रति झुका रहता है, और इसलिए उन्हें असमान बाँटता है: सूर्य की ओर झुके गोलार्ध में हर अक्षांश वृत्त का आधे से अधिक भाग प्रकाश में रहता है, और यह अंश अक्षांश के साथ बढ़ता है, अतः दिन भूमध्य रेखा से ध्रुव की ओर लंबे होते जाते हैं; दूर झुके गोलार्ध में आधे से कम प्रकाशित रहता है और दिन ध्रुव की ओर छोटे होते जाते हैं। भूमध्य रेखा, महावृत्त होने से, सदा आधी कटती है और सदा 12 घंटे का दिन रखती है। पृथ्वी के परिक्रमण के साथ हर गोलार्ध का झुकाव सूर्य की ओर से दूर की ओर बदलता है, अतः एक ही अक्षांश वृत्त पर एक संक्रांति में सबसे लंबा दिन और दूसरी में सबसे छोटा होता है, विषुवों पर बराबर दिन-रात के साथ; कोलकाता में दिन जून में लगभग 13½ घंटे से दिसंबर में 10¾ घंटे तक बदलता है, 60° उ. पर 18½ से 5½ तक। ध्रुवीय वृत्तों के परे संक्रांतियों पर प्रदीप्ति वृत्त अक्षांश वृत्तों तक पहुँच ही नहीं पाता, जिससे ग्रीष्म में 24 घंटे के दिन और शीत में 24 घंटे की रातें होती हैं जो ध्रुव की ओर लंबी होती जाती हैं। स्वयं ध्रुवों पर सूर्य वर्ष में एक बार वसंत विषुव पर उगता है, छह महीने आकाश में चक्कर लगाता है, संक्रांति पर 23½° तक चढ़ता है, और शरद विषुव पर छह महीने की रात के लिए डूब जाता है; दोनों ध्रुवों के छह महीने के दिन विपरीत समयों पर होते हैं।
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Describe the heat zones of the earth. / पृथ्वी के ताप कटिबंधों का वर्णन कीजिए।
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The earth is divided by the tropics and the polar circles into five heat zones according to the angle of the sun's rays and the length of day. The Torrid Zone lies between the Tropic of Cancer (23½° N) and the Tropic of Capricorn (23½° S); here the sun is overhead at least once a year, the rays are nearly vertical throughout the year and day and night are nearly equal, so it is hot all year with only wet and dry seasons; peninsular India, the Amazon, the Congo and Indonesia lie in it. The North and South Temperate Zones lie between the tropics and the polar circles (23½° to 66½°); the sun is never overhead but never absent, the rays fall at a slant that changes with the season and day-length varies widely, giving four distinct seasons and moderate temperatures; northern India, Europe, the USA, China and Argentina lie here, and most of humanity. The North and South Frigid Zones lie within the Arctic and Antarctic Circles (66½° to 90°); the sun never rises high and does not rise at all for part of the year, the rays are always very oblique and the surface is ice, so they are bitterly cold with a short summer and a long dark winter; Greenland, Siberia and Antarctica belong to them. The zones are approximate because altitude, oceans and winds also govern temperature. / पृथ्वी सूर्य की किरणों के कोण और दिन की लंबाई के अनुसार कटिबंधों और ध्रुवीय वृत्तों द्वारा पाँच ताप कटिबंधों में बँटी है। उष्ण कटिबंध कर्क रेखा (23½° उ.) और मकर रेखा (23½° द.) के बीच है; यहाँ सूर्य वर्ष में कम से कम एक बार सिर के ऊपर होता है, किरणें वर्ष भर लगभग लंबवत रहती हैं और दिन-रात लगभग बराबर, इसलिए यह वर्ष भर गर्म रहता है और केवल आर्द्र तथा शुष्क ऋतुएँ होती हैं; प्रायद्वीपीय भारत, अमेज़न, कांगो और इंडोनेशिया इसमें हैं। उत्तरी और दक्षिणी शीतोष्ण कटिबंध कटिबंधों और ध्रुवीय वृत्तों के बीच (23½° से 66½°) हैं; सूर्य कभी सिर के ऊपर नहीं होता पर कभी अनुपस्थित भी नहीं, किरणें ऋतु के साथ बदलते तिरछेपन से गिरती हैं और दिन की लंबाई बहुत बदलती है, जिससे चार स्पष्ट ऋतुएँ और संतुलित तापमान मिलते हैं; उत्तरी भारत, यूरोप, अमेरिका, चीन और अर्जेंटीना यहाँ हैं, और अधिकांश मानवता। उत्तरी और दक्षिणी शीत कटिबंध आर्कटिक और अंटार्कटिक वृत्तों के भीतर (66½° से 90°) हैं; सूर्य कभी ऊँचा नहीं चढ़ता और वर्ष के कुछ भाग में उगता ही नहीं, किरणें सदा बहुत तिरछी होती हैं और सतह बर्फ है, इसलिए वे छोटी ग्रीष्म और लंबी अँधेरी शीत के साथ कड़ाके की ठंड वाले हैं; ग्रीनलैंड, साइबेरिया और अंटार्कटिका इनमें हैं। ये कटिबंध अनुमानित हैं क्योंकि ऊँचाई, महासागर और पवनें भी तापमान को नियंत्रित करती हैं।
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Distinguish between rotation and revolution of the earth. / पृथ्वी के घूर्णन और परिक्रमण में अंतर बताइए।
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Rotation is the spinning of the earth on its own axis, whereas revolution is its movement round the sun along its orbit. Rotation takes place about the axis through the poles, from west to east; revolution takes place along an elliptical orbit with the sun at one focus, anticlockwise as seen from above the North Pole. Rotation is completed in 24 hours (23 hours 56 minutes 4 seconds relative to the stars); revolution in 365 days 5 hours 48 minutes 46 seconds. The speed of rotation is about 1,670 km per hour at the equator and falls to zero at the poles; the speed of revolution is about 30 km per second for the whole earth. Rotation causes day and night, the apparent daily movement of the sun and stars, the difference of time between places, the Coriolis deflection of winds and currents, the tides and the equatorial bulge; revolution, together with the tilt of the axis, causes the year, the seasons, the changing length of day and night, the solstices and equinoxes, the apparent migration of the sun between the tropics and the heat zones. Rotation is proved by the Foucault pendulum and the Coriolis effect; revolution by the seasons, stellar parallax and the changing constellations of the night sky. / घूर्णन पृथ्वी का अपनी धुरी पर घूमना है, जबकि परिक्रमण उसकी कक्षा पर सूर्य के चारों ओर उसकी गति है। घूर्णन ध्रुवों से गुज़रती धुरी के चारों ओर पश्चिम से पूर्व होता है; परिक्रमण एक दीर्घवृत्ताकार कक्षा पर होता है जिसके एक नाभि पर सूर्य है, उत्तरी ध्रुव के ऊपर से देखने पर वामावर्त। घूर्णन 24 घंटे में पूरा होता है (तारों के सापेक्ष 23 घंटे 56 मिनट 4 सेकंड); परिक्रमण 365 दिन 5 घंटे 48 मिनट 46 सेकंड में। घूर्णन की गति भूमध्य रेखा पर लगभग 1,670 किमी प्रति घंटा है और ध्रुवों पर शून्य हो जाती है; परिक्रमण की गति पूरी पृथ्वी के लिए लगभग 30 किमी प्रति सेकंड है। घूर्णन दिन-रात, सूर्य और तारों की आभासी दैनिक गति, स्थानों के बीच समय का अंतर, पवनों और धाराओं का कोरिओलिस विक्षेपण, ज्वार-भाटा और भूमध्यरेखीय उभार उत्पन्न करता है; परिक्रमण, धुरी के झुकाव के साथ, वर्ष, ऋतुएँ, दिन-रात की बदलती लंबाई, संक्रांतियाँ और विषुव, कटिबंधों के बीच सूर्य का आभासी प्रवास और ताप कटिबंध उत्पन्न करता है। घूर्णन फूको के लोलक और कोरिओलिस प्रभाव से सिद्ध होता है; परिक्रमण ऋतुओं, तारकीय लंबन और रात के आकाश के बदलते तारामंडलों से।
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