Chapter 2 — Movements of the Earth

The Earth has two principal movements, and the whole chapter rests on keeping them clearly apart. Rotation is the spinning of the Earth on its own axis, like a top, which it completes once in about 24 hours and which produces day and night. Revolution is the journey of the Earth round the Sun along its orbit, which it completes once in about 365 and a quarter days and which, together with the tilt of the axis, produces the seasons. Both movements go on at the same time: while the Earth spins, it is also travelling through space at about 30 kilometres every second, so that in one day of spinning it has moved about 26 lakh kilometres along its orbit.

Both movements are in the same direction, from west to east, that is anticlockwise when seen from above the North Pole. This is why the Sun, the Moon and the stars all appear to rise in the east and set in the west: they are not moving; we are turning towards the east beneath them. The Earth is also carried along with the whole solar system round the centre of the Milky Way, and it has slower wobbles of its axis, but these are not part of the Class 9 course.

The axis is the imaginary line through the centre of the Earth about which it rotates. It meets the surface at the two poles, the North Pole and the South Pole. The axis is not upright with respect to the Earth's path round the Sun; it leans over at an angle of 66 and a half degrees to the plane of the orbit, or, put the other way, it is tilted 23 and a half degrees from the perpendicular to that plane. The plane of the orbit is called the plane of the ecliptic. The axis always points the same way in space, towards the Pole Star, wherever the Earth is in its orbit; this is called the parallelism of the axis, and it is the key to the seasons.

A simple way to remember the difference: rotation is a daily movement about an axis inside the Earth, and revolution is a yearly movement about a point, the Sun, outside the Earth. The effects of rotation are daily, day and night and the tides; the effects of revolution are yearly, the seasons and the changing length of days. The student should be able to draw both movements in one diagram: a tilted Earth spinning in place while moving along an ellipse round the Sun.

The Earth rotates on its axis from west to east. Seen from above the North Pole the spin is anticlockwise; seen from above the South Pole it is clockwise. One complete rotation measured against the distant stars takes 23 hours 56 minutes and 4 seconds, called a sidereal day. But because the Earth has also moved a little along its orbit during that time, it must turn for about four minutes more before the Sun returns to the same position overhead; this interval of 24 hours is the solar day, on which clocks are based. The four minutes add up over the year to one extra rotation relative to the stars, so a year has 366 and a quarter sidereal days but 365 and a quarter solar days.

The speed of rotation is not the same everywhere. Every point on the Earth completes one circle in the same 24 hours, but the circle is largest at the equator and shrinks to a point at the poles. At the equator the surface must cover the whole circumference of about 40,077 kilometres in 24 hours, a speed of about 1,670 kilometres per hour, faster than a passenger aircraft. At 30 degrees latitude the speed is about 1,446 kilometres per hour, at 60 degrees about 835, and at the poles it is zero; a person standing exactly at the pole simply turns round once a day. The rule is that the speed at any latitude equals the speed at the equator multiplied by the cosine of the latitude. Kolkata, at about 22.5 degrees north, moves at about 1,540 kilometres per hour.

We feel none of this motion because everything around us, the ground, the air, the buildings, moves with us at the same steady speed, just as a passenger in a smoothly moving train does not feel the train's speed. Only a change of speed would be felt, and the Earth's rotation does not change. The angular speed, however, is the same everywhere: the Earth turns through 360 degrees in 24 hours, that is 15 degrees in one hour or one degree in four minutes. This simple relation between angle and time is the foundation of the whole system of longitude and local time studied in the next chapter.

The rotation of the Earth is slowly getting slower because of the drag of the tides; the day lengthens by about two thousandths of a second per century. Six hundred million years ago a day was only about 21 hours long. For ordinary purposes, however, the length of the day may be taken as fixed.

For most of history people believed that the Earth stood still and the sky turned round it, because that is what the eyes suggest. The daily rising and setting of the Sun could be explained either way: a turning sky over a still Earth, or a still sky over a turning Earth. Several proofs settle the matter.

Foucault's pendulum. In 1851 the French physicist Léon Foucault hung a heavy iron ball on a wire 67 metres long from the dome of the Panthéon in Paris and set it swinging. A freely swinging pendulum keeps swinging in the same plane in space; nothing can turn it. Yet over the hours the direction of the swing, marked by a pointer scratching in sand on the floor, slowly turned clockwise. Since nothing had turned the pendulum, the floor beneath it must have turned. At the poles the plane appears to turn through a full 360 degrees in 24 hours; at the equator it does not turn at all; at other latitudes it turns at a rate proportional to the sine of the latitude. Foucault pendulums can be seen today in science museums and the experiment can be repeated by anyone.

Deflection of moving bodies. Winds, ocean currents, rivers and even artillery shells moving over long distances are deflected to the right of their path in the northern hemisphere and to the left in the southern hemisphere. This is the Coriolis effect, and it can only be produced by a rotating surface. The trade winds, which should blow straight towards the equator, blow from the north east and the south east instead; cyclones spin anticlockwise north of the equator and clockwise south of it. All of this is exactly what rotation predicts.

Flattening at the poles. The Earth's equatorial diameter is 43 kilometres greater than its polar diameter. Only a rotating body bulges at its equator in this way, because the outward centrifugal effect is greatest where the speed of rotation is greatest.

Movement of the stars and satellites. A camera pointed at the Pole Star for several hours records the other stars as circular arcs round it, all sweeping through 15 degrees per hour, the exact rate of the Earth's rotation. An artificial satellite launched in a fixed orbit passes over a point farther west on each round, because the Earth has turned eastward beneath it. Finally, the sequence of sunrise from east to west across the globe, observed and timed every day, fits a turning Earth and nothing else.

The most obvious effect of rotation is the alternation of day and night. The Sun can light only the half of the Earth that faces it at any moment; the other half is in darkness. As the Earth turns from west to east, each place is carried out of the dark half into the lit half, which we call sunrise, across the lit half, and back into the dark half at sunset. If the Earth did not rotate, one half would have perpetual day and become scorching hot while the other half would lie in perpetual night and freeze, and life as we know it would be impossible. Rotation spreads the Sun's heat round the whole globe every 24 hours.

The line that separates the lighted half from the dark half is called the circle of illumination. It is a great circle, dividing the Earth into two equal halves, and it moves continuously westward over the surface as the Earth turns eastward beneath it. The circle of illumination is not the same as the axis or the meridians; because the axis is tilted, the circle of illumination usually cuts the parallels of latitude unequally, and only on the equinoxes does it pass through both poles. Places on the eastern edge of the lit half are seeing sunset, and places on the western edge are seeing sunrise.

Because the Earth's atmosphere scatters sunlight, the change from day to night is not sudden. For a period before sunrise and after sunset, when the Sun is below the horizon but its light still reaches the upper air, the sky remains faintly lit. The light before sunrise is dawn and the light after sunset is dusk; together they are called twilight. Twilight is short near the equator, where the Sun rises and sets steeply, about half an hour, and long in high latitudes, where the Sun crosses the horizon at a slant; in northern Europe in summer it may last all night. In Kolkata twilight lasts a little over half an hour.

The length of the lit period at any place, the day, depends on where the circle of illumination cuts that place's parallel of latitude, which in turn depends on the season. This is taken up under the effects of revolution. For the moment the student should remember: rotation causes day and night to alternate, the circle of illumination is the boundary between them, and twilight is the gradual passage across that boundary produced by the scattering of light in the atmosphere.

Because the Earth turns through 360 degrees in 24 hours, every hour it turns through 15 degrees of longitude and every four minutes through one degree. This is the basis of local time. At any place the moment when the Sun is highest in the sky, exactly on the meridian, is local noon, and the local clock is set from it. Since the Sun crosses the meridians one after another as the Earth rotates eastward, local noon comes earlier at places farther east and later at places farther west. Two places on the same meridian have the same local time, however far apart they are north and south; two places on different meridians always have different local times.

The rule for calculation is simple. Find the difference in longitude between the two places, multiply by four minutes for each degree, and add the result if the place sought is to the east or subtract it if the place is to the west. For example, Kolkata lies at about 88 degrees east and Mumbai at about 73 degrees east, a difference of 15 degrees; so local noon at Kolkata comes one hour before local noon at Mumbai. When the Sun is overhead at Kolkata the local time at Mumbai is only 11 a.m.

Local time was sufficient when people travelled slowly, but railways and the telegraph made the confusion of hundreds of local times intolerable. Countries therefore adopt a standard time, the local time of one chosen meridian applied to the whole country. India uses the local time of the meridian of 82 and a half degrees east, which passes near Mirzapur in Uttar Pradesh, giving Indian Standard Time five and a half hours ahead of Greenwich. Because India stretches from about 68 to 97 degrees east, nearly two hours of local time, the Sun rises in Arunachal Pradesh long before it rises in Gujarat although both keep the same clock time.

The Earth's rotation also gives us our unit of time. The mean solar day of 24 hours, divided into hours, minutes and seconds, is the day of ordinary life, and the year, the month and the week are counted in these days. The whole subject of longitude, time zones and the International Date Line is dealt with in the next chapter; here it is enough to see that all of it follows from a single fact, that the Earth rotates through 15 degrees every hour.

Any body that moves freely over the surface of the rotating Earth, whether a parcel of air, a mass of ocean water, a river or a rocket, appears to be pushed sideways from its intended course. In the northern hemisphere the deflection is to the right of the direction of motion; in the southern hemisphere it is to the left. This apparent force is named the Coriolis effect after the French scientist Gaspard Gustave de Coriolis, who explained it mathematically in 1835. It is not a real push; it arises because the ground itself is turning under the moving body.

The reason can be understood from the speed of rotation. A point on the equator moves eastward at about 1,670 kilometres per hour, while a point at 60 degrees north moves at only 835. Suppose a mass of air starts from the equator heading straight north. It carries its eastward speed of 1,670 kilometres per hour with it. As it moves into latitudes where the ground is moving more slowly eastward, the air is moving east faster than the ground beneath it and so drifts to the east, that is to the right of its northward path. Air moving from high latitudes towards the equator has less eastward speed than the ground it reaches and lags behind to the west, which is again to the right of its southward path. In the southern hemisphere the same reasoning gives deflection to the left. The effect is zero at the equator and greatest at the poles, and it grows with the speed of the moving body.

The consequences are seen everywhere in physical geography. The trade winds, blowing towards the equator, become north-easterly north of it and south-easterly south of it. The westerlies of the middle latitudes are south-westerly in the northern hemisphere and north-westerly in the southern. Cyclones in the Bay of Bengal spin anticlockwise, while those off Australia spin clockwise. Ocean currents form great circling gyres, clockwise in the North Atlantic and the North Pacific and anticlockwise in the southern oceans. Even large rivers in the northern hemisphere tend to erode their right banks more than their left, a rule stated by the Russian scientist Karl von Baer.

Engineers must allow for the effect too. Long range artillery and rockets are aimed with a correction for it, and aircraft on long flights would drift off course without it being taken into account. The Coriolis effect is one of the clearest proofs that the Earth rotates, and it is the reason why the winds and currents of the world follow curved rather than straight paths.

Twice every day the level of the sea along the coast rises and falls. The rising of the water is the flood tide or high tide and the falling is the ebb tide or low tide. Anyone who has stood on the bank of the Hooghly at Kolkata has seen the water rise by several metres and then drain away again. Tides are caused by the gravitational pull of the Moon, and to a smaller extent of the Sun, on the waters of the ocean, but it is the rotation of the Earth that makes them a daily event, which is why they belong in this chapter.

The Moon pulls the water on the side of the Earth nearest to it into a bulge. On the opposite side of the Earth, farthest from the Moon, the Moon's pull is weaker than at the Earth's centre, so the water there is left behind and forms a second bulge. Thus at any moment there are two high tides on opposite sides of the Earth and two low tides in between them. These bulges stay roughly in line with the Moon while the Earth rotates beneath them. In one rotation of 24 hours a place on the coast passes under both bulges and both hollows, so it has two high tides and two low tides. Because the Moon also moves along its orbit, the Earth has to turn about 50 minutes longer to bring the same place back under the Moon, so the tides come about 12 hours 25 minutes apart and about 50 minutes later each day.

The Sun is much more massive than the Moon but it is 400 times farther away, so its tide raising force is less than half the Moon's. When the Sun, the Moon and the Earth are in a straight line, at new moon and full moon, the two pulls add together and the tides are unusually high and low; these are spring tides, which have nothing to do with the season. When the Moon is at right angles to the Sun as seen from the Earth, at the first and third quarters, the pulls partly cancel and the range is small; these are neap tides. Spring and neap tides therefore alternate about every seven days.

Tides matter to West Bengal. The tidal bore of the Hooghly, a wall of water that rushes up the river with the rising tide, once endangered small boats. Large ships enter the port of Kolkata and Haldia on the high tide. The Sundarbans are flooded and drained twice daily, sustaining the mangrove forest and its fisheries. Tidal energy is a possible source of electricity, and tidal currents help to scour silt from harbours.

While it spins, the Earth also travels round the Sun. This movement is revolution, and the path followed is the orbit. Johannes Kepler showed in 1609 that the orbit is not a circle but an ellipse, a slightly flattened oval, with the Sun at one focus rather than at the centre. As a result the distance between the Earth and the Sun changes through the year. The Earth is nearest to the Sun, at about 14.7 crore kilometres, on about 3 January; this position is called perihelion, from Greek words meaning near the Sun. It is farthest from the Sun, at about 15.2 crore kilometres, on about 4 July; this position is aphelion, meaning away from the Sun. The difference of about 50 lakh kilometres is only about three per cent of the distance, so the orbit is very nearly a circle and this variation has only a small effect on the seasons. Notice that the Earth is closest to the Sun in the northern winter, which shows that distance is not the cause of the seasons.

The Earth moves along its orbit at an average speed of about 30 kilometres per second, a little faster near perihelion and a little slower near aphelion, as Kepler's second law requires. The whole orbit is about 94 crore kilometres long. One complete revolution measured against the stars takes 365 days 6 hours 9 minutes, the sidereal year; measured from one spring equinox to the next it takes 365 days 5 hours 48 minutes 46 seconds, the tropical year, on which the calendar is based. The direction of revolution is anticlockwise as seen from above the North Pole, the same as the direction of rotation.

The plane containing the orbit is the plane of the ecliptic. The Earth's axis is tilted 66 and a half degrees to this plane and keeps that tilt, always pointing towards the Pole Star, throughout the year. Because the axis remains parallel to itself at every point in the orbit, first the North Pole and then the South Pole leans towards the Sun as the year goes round. This combination of a fixed tilt and a circling path is what produces the seasons.

The student should be able to draw the orbit as an ellipse, mark the Sun at one focus, show perihelion and aphelion with their dates and distances, and draw the Earth at four positions with its axis tilted the same way in each. The proof that the Earth revolves is the yearly change of the constellations seen at night, the apparent shift of nearby stars called parallax, and the aberration of starlight, but for the examination the effects of revolution matter more than its proofs.

A calendar must fit whole days to a year that is not a whole number of days. The tropical year, the time from one spring equinox to the next, is 365 days 5 hours 48 minutes and 46 seconds, which is 365.2422 days. If the calendar year were always 365 days, the extra quarter day would be lost each year, and after four years the calendar would be one day ahead of the seasons; after 120 years it would be a month ahead, and midsummer would fall in what the calendar called July and then August. To prevent this drift the extra quarter days are saved up and added as one whole day, 29 February, every fourth year. A year with this extra day has 366 days and is called a leap year.

The rule for finding a leap year is: a year exactly divisible by four is a leap year, so 2020, 2024 and 2028 are leap years and 2023 is not. But a quarter day added every four years is slightly too much, because the true excess is 5 hours 48 minutes, not 6 hours; the difference of 11 minutes 14 seconds a year comes to about three days in four hundred years. To correct this, the century years, which are all divisible by four, are made leap years only if they are divisible by 400. Thus 1700, 1800 and 1900 were not leap years, but 1600 and 2000 were, and 2100 will not be. This refinement was introduced by Pope Gregory XIII in 1582 and the resulting calendar is the Gregorian calendar, now used by almost every country including India for civil purposes. It is accurate to about one day in 3,300 years.

Before the reform the Julian calendar, introduced by Julius Caesar in 45 BC, had made every fourth year a leap year without the century rule, and by 1582 it had drifted ten days from the seasons; ten days were dropped, so that 4 October was followed by 15 October. Britain and its colonies, including India under the East India Company, adopted the change only in 1752, dropping eleven days.

The Indian national calendar, the Saka calendar, and the Bengali calendar used for festivals in West Bengal are also solar calendars that keep in step with the seasons, and the Bengali new year, Poila Boishakh, falls in mid April at the time when the Sun enters the sign of Mesha. Lunar calendars such as the Islamic calendar follow the Moon and drift through the seasons by about eleven days every year. All calendar problems arise from the same simple fact that the Earth's revolution does not contain a whole number of its rotations.

If the Earth's axis stood upright, at right angles to the plane of its orbit, the Sun would always be overhead at the equator, every place would have twelve hours of day and twelve of night throughout the year, and there would be no seasons at all. The seasons exist because of two facts about the axis, its inclination and its parallelism.

Inclination. The axis is tilted at 66 and a half degrees to the plane of the orbit, which is the same as saying it is tilted 23 and a half degrees away from the perpendicular. The figure 23 and a half degrees is the one that appears again and again in geography: it is the latitude of the Tropic of Cancer and the Tropic of Capricorn, the farthest north and south that the overhead Sun ever reaches, and 90 minus 23 and a half, that is 66 and a half degrees, is the latitude of the Arctic and Antarctic Circles, the limits of the midnight Sun. The plane of the equator is thus tilted at 23 and a half degrees to the plane of the ecliptic.

Parallelism. As the Earth travels round the Sun, the axis does not swing about; it stays pointing in the same direction in space, towards the Pole Star, at every point in the orbit. The axis at one position of the Earth is therefore parallel to the axis at every other position, which is why this property is called the parallelism of the axis. Its consequence is that in one half of the orbit the North Pole leans towards the Sun and in the other half it leans away, and the same for the South Pole in reverse.

Consider the effect. On 21 June the North Pole is tilted towards the Sun as far as it can be, so the Sun's vertical rays fall on the Tropic of Cancer, the northern hemisphere receives more direct heat and longer days, and it is summer there. Six months later, on 22 December, the Earth is on the opposite side of its orbit, the axis still points the same way in space, so now the South Pole leans towards the Sun, the vertical rays fall on the Tropic of Capricorn, and the northern hemisphere has its winter. In between, on 21 March and 23 September, neither pole leans towards the Sun and the rays are vertical at the equator.

Thus the tilt gives unequal heating, and parallelism makes that inequality shift from one hemisphere to the other in a yearly cycle. Together with revolution these two properties of the axis are the complete cause of the seasons; the small change in the Earth's distance from the Sun plays almost no part.

The year divides naturally into four positions of the Earth in its orbit, marked by two solstices and two equinoxes.

Summer solstice, 21 June. The North Pole leans towards the Sun. The Sun's vertical rays fall on the Tropic of Cancer at 23 and a half degrees north. The northern hemisphere has its longest day and shortest night; the length of day increases northward until, within the Arctic Circle, the Sun does not set at all for twenty four hours. The southern hemisphere has its shortest day, and the Antarctic Circle has twenty four hours of night. It is summer in the north and winter in the south. The word solstice means Sun standing still, because on this day the Sun's noon position stops moving north and begins to return south.

Autumnal equinox, 23 September. Neither pole leans towards the Sun. The vertical rays fall on the equator, the circle of illumination passes through both poles and every place on the Earth has twelve hours of day and twelve of night; equinox means equal night. It is autumn in the northern hemisphere and spring in the southern.

Winter solstice, 22 December. The South Pole leans towards the Sun. The vertical rays fall on the Tropic of Capricorn at 23 and a half degrees south. The northern hemisphere has its shortest day and longest night, and the Arctic Circle has twenty four hours of darkness; the southern hemisphere has its longest day and the Antarctic Circle continuous daylight. It is winter in the north and summer in the south.

Vernal or spring equinox, 21 March. Again the Sun is vertical at the equator and day and night are equal everywhere. It is spring in the northern hemisphere and autumn in the southern.

The northern hemisphere is warmer in summer for two reasons that work together. First, the Sun stands higher in the sky, so its rays strike the ground more directly and are concentrated on a smaller area; slanting winter rays spread the same energy over a larger area and also pass through more atmosphere. Second, the days are longer, so the ground is heated for more hours and cooled for fewer. In winter both effects are reversed. The hottest and coldest weather comes a few weeks after the solstices because land and sea take time to heat and cool.

West Bengal, lying between about 21 and 27 degrees north, is entirely in the northern hemisphere. Its longest day, at Kolkata about 13 hours 30 minutes, is in June and its shortest, about 10 hours 45 minutes, in December. The cycle is the reverse in Australia and South Africa, where Christmas falls in high summer.

Because the Earth's axis is tilted 23 and a half degrees and keeps that tilt as the Earth revolves, the latitude at which the Sun stands vertically overhead at noon changes through the year. To an observer on the Earth the Sun appears to move slowly north and south between two limits. This is the apparent annual movement of the Sun, and the two limits are the Tropic of Cancer at 23 and a half degrees north and the Tropic of Capricorn at 23 and a half degrees south. The Sun is never overhead beyond these lines; from Kolkata, at 22 and a half degrees north, the Sun does pass overhead, twice a year, but from Delhi, at 28 and a half degrees, it never does.

The cycle runs as follows. On 21 March the overhead Sun is at the equator. It then moves north, reaching the Tropic of Cancer on 21 June, when it seems to pause, which is the meaning of the word solstice, and turns back. It crosses the equator southward on 23 September, reaches the Tropic of Capricorn on 22 December, pauses and turns north again, crossing the equator on 21 March to complete the cycle. The Sun therefore takes six months to travel from one Tropic to the other, moving on average about a quarter of a degree of latitude a day, and every latitude between the Tropics has the Sun overhead twice a year, once on the way north and once on the way south; the Tropics themselves have it overhead only once.

The names come from astronomy. About two thousand years ago the Sun entered the constellation of Cancer, the Crab, at the June solstice and the constellation of Capricornus, the Goat, at the December solstice, so the lines were named after them. The word tropic itself comes from a Greek word meaning turning, because the Sun turns back there. The zone between the two Tropics is the torrid zone, where the Sun is always high and the climate hot. Between each Tropic and the polar circle of the same hemisphere is a temperate zone, where the Sun is never overhead but never absent for a whole day. Beyond the Arctic and Antarctic Circles are the frigid zones, which have at least one day of continuous sunlight and one of continuous darkness a year.

The apparent movement of the Sun is easily seen from the changing noon shadow. At Kolkata a vertical pole casts a short northward shadow at noon in December, a shorter one in March, no shadow at all on the two days when the Sun passes overhead in late May and July, and a small southward shadow in June. Marking the noon shadow month by month is a good project for verifying the whole idea.

Because the circle of illumination is tilted relative to the axis for most of the year, it cuts each parallel of latitude into unequal parts, a lit arc and a dark arc. The length of the day at any place is proportional to the lit arc of its parallel, and this changes with the season and the latitude in a regular way.

At the equator the circle of illumination always bisects the parallel, because every great circle bisects every other great circle. So the equator has twelve hours of day and twelve hours of night throughout the year, and no seasons of daylight at all. This is why places like Singapore have sunrise and sunset at almost the same clock time all year.

Between the equator and the poles the inequality grows with latitude. On 21 June the lit arc of every northern parallel is more than half, and the excess grows northward: at 20 degrees north the day is about 13 hours 12 minutes, at 40 degrees about 14 hours 50 minutes, at 60 degrees about 18 hours 30 minutes, and at 66 and a half degrees, the Arctic Circle, the whole parallel is in the light and the day is 24 hours. Beyond the circle the Sun does not set for days, weeks or months, and at the North Pole it is above the horizon continuously for six months, from 21 March to 23 September. Meanwhile every southern parallel has the reverse: short days, and continuous night within the Antarctic Circle.

On 22 December the picture is exactly reversed, with long nights in the north and the midnight Sun in the south. On the two equinoxes the circle of illumination passes through both poles, every parallel is cut in half, and day and night are equal all over the world.

The table gives the approximate length of the longest day at a few latitudes.

The midnight Sun is seen in Norway, northern Russia, Alaska and northern Canada in June, when tourists travel to see the Sun circling the sky without setting. The long polar night is broken only by twilight and the aurora. These extremes are the direct result of the tilt of the axis; a vertical axis would give twelve hour days everywhere for ever.

Examination questions on this chapter ask for definitions, differences, causes and effects, and one labelled diagram, usually of the seasons. The points below are arranged so that each can be written out as an answer.

Effects of rotation. First, the alternation of day and night, because only the half facing the Sun is lit and the turning Earth brings each place into and out of the light every 24 hours. Second, the difference of local time between places on different meridians, since the Earth turns 15 degrees an hour and one degree in four minutes. Third, the deflection of winds and ocean currents to the right in the northern hemisphere and to the left in the southern, the Coriolis effect, which shapes the pattern of the trade winds, the westerlies, cyclones and the ocean gyres. Fourth, the daily occurrence of two high tides and two low tides as each place passes under the tidal bulges. Fifth, the shape of the Earth, flattened at the poles and bulging at the equator. Sixth, the apparent daily movement of the Sun, Moon and stars from east to west.

Effects of revolution combined with the tilt of the axis. First, the cycle of seasons, summer and winter alternating between the hemispheres. Second, the apparent movement of the Sun between the Tropics, with vertical rays at the Tropic of Cancer on 21 June, at the equator on 21 March and 23 September, and at the Tropic of Capricorn on 22 December. Third, the varying length of day and night at all latitudes except the equator, with the midnight Sun and the polar night within the polar circles. Fourth, the division of the Earth into torrid, temperate and frigid heat zones. Fifth, the length of the year and the need for the leap year.

Differences to remember. Rotation is the spin on the axis, once in 24 hours, causing daily effects; revolution is the movement round the Sun, once in 365 and a quarter days, causing yearly effects. Perihelion, about 3 January, is the nearest point to the Sun; aphelion, about 4 July, is the farthest. A solstice, 21 June or 22 December, has the greatest inequality of day and night; an equinox, 21 March or 23 September, has equal day and night everywhere. Spring tides occur at new and full moon; neap tides at the quarters.

Common errors. The seasons are not caused by the changing distance from the Sun; the Earth is closest in January, in the northern winter. The circle of illumination is not the equator or a meridian; it is the boundary of light and dark. The axis is tilted 66 and a half degrees to the orbital plane, not to the vertical; the tilt from the vertical is 23 and a half degrees. Learn the four dates with their positions of the overhead Sun; they carry marks in almost every year's paper.