Chapter 10 — ग्रह रूप में पृथ्वी

Everything that exists — all matter, all energy, all space and all time — is the universe. It is unimaginably large: light, travelling at about 3 lakh kilometres every second, takes more than 13,000 crore years to cross the part of it we can observe. Within this vastness matter is not spread evenly but gathered into enormous islands of stars called galaxies, each holding thousands of crores of stars bound together by gravity, with gas and dust between them. There are perhaps 200,000 crore galaxies in the observable universe.

The origin of the universe. The accepted scientific account is the Big Bang theory, proposed by Georges Lemaître in 1927 and supported by Edwin Hubble's discovery in 1929 that all galaxies are moving away from one another, so that the universe is expanding. About 1,380 crore (13.8 billion) years ago the entire universe was concentrated in a point of unimaginable density and temperature; it expanded suddenly and has been expanding and cooling ever since. Within the first minutes the lightest elements, hydrogen and helium, formed; after some crores of years the gas gathered under gravity into the first stars and galaxies. Heavier elements such as carbon, oxygen and iron — the stuff of planets and of our bodies — were made inside stars and scattered when the stars exploded. The faint glow of the Big Bang, the cosmic microwave background radiation, was detected in 1965 and is its strongest proof.

Our galaxy, the Milky Way. The sun belongs to a spiral galaxy called the Milky Way or Akash Ganga, which appears on a clear dark night as a hazy band of light across the sky — the light of thousands of crores of distant stars in the plane of the galaxy. The Milky Way is a flattened disc about one lakh light years across and about 1,000 light years thick, with a bulge at the centre and spiral arms winding outward. It contains 10,000 to 40,000 crore stars. The sun lies in one of the spiral arms (the Orion arm) about 26,000 light years from the centre, and it goes round the centre once in about 22.5 crore years at a speed of about 220 km per second. Our nearest neighbour galaxy of similar size is Andromeda, 25 lakh light years away, visible to the naked eye as a faint smudge.

Units of distance. Because the distances are so great, astronomers use the light year, the distance light travels in one year, about 9.46 lakh crore kilometres (9.46 × 10¹² km), and for the solar system the astronomical unit, the average distance between the earth and the sun, about 15 crore kilometres. The nearest star after the sun, Proxima Centauri, is 4.2 light years away; its light left it four years before we see it.

Stars. A star is a huge ball of hot gas, mainly hydrogen, which shines by nuclear fusion in its core, where hydrogen is converted into helium with the release of enormous energy. Stars are born in clouds of gas and dust called nebulae, live for crores or thousands of crores of years, and die as white dwarfs, neutron stars or black holes according to their mass. Groups of stars that seem to form patterns are called constellations: Ursa Major (Saptarshi), Orion (Kalpurush), Scorpius; the Pole Star (Dhruva tara) stands almost exactly above the north pole of the earth and has guided travellers for thousands of years.

The solar system is the family of the sun: the eight planets, their satellites, the dwarf planets, and the crores of asteroids, comets and meteoroids that all revolve round the sun under its gravity. It formed about 460 crore years ago, according to the nebular hypothesis first suggested by Kant and Laplace, from a rotating cloud of gas and dust; the centre collapsed into the sun and the surrounding disc gathered into planets.

The sun. The sun is a medium-sized yellow star, a ball of glowing gas — about 71 per cent hydrogen and 27 per cent helium — with a diameter of about 13.9 lakh km, 109 times that of the earth, and a mass 3.3 lakh times the earth's, holding 99.8 per cent of all the mass in the solar system. Its surface temperature is about 5,500 °C and its core about 1.5 crore °C, where hydrogen fuses into helium and releases the energy that reaches us as light and heat. The visible surface is the photosphere; above it lie the chromosphere and the corona, seen during a total eclipse. Dark, cooler patches called sunspots come and go in an 11-year cycle. The sun rotates once in about 25 days at its equator. Its light takes 8 minutes 20 seconds to reach the earth, 15 crore km away.

The planets. A planet is a body that revolves round the sun, is large enough for its gravity to make it round, and has cleared its orbit of other objects. There are eight, in order from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — remembered by the phrase My Very Educated Mother Just Served Us Noodles. They all revolve in the same direction, anticlockwise seen from above the north pole, in nearly circular orbits that lie almost in one plane, the ecliptic. Pluto, counted as the ninth planet from 1930, was reclassified in 2006 as a dwarf planet because it has not cleared its orbit; Ceres in the asteroid belt, Eris, Haumea and Makemake are other dwarf planets.

Two groups. The four inner or terrestrial planets — Mercury, Venus, Earth, Mars — are small, dense and rocky, with solid surfaces, few or no satellites and thin or no rings. They lie within 23 crore km of the sun. The four outer or Jovian planets — Jupiter, Saturn, Uranus, Neptune — are giants made mostly of hydrogen, helium, methane and ammonia, with no solid surface, many satellites and rings, and they lie far out beyond the asteroid belt. Between the two groups, between Mars and Jupiter, lies the asteroid belt of rocky fragments that never formed a planet.

Other members. Satellites or moons revolve round planets; the earth has one, Mars two, Jupiter more than 90, Saturn more than 140. Comets are balls of ice and dust from the outer solar system that grow a glowing tail pointing away from the sun as they approach it; Halley's comet returns every 76 years, last in 1986. Meteoroids are small rocks; when they enter the atmosphere and burn they are seen as meteors or shooting stars, and any piece that reaches the ground is a meteorite, such as the one that made the Lonar crater in Maharashtra. Beyond Neptune lie the Kuiper belt of icy bodies and, far beyond, the Oort cloud from which comets come.

Each planet has a character of its own, and the examination often asks for their special features.

Mercury is the nearest to the sun (5.8 crore km) and the smallest planet, only slightly larger than our moon, with a diameter of 4,880 km. It has almost no atmosphere, so its day side reaches 430 °C and its night side falls to −180 °C. It goes round the sun fastest, in 88 days, but rotates slowly, once in 59 days. It has no satellite and its surface is cratered like the moon.

Venus, the second planet, is nearly the earth's twin in size (12,104 km) and mass, and is the brightest object in the sky after the sun and moon — the morning star and the evening star (Shukra). It is wrapped in thick clouds of carbon dioxide and sulphuric acid that trap heat in a runaway greenhouse effect, making it the hottest planet at about 465 °C, hotter than Mercury. It rotates backward (clockwise) and so slowly, in 243 days, that its day is longer than its year of 225 days. It has no satellite.

Earth, the third planet at about 15 crore km, is the largest of the inner planets (12,756 km), the only one with liquid water on its surface, an oxygen-rich atmosphere and life. Seen from space it is the blue planet, because 71 per cent of its surface is ocean. It rotates in 24 hours, revolves in 365¼ days and has one satellite, the moon.

Mars, the fourth planet, is the red planet (Mangal), its colour due to iron oxide in its soil. It is half the earth's size (6,792 km), has a thin carbon dioxide atmosphere, polar ice caps that grow and shrink with the seasons, the largest volcano in the solar system (Olympus Mons, 22 km high) and a great canyon, Valles Marineris. Its day is 24½ hours, its year 687 days, and it has two tiny moons, Phobos and Deimos. Dry river beds show that water once flowed on it, and it is the planet most explored by spacecraft; India's Mangalyaan reached its orbit in 2014.

Jupiter is the largest planet (Brihaspati), with a diameter of 1,42,984 km, eleven times the earth's, and a mass two and a half times all the other planets together. It is a ball of hydrogen and helium with bands of coloured cloud and the Great Red Spot, a storm larger than the earth that has raged for at least 300 years. It rotates fastest of all, in under 10 hours, and takes 12 years to go round the sun. It has more than 90 moons; the four big ones — Io, Europa, Ganymede, Callisto — were discovered by Galileo in 1610, and Ganymede is the largest moon in the solar system.

Saturn (Shani), the second largest (1,20,536 km), is famous for its bright rings of ice and rock, 2.8 lakh km across but only about a kilometre thick. It is the least dense planet — it would float on water. Its year is 29½ years, its day 10½ hours, and it has more than 140 moons; Titan, the largest, has a thick atmosphere.

Uranus, discovered by William Herschel in 1781, is a blue-green ice giant (51,118 km) that rotates on its side, its axis tilted 98°, so that it rolls along its orbit; its year is 84 years. It has faint rings and 27 known moons.

Neptune, the outermost planet, was found in 1846 by calculation before it was seen. It is deep blue, with the fastest winds in the solar system (2,000 km per hour), a diameter of 49,528 km, a year of 165 years and 14 moons, of which Triton orbits backward. It lies about 450 crore km from the sun, and sunlight takes over four hours to reach it.

To someone standing in a field the earth looks flat, and for most of human history people believed it was. The earliest civilisations pictured it as a flat disc or square floating on water and covered by the dome of the sky. The Babylonians drew the world as a flat circle surrounded by a river; the ancient Egyptians thought of it as a flat plate; in Indian mythology the earth rested on the backs of elephants standing on a tortoise; the Norse peoples imagined a flat disc with a great tree at its centre; and the Chinese thought of a square earth under a round heaven. Sailors feared that a ship going too far would fall off the edge.

The Greek revolution. The Greeks were the first to argue from observation that the earth is a sphere. Pythagoras (about 500 BCE) is credited with suggesting a spherical earth, perhaps because the sphere was thought the perfect shape. Aristotle (about 350 BCE) gave the first real proofs: the earth's shadow on the moon during a lunar eclipse is always circular, and only a sphere throws a circular shadow in every position; travellers going south see new stars rise above the horizon and familiar ones sink, which could not happen on a flat earth; and elephants were found both in India in the east and in Africa in the west, so, he reasoned, the two must be closer round the back than the flat map suggested. Eratosthenes (about 240 BCE), librarian at Alexandria, not only accepted the sphere but measured its circumference, a story told in a later topic. Ptolemy (about 150 CE) built his geography and his star maps on a spherical earth, and his book passed to the Arabs and back to medieval Europe.

India. The astronomer Aryabhata (476–550 CE) stated in his Aryabhatiya that the earth is a sphere that rotates on its axis, and that the apparent movement of the stars is caused by this rotation — a thousand years before Copernicus. Varahamihira and Brahmagupta in the sixth and seventh centuries also held the earth to be round, and Brahmagupta described gravity as an attraction towards the earth's centre.

The Middle Ages and after. Arab geographers such as Al-Biruni, who visited India in the eleventh century and computed the earth's radius from the dip of the horizon seen from a mountain, kept the knowledge alive. In Europe, though educated people knew the earth was round, the idea was not proved by experience until Magellan's expedition (1519–1522) sailed westward from Spain and returned from the east, going right round the earth. Since then no serious doubt has remained, and in the twentieth century photographs from rockets and satellites, and the sight of the whole blue globe from the moon in 1968, made the sphere visible to all.

The story shows how science works: an idea from reasoning (Pythagoras), tested by observation (Aristotle), measured (Eratosthenes), confirmed by experience (Magellan) and finally seen directly (space photographs). The next topic sets out the evidence in order.

The following proofs, taken together, leave no doubt that the earth is round like a ball and not flat.

1. Circumnavigation. Magellan's ships sailed west from Spain in 1519 and, after his death in the Philippines, Elcano brought the Victoria home from the east in 1522: they had gone right round the world. Since then countless ships, aircraft and satellites have done the same. On a flat earth a traveller going always in one direction would reach an edge; on a sphere he returns to where he started.

2. Ships on the horizon. When a ship sails away from a port it does not simply grow smaller; first the hull disappears, then the deck, then the funnel and finally the mast top, as if it were going down a hill. A ship approaching appears mast first. This can only happen if the surface of the sea curves away from the observer. On a flat sea the whole ship would shrink evenly and vanish as a dot.

3. The lunar eclipse. A lunar eclipse occurs when the earth comes between the sun and the moon and its shadow falls on the moon. The edge of the shadow is always an arc of a circle, whatever the position of the earth. Only a sphere throws a circular shadow from every direction; a flat disc would sometimes throw an oval or a line.

4. The rising and setting of the sun at different times. When it is sunrise in Kolkata it is still night in Mumbai and midday in Japan; the sun rises about four minutes earlier for every degree of longitude one goes east. On a flat earth the sun would rise and set for everyone at the same moment. The time difference across the earth is explained only by a curved surface turning on its axis.

5. The altitude of the Pole Star. The Pole Star stands above the north pole. At the equator it is seen on the horizon; as one travels north it rises higher in the sky, and at the north pole it is overhead; south of the equator it cannot be seen at all. The angle of the Pole Star above the horizon equals the observer's latitude, which is possible only on a sphere.

6. The circular horizon. From a ship at sea or a high tower the horizon is always a circle, and it widens as one climbs higher: from a height of 10 m the horizon is about 11 km away, from 100 m about 36 km. On a flat earth the horizon would be the same from every height, and only a sphere shows a circular edge in every direction.

7. Sunrise seen first from a height. The top of a tall building or a mountain peak catches the first rays of the rising sun before the plain below, and keeps the last rays of the setting sun. This is because the curved surface hides the sun from the lower ground first.

8. Aerial and space photographs. The most direct proof: photographs from high-flying aircraft show the curve of the horizon, and photographs from satellites and from the Apollo spacecraft of 1968–72 show the whole earth as a blue ball.

9. Analogy with other heavenly bodies. The sun, the moon and all the planets seen through a telescope are spheres; it would be strange if the earth alone were flat.

10. Surveying and the Bedford Level. Surveyors find that the surface falls away from a straight line of sight by about 8 cm in the first kilometre and four times that in two kilometres; long bridges and canals must be built with this curvature allowed for.

The earth is round, but it is not a perfect sphere. Careful measurement shows two departures from the sphere, and the chapter gives the earth two more exact names.

Oblate spheroid. Because the earth spins on its axis once a day, every part of it is pulled outward by the centrifugal effect of the rotation, and the pull is greatest at the equator, where the speed of rotation is highest (about 1,670 km per hour), and zero at the poles. Over the ages this has made the earth bulge at the equator and flatten at the poles. Isaac Newton predicted this in 1687 from his theory of gravitation, and the French Academy sent expeditions in 1735 to Lapland near the Arctic and to Peru near the equator to measure a degree of latitude at each; the degree was longer near the pole, proving that the earth is flattened there. Such a shape, a sphere slightly squashed along its axis, is an oblate spheroid or oblate ellipsoid. The figures are: equatorial diameter 12,756 km, polar diameter 12,714 km, a difference of about 42 km. The equatorial circumference is 40,075 km and the polar (meridional) circumference 40,008 km, a difference of about 67 km. The flattening is only 1 part in 298, so a globe 1 m across would be flattened by about 3 mm — invisible to the eye, which is why the earth looks a perfect sphere from space.

Geoid. Even the oblate spheroid is an idealisation. Measurements by satellites since 1958 show that the earth's surface — more exactly the surface of mean sea level extended under the continents — has small irregularities of its own: the northern hemisphere is very slightly more pointed and the southern very slightly flatter, and there are bumps and hollows of a few tens of metres where the crust and mantle are denser or lighter. For this reason the exact shape is called simply a geoid, from the Greek for earth-shaped, meaning the shape the earth actually has and nothing else. The geoid is defined as the surface of equal gravitational potential that coincides with mean sea level; it is the reference from which heights are measured. Some writers have described the geoid as slightly pear-shaped, but the pear is an exaggeration of differences of only a few metres.

Why the difference matters. For everyday purposes the earth is a sphere: a globe is made round, distances on maps are calculated on a sphere, and the difference is far smaller than the height of Mount Everest. But for satellites, for the Global Positioning System, for measuring gravity and for the exact survey of a country the oblate spheroid and the geoid must be used; the Survey of India uses a reference ellipsoid (the Everest spheroid, named after George Everest) fitted to India.

Summary of shapes. Sphere — the first approximation, a perfect ball. Oblate spheroid — flattened at the poles and bulging at the equator because of rotation, 42 km shorter through the poles. Geoid — the actual irregular shape given by the surface of mean sea level, the earth's own shape.

The dimensions of the earth. The earth is the fifth largest planet and the largest of the four rocky ones. Its main measurements are:

The difference between the highest mountain and the deepest trench is about 20 km, which on a globe 1 m across would be 1.5 mm — the earth is smoother than a billiard ball in proportion.

Eratosthenes' measurement (about 240 BCE). The first and one of the most elegant measurements of the earth was made by Eratosthenes, the Greek scholar in charge of the library at Alexandria in Egypt. He knew that at Syene (modern Aswan), far to the south, the sun at noon on the day of the summer solstice shone straight down a deep well and a vertical stick cast no shadow: the sun was directly overhead. At Alexandria on the same day at noon a vertical stick did cast a shadow, and he measured the angle between the stick and the sun's rays as about 7.2°, one-fiftieth of a full circle of 360°. Since the sun is so far away that its rays reach the earth parallel, this angle must equal the angle at the centre of the earth between Alexandria and Syene, that is, the difference in their latitudes. The distance between the two cities, he estimated from the time taken by camel caravans, was 5,000 stadia. Then the whole circumference, being 50 times the arc between them, was 50 × 5,000 = 2,50,000 stadia. Taking the stadion at about 157 m, this is about 39,250 km — within a few per cent of the true 40,008 km. The reasoning is the same as that used today: circumference = (360 ÷ angle between the two places) × distance between them.

Later measurements. The Indian astronomer Aryabhata gave the earth's circumference as 4,967 yojanas, which on one reading of the yojana is close to the true value. Al-Biruni in about 1030 CE, standing on a hill in Punjab, measured the dip of the horizon and calculated the radius of the earth as about 6,340 km, only 0.5 per cent low. The French geodesists of the eighteenth century measured arcs of meridian with surveying chains, the Great Trigonometrical Survey of India under Lambton and Everest measured the great arc from Cape Comorin to the Himalaya between 1802 and 1841, and today satellites and laser ranging give the figures to a few centimetres.

Of the eight planets, and of the thousands of planets now known round other stars, the earth is the only one on which life is known to exist. Seen from space it is unmistakable — a blue-and-white marble with brown and green continents — and astronauts call it the blue planet. Several conditions combine to make it habitable, and the absence of any one of them elsewhere makes those planets dead.

1. The right distance from the sun. The earth lies about 15 crore km from the sun, in the narrow band that astronomers call the habitable zone or Goldilocks zone — neither too hot nor too cold, but just right for water to remain liquid. Venus, 30 per cent nearer, is a furnace; Mars, 50 per cent farther, is frozen. The average surface temperature of the earth is about 15 °C.

2. Liquid water. Water covers 71 per cent of the surface in oceans, seas, lakes and rivers, and is present as vapour in the air and as ice at the poles. Every living thing is mostly water and every life process happens in it. The earth is the only planet where water exists in all three states at the surface; the hydrological cycle — evaporation, condensation, rain, rivers, back to the sea — keeps it moving and fresh.

3. The atmosphere. The earth is wrapped in a blanket of air about 78 per cent nitrogen, 21 per cent oxygen, with argon, carbon dioxide and water vapour. Oxygen, produced by plants over 200 crore years, lets animals breathe; carbon dioxide feeds plants; the ozone layer in the stratosphere screens out the sun's harmful ultraviolet rays; and the greenhouse effect of water vapour and carbon dioxide keeps the surface about 33 °C warmer than it would otherwise be, while not running away as on Venus. The atmosphere also burns up most meteors, spreads heat by winds and carries the rain.

4. The right size and gravity. The earth is massive enough for its gravity to hold on to its atmosphere and water — the moon and Mercury are too small and have lost theirs — but not so massive as to keep a crushing hydrogen atmosphere like Jupiter.

5. The magnetic field. The earth's liquid iron outer core, spinning, produces a magnetic field that reaches far into space and deflects the solar wind, the stream of charged particles from the sun that would otherwise strip away the atmosphere and bathe the surface in radiation. Mars, which has no such field, lost most of its air. The field also gives us the compass and the aurora.

6. Rotation and the tilt of the axis. The earth turns once in 24 hours, a period short enough to keep day and night from becoming extremes of heat and cold, and its axis is tilted at 23½°, giving the seasons that spread warmth over both hemispheres in turn.

7. Geological activity and the moon. The moving plates of the crust recycle carbon and nutrients and build the continents; the large moon steadies the tilt of the axis and drives the tides that helped life move from the sea to the land.

The biosphere. The thin zone where land, water and air meet, a few kilometres deep, is the biosphere, the home of all life from bacteria to blue whales. It is very small in relation to the planet — on a globe 1 m across it would be a film thinner than a coat of paint — and it is now being changed by human activity faster than at any time in its history. Protecting it is the practical reason for studying this chapter.

The moon (Chandra) is the earth's only natural satellite and its nearest neighbour in space, and it is the only other world on which human beings have walked. It is important to geography for the tides, the calendar, eclipses and as the yardstick of all our ideas about the planets.

Size and distance. The moon has a diameter of 3,476 km, about a quarter of the earth's, and a mass 1/81 of the earth's; its gravity is one-sixth of ours, so a person weighing 60 kg on the earth would weigh 10 kg there. It is about 3,84,400 km from the earth on average — about 30 earth-diameters away, a distance light crosses in 1.3 seconds and the Apollo spacecraft in three days. Because its orbit is elliptical, the distance varies between about 3,56,000 km (perigee) and 4,06,000 km (apogee); a full moon at perigee is a supermoon, appearing about 14 per cent larger.

Movements. The moon revolves round the earth once in 27 days 8 hours (the sidereal month), but because the earth is meanwhile moving round the sun, the interval from one new moon to the next is 29½ days (the synodic month), the basis of the lunar calendar. The moon also rotates on its axis in exactly the same time as it revolves, so it always turns the same face to the earth; the far side was first seen by the Soviet Luna 3 spacecraft in 1959.

Phases. The moon has no light of its own; it shines by reflecting sunlight. As it goes round the earth we see different amounts of its lit half: new moon (amavasya, the moon between earth and sun, dark), crescent, first quarter (half lit), gibbous, full moon (purnima, the earth between sun and moon, fully lit), then waning through gibbous, last quarter and crescent back to new. The bright fortnight is the shukla paksha and the dark fortnight the krishna paksha of the Indian calendar.

Surface and conditions. The moon has no atmosphere and no liquid water, so there is no weather, no sound, no blue sky, and the temperature swings from 127 °C in the day to −173 °C at night. The surface has dark plains of solidified lava called maria (seas, though dry), bright cratered highlands, mountain ranges and millions of craters made by meteorite impacts that no air ever slowed. The largest, Bailly, is 300 km across. In 2008 India's Chandrayaan-1 found water molecules in the lunar soil, and in 2023 Chandrayaan-3 landed near the south pole, the first craft to do so.

Origin. The most accepted theory is that soon after the earth formed, about 450 crore years ago, a Mars-sized body struck it and the debris thrown into orbit gathered into the moon — the giant-impact hypothesis, which explains why the moon's rocks resemble the earth's mantle.

Effects on the earth. The moon's gravity pulls the ocean water into two bulges and causes the tides, two high and two low each day, strongest (spring tides) at new and full moon when sun and moon pull together. The moon steadies the tilt of the earth's axis and so keeps the seasons regular. When it passes between earth and sun it causes a solar eclipse; when the earth's shadow falls on it, a lunar eclipse. Eclipses happen only at new or full moon and only when the moon is near the plane of the earth's orbit, which is why they do not happen every month. Human landings between 1969 and 1972 brought back 382 kg of moon rock, and the moon remains the first step of every plan to explore beyond the earth.

This chapter introduces, and the next chapter develops, the two motions of the earth, because they are part of what it means to be a planet and they help to explain the earth's shape.

Rotation. The earth spins on its own axis, an imaginary line through the north and south poles, from west to east, once in 23 hours 56 minutes 4 seconds relative to the stars — the sidereal day — which, allowing for the earth's movement round the sun, gives the 24-hour solar day. The spin causes day and night, the apparent rising of the sun in the east and setting in the west, the deflection of winds and currents, the difference of time between places, and, as the previous topic showed, the equatorial bulge. The speed of rotation at the equator is about 1,670 km per hour, falling to zero at the poles; Kolkata at 22½° N moves at about 1,540 km per hour.

Revolution. At the same time the earth travels round the sun in an orbit that is an ellipse, not a circle, with the sun at one focus, once in 365 days 5 hours 48 minutes 46 seconds — the tropical year, which the calendar makes up with a leap day every fourth year. The speed of revolution is about 30 km per second, or 1,07,000 km per hour, and the orbit is about 94 crore km round. Because the orbit is an ellipse, the earth's distance from the sun varies: it is nearest (perihelion, 14.71 crore km) about 3 January and farthest (aphelion, 15.21 crore km) about 4 July. The difference is only 3 per cent and it is not the cause of the seasons; the northern hemisphere winter comes when the earth is nearest the sun.

The tilt of the axis. The earth's axis is not upright to the plane of its orbit but inclined at 23½° to the vertical, that is at 66½° to the plane of the orbit, and it keeps pointing in the same direction (towards the Pole Star) throughout the year. This tilt, combined with revolution, causes the seasons, the varying length of day and night, and the apparent movement of the sun between the Tropic of Cancer and the Tropic of Capricorn. These are the subjects of the next chapter.

The earth among the planets. All eight planets rotate and revolve, but the earth's combination of a 24-hour day, a nearly circular orbit and a moderate tilt is unusually kind to life: a day of 59 earth-days as on Mercury would roast one side and freeze the other, and a tilt of 98° as on Uranus would give each pole 42 years of darkness. The earth also shares the sun's motion: with the whole solar system it circles the centre of the galaxy at 220 km per second, so that in space nothing stands still.

Why the orbit is an ellipse. Kepler discovered in 1609 that every planet moves in an ellipse with the sun at one focus, and that it moves faster when nearer the sun; Newton in 1687 showed that this follows from gravitation, the same force that makes an apple fall. The earth stays in orbit because its forward motion and the sun's pull are exactly balanced: if the sun's gravity vanished, the earth would fly off in a straight line; if the earth stopped moving, it would fall into the sun.

For all of history until 1957 the earth could be studied only from its own surface. The launch of the first artificial satellite, Sputnik 1, by the Soviet Union on 4 October 1957 opened the space age, and since then the view from above has transformed geography.

Artificial satellites. A satellite is put into orbit by a rocket that gives it a speed of about 28,000 km per hour; at that speed its fall towards the earth exactly matches the curve of the earth, so it circles endlessly. Satellites are of several kinds. Remote-sensing or earth-observation satellites carry cameras and sensors that photograph the earth in visible light and in infrared and microwave bands, mapping forests, crops, floods, cyclones, glaciers and cities; India's IRS series, Resourcesat and Cartosat, are among the best in the world. Weather satellites such as INSAT and Kalpana-1 watch clouds and track cyclones in the Bay of Bengal, giving the warnings that have cut cyclone deaths so sharply. Communication satellites in geostationary orbit, 35,786 km above the equator, relay television, telephone and internet. Navigation satellites — the American GPS, the Russian GLONASS, the European Galileo and India's own NavIC — let a mobile phone find its position to within a few metres. Scientific satellites measure the earth's gravity, magnetism, shape and the height of the sea.

What space has taught us about the earth. Photographs from Apollo 8 in 1968 and the famous Blue Marble photograph from Apollo 17 in 1972 showed the whole earth as a small blue sphere — the first time humanity saw its home entire. Satellite measurements refined the geoid, revealed the bumps in the earth's shape, and fixed its dimensions to centimetres. Satellites measure the rise of the sea (about 3.3 mm a year), the shrinking of Arctic ice, the loss of forests in the Amazon and the Sundarbans, the movement of the plates (India moves north-east at about 5 cm a year), and the growth of cities at night. They map the ocean floor by measuring the height of the sea surface, which follows the hidden relief below.

India in space. The Indian Space Research Organisation (ISRO), set up in 1969 under Vikram Sarabhai, launched Aryabhata in 1975, the Rohini satellite by its own SLV-3 rocket in 1980, and now flies the PSLV and GSLV launchers from Sriharikota in Andhra Pradesh. Chandrayaan-1 (2008) found water on the moon, Mangalyaan (2014) reached Mars orbit at the first attempt, Chandrayaan-3 (2023) landed near the moon's south pole and Aditya-L1 (2023) studies the sun. Rakesh Sharma flew in space in 1984, and Kalpana Chawla and Sunita Williams, of Indian origin, flew for NASA.

Geographic Information Systems and the modern map. Satellite images, GPS positions and census data are now combined in computers as layered digital maps — Geographic Information Systems — used for planning cities, tracking disease, managing disasters and guiding vehicles. The map on a mobile phone that shows where you are is the descendant of Eratosthenes' stick and shadow: the same earth, measured from above instead of below.

A good way to understand the earth is to set it beside the two planets nearest to it, Venus and Mars, and beside the moon. The comparison shows how narrow the conditions for life are.

Earth and Venus. Venus is almost the earth's twin in size (12,104 km against 12,756 km), mass (82 per cent) and density, and it is made of the same rocky materials; astronomers once imagined jungles beneath its clouds. But it lies 30 per cent closer to the sun, and as it warmed its oceans boiled away, the water vapour and carbon dioxide trapped ever more heat, and the greenhouse effect ran away. Today its atmosphere is 96 per cent carbon dioxide at 90 times the earth's pressure, its clouds are sulphuric acid, and its surface is 465 °C, hot enough to melt lead, day and night, pole and equator alike. It rotates backward and very slowly, has no moon and no magnetic field. Venus is the warning of what a greenhouse effect can do.

Earth and Mars. Mars is half the earth's size and a tenth of its mass, and 50 per cent farther from the sun. It has a 24½-hour day, a 25° tilt and seasons, polar ice caps, volcanoes, canyons, dust storms and dry river beds — of all the planets it is the most earth-like, and long ago it had rivers and lakes. But its small size meant weak gravity and a core that cooled and stopped generating a magnetic field; the solar wind stripped away most of its atmosphere, which is now thin carbon dioxide at less than 1 per cent of the earth's pressure, and its water froze into the ground or escaped. The average temperature is −60 °C. Mars shows what happens to a planet too small to hold its air.

Earth and the moon. The moon is a quarter of the earth's diameter, has one-sixth of its gravity, no atmosphere, no water at the surface and no magnetic field; its surface swings through 300 °C between day and night and is pitted by every meteorite that ever struck it, because nothing ever eroded the craters away. The earth has been struck as often, but wind, water and moving plates have erased almost every scar. The moon is the earth without air, water and internal activity — a museum of the early solar system.

What the comparison teaches.

• Distance from the sun sets the temperature range; a small difference tips a planet into furnace or freezer.
• Size sets gravity, and gravity decides whether a planet keeps its atmosphere and water.
• An active interior gives a magnetic field that shields the atmosphere and plates that recycle the crust.
• An atmosphere with the right greenhouse effect keeps water liquid — too little and the world freezes, too much and it boils.
• Water in liquid form is the single condition that all life requires.

The earth passes all these tests, and it is the only body we know that does. The lesson of the chapter is that the earth is not merely a planet among planets: it is a rare and delicately balanced one, and its balance — its atmosphere, its climate, its water — is now partly in human hands. Geography, the study of the earth as the home of humankind, begins with understanding how special that home is.