Chapter 1 — Earth as a Planet

Everything that exists, all matter, all energy and all space, is called the universe. It is unimaginably large, and astronomers believe it has been expanding since a great burst of energy about 13.8 billion years ago, which they call the Big Bang. Within the universe, stars are not scattered evenly; they gather in enormous groups called galaxies. A galaxy may hold a hundred billion stars, and there are more than a hundred billion galaxies. Our own galaxy is the Milky Way, called Akashganga in Indian tradition because on a dark night it appears as a pale river of light across the sky. The Milky Way is a spiral galaxy about one lakh light years across, and our Sun lies in one of its arms, far from the centre.

A light year is not a unit of time but of distance: it is the distance that light, travelling at about 3,00,000 kilometres every second, covers in one year, which is roughly 9.46 trillion kilometres. Distances between stars are so great that kilometres become useless, so astronomers use the light year. The nearest star after the Sun, Proxima Centauri, is more than four light years away.

The Sun is a medium sized star, a huge ball of glowing gas, mostly hydrogen and helium. In its core hydrogen atoms fuse into helium, releasing enormous energy as heat and light. Its surface temperature is about 6,000 degrees Celsius and its core is about 15 million degrees. The Sun's diameter is about 13,92,000 kilometres, roughly 109 times that of the Earth, and its mass is about 3,30,000 times the Earth's mass; it holds 99.8 per cent of all the matter in the solar system. Light from the Sun takes about 8 minutes 20 seconds to reach us.

Because the Sun is so massive, its gravity holds the planets, dwarf planets, asteroids, comets and meteoroids in orbit around it. This whole family is the solar system. For geography the Sun matters most as the single source of energy that drives the winds, the ocean currents, the water cycle, weathering and all life on the Earth. Without the Sun's steady output over billions of years the Earth would be a frozen, dead rock. So the study of the Earth as a planet begins with its star.

The solar system consists of the Sun at the centre and everything bound to it by gravity. Its chief members are the eight planets, their satellites or moons, the dwarf planets, the asteroids, the comets and the meteoroids. A planet is a body that goes round the Sun, is large enough for its own gravity to pull it into a nearly round shape, and has cleared its orbit of other objects. Planets do not have light of their own; they shine only by reflecting sunlight, which is why the word planet, from the Greek for wanderer, was given to the bright points that move against the fixed stars.

In order of increasing distance from the Sun the eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A common way to remember the order is the sentence My Very Educated Mother Just Served Us Noodles. Jupiter is the largest, with a diameter of about 1,43,000 kilometres, more than eleven times that of the Earth, and Mercury is the smallest. Until 2006 Pluto was counted as the ninth planet, but because it has not cleared its orbit it is now classed as a dwarf planet together with Ceres, Eris, Haumea and Makemake.

A satellite is a body that revolves around a planet. The Earth has one natural satellite, the Moon; Mars has two small ones, Phobos and Deimos; Jupiter and Saturn have dozens each; Mercury and Venus have none. Between the orbits of Mars and Jupiter lies the asteroid belt, a ring of rocky fragments ranging from dust to bodies hundreds of kilometres across; Ceres, the largest, is about 950 kilometres in diameter.

Comets are bodies of ice, dust and frozen gas that follow very elongated orbits. When one approaches the Sun the ice turns to vapour and forms a glowing head and a long tail that always points away from the Sun because of the solar wind. Halley's Comet returns about every 76 years and was last seen in 1986. Meteoroids are small pieces of rock or metal moving through space; when one enters the Earth's atmosphere it burns from friction and appears as a streak of light, a meteor or shooting star, and any piece that survives to reach the ground is a meteorite. The Lonar crater in Maharashtra was made by such an impact.

The eight planets fall naturally into two groups separated by the asteroid belt. The four planets closest to the Sun, Mercury, Venus, Earth and Mars, are called the inner planets or terrestrial planets, from the Latin terra meaning earth, because they resemble the Earth in being made of rock and metal. The four beyond the belt, Jupiter, Saturn, Uranus and Neptune, are the outer planets or Jovian planets, after Jove, another name of Jupiter, because they resemble Jupiter in being huge and gaseous.

The terrestrial planets are small and dense. They have solid rocky surfaces with mountains, valleys and craters, iron cores, and thin atmospheres or none at all. Their density is high, about 4 to 5.5 grams per cubic centimetre; the Earth, at 5.5, is the densest planet of all. They have few or no satellites, three moons among the four of them, and no rings. Because they are close to the Sun they are warm, and they complete their orbits quickly: Mercury goes round in 88 days and Mars in 687 days.

The Jovian planets are enormous but light. Jupiter alone has more than twice the mass of all the other planets combined, yet its density is only 1.3 grams per cubic centimetre, and Saturn, at 0.7, is lighter than water and would float in a large enough ocean. They have no solid surface; beneath thick atmospheres of hydrogen, helium, methane and ammonia lie layers of liquid and metallic hydrogen and a small rocky core. All four have ring systems, Saturn's being the most magnificent, and many satellites, Jupiter with more than ninety and Saturn with more than a hundred. Far from the Sun they are bitterly cold, and their years are long: Jupiter takes almost 12 Earth years to orbit the Sun and Neptune about 165 years.

The reason for the difference lies in how the solar system formed. Close to the young Sun, heat drove away the light gases, so only rock and metal condensed into small bodies. Far away, where it was cold, ice and gas survived and the planets grew large enough to capture huge envelopes of hydrogen and helium. Understanding this division helps the student see why the Earth, a small rocky planet at the right distance, became habitable while its neighbours did not.

The Earth is the third planet from the Sun and the fifth largest of the eight. It lies on average about 15 crore kilometres (149.6 million km) from the Sun, a distance that astronomers call one astronomical unit and use to measure other distances in the solar system. Neptune, for instance, is about 30 astronomical units away. The Earth is not the largest planet, nor the closest to the Sun, nor the oldest; yet it is the only body we know of anywhere on which life exists. Seen from space it appears as a blue and white ball, blue from the oceans that cover 71 per cent of its surface and white from the clouds, which is why it is called the blue planet.

Several conditions together make the Earth habitable. First is its distance from the Sun. Mercury and Venus are too close and too hot; Venus, wrapped in a thick blanket of carbon dioxide, has a surface temperature of about 460 degrees Celsius. Mars and the planets beyond are too cold. The Earth lies in the narrow band, sometimes called the Goldilocks zone, where the average surface temperature, about 15 degrees Celsius, allows water to remain liquid.

Second is its atmosphere, a layer of gases held by the Earth's gravity, made up of 78 per cent nitrogen, 21 per cent oxygen and small amounts of argon, carbon dioxide and water vapour. Oxygen supports breathing and burning; the ozone in the upper atmosphere shields life from harmful ultraviolet rays; carbon dioxide and water vapour trap enough heat to keep nights from freezing; and the atmosphere burns up most meteoroids before they strike the ground.

Third is water. The Earth alone has large bodies of liquid water on its surface, without which no known form of life can exist. Fourth is its size and mass, which give gravity strong enough to hold the atmosphere and oceans but not so strong as to crush living things. Fifth, the Earth has a magnetic field produced by its molten iron core that turns aside charged particles streaming from the Sun. Finally, its steady rotation spreads heat evenly between day and night. Change any one of these and the Earth would be as barren as its neighbours.

To a person standing in an open field the Earth looks flat, stretching away to a straight horizon on every side. It is not surprising that the earliest peoples believed the Earth to be a flat disc. The ancient Babylonians thought of it as a flat plate floating on water; some early Greeks imagined a flat Earth surrounded by the river Oceanus; and in many old traditions the sky was a solid dome resting on the edges of a flat land. In India too there were ideas of a flat Earth resting on the backs of elephants standing on a tortoise, a story that still survives in folk tales. These were natural guesses made from limited experience, before anyone had travelled far enough to see the truth.

The change began with the Greeks. The mathematician Pythagoras in the sixth century BC suggested that the Earth was a sphere, partly because he thought the sphere the most perfect shape. Aristotle, about 350 BC, gave real proofs: he noted that the shadow of the Earth on the Moon during a lunar eclipse was always curved, and that travellers going south saw new stars rising above the horizon that were never visible in the north. Both are impossible on a flat Earth. About 240 BC Eratosthenes, the librarian of Alexandria, went further and actually measured the circumference of the sphere with remarkable accuracy.

Indian astronomers reached the same conclusion. Aryabhata, writing in AD 499, described the Earth as a sphere hanging in space and even proposed that it rotates on its axis, explaining the daily movement of the stars. Later Islamic scholars such as Al-Biruni, who visited India in the eleventh century, measured the Earth's radius using the dip of the horizon from a mountain top.

Yet the flat Earth idea did not die easily among ordinary people. It was the great voyages of the fifteenth and sixteenth centuries that settled the matter for everyone. When Ferdinand Magellan's expedition set out westward from Spain in 1519 and, after his death, one ship returned from the east in 1522, it had sailed right round the world, which is possible only on a globe. Since then no serious doubt has remained, and the twentieth century added the final proof: photographs of the whole Earth taken from space.

Several observations that any careful person can make show that the surface of the Earth is curved in every direction, which is the property of a sphere.

1. A ship on the horizon. When a ship sails away from the shore it does not simply grow smaller until it vanishes as a dot, as it would on a flat plain. Instead the hull disappears first, then the deck, and finally the tip of the mast, as though the ship were sinking. A ship approaching the shore appears in the reverse order, mast first. This happens because the curved surface of the sea rises between the observer and the ship, hiding the lower parts first. With a telescope the hull cannot be brought back into view, proving that it is really hidden by the curvature and not merely too small to see.

2. The widening horizon. From the ground the horizon lies a few kilometres away, but from a hill top or a tall building one can see much farther, and from an aeroplane farther still. The higher one goes the more of the curved surface comes into view. On a flat Earth height would make little difference to the distance seen. The formula for the distance to the horizon in kilometres is about 3.57 times the square root of the height in metres, so from 100 metres up the horizon is about 36 kilometres away.

3. The circular horizon. From a ship in the open sea the horizon appears as a perfect circle all around, whatever direction one looks. Only a spherical surface presents a circular horizon from every point on it.

4. Sunrise and sunset at different times. The Sun rises in Arunachal Pradesh about two hours before it rises in Gujarat, and in Japan before it rises in India. If the Earth were flat the whole world would see sunrise at the same moment. Because the surface is curved, places to the east are turned towards the Sun earlier. The system of time zones, with India five and a half hours ahead of Greenwich, rests entirely on this fact.

5. Different stars at different latitudes. The Pole Star stands high in the sky at Srinagar, lower at Kanyakumari and disappears below the horizon in the southern hemisphere, where the Southern Cross becomes visible instead. As one travels north or south the sky changes because one is moving over a curved surface, and the angle of the Pole Star above the horizon equals the observer's latitude.

To the everyday evidence, three further proofs may be added, each of which is decisive in its own way.

The lunar eclipse. A lunar eclipse occurs when the Earth comes exactly between the Sun and the Moon, so that the Earth's shadow falls on the Moon. As the shadow creeps across the Moon's face its edge is always seen as an arc of a circle. This was Aristotle's proof and it remains the simplest. A flat disc could cast a circular shadow only when facing the Sun squarely; at other angles its shadow would be an ellipse or a straight line. Lunar eclipses occur at all hours of the night and in all seasons, with the Earth presenting itself to the Sun at every possible angle, and yet the shadow is circular every time. Only one shape casts a circular shadow in every direction: the sphere.

Circumnavigation. If a traveller keeps moving in one direction, say always west, and eventually comes back to the starting point from the east, the surface must be closed and curved like a globe; on a flat Earth the traveller would reach an edge or go on forever. Magellan's expedition made the first such voyage between 1519 and 1522. Since then countless ships, aircraft and even solo sailors have gone round the world, and every one has confirmed the result. Air routes and shipping lanes are drawn on the assumption of a globe and they work.

Photographs from space. The final and most direct proof came in the twentieth century. Rockets carrying cameras first photographed the curved horizon in 1946; in 1961 Yuri Gagarin became the first person to see the whole planet from orbit; and in 1972 the crew of Apollo 17 took the famous photograph called the Blue Marble, showing the complete round Earth with Africa and Antarctica in view. Today satellites photograph the Earth every hour and the images are used for weather forecasting and mapping. The pictures show a round body from every side, which settles the question beyond all argument.

The Bedford Level experiment. In 1838 a measurement was made along a straight, still stretch of canal in England, with three poles of equal height fixed at intervals of about 5 kilometres. A telescope sighted along the tops of the first and last poles showed the middle pole standing higher, because the curved surface of the water lifted it. Careful repetitions of the test, allowing for the bending of light in air, confirmed the curvature.

Together, the shadow on the Moon, the round-the-world journey and the photograph from orbit show the same thing by three completely different methods, which is why geographers treat the sphericity of the Earth as an established fact rather than a theory.

Once it was accepted that the Earth is round, the next question was whether it is a perfect sphere. Isaac Newton in 1687 argued from his theory of gravitation that it could not be. A rotating body of soft or fluid material is thrown outward most strongly at its equator, where the speed of rotation is greatest, and so bulges there while flattening at the poles. The Earth spins once in about 24 hours, and its equator moves at more than 1,600 kilometres per hour, so a small but real bulge should exist. French expeditions in the 1730s measured the length of one degree of latitude near the equator in Peru and near the pole in Lapland and found that a degree is longer near the pole, which is what a flattened Earth should show. Newton was right.

The exact figures confirm it. The equatorial diameter of the Earth is about 12,757 kilometres and the polar diameter about 12,714 kilometres, a difference of 43 kilometres. The equatorial circumference is about 40,077 kilometres and the polar circumference about 40,009 kilometres, a difference of 68 kilometres. The flattening, the difference of the two radii divided by the equatorial radius, is about one part in 298. This is very small; if the Earth were shrunk to the size of a football the bulge would be too small to feel, and the Earth would be smoother than the football. But over 40,000 kilometres it matters for maps, satellites and navigation.

A sphere flattened at the poles is called an oblate spheroid. The Earth departs from even this shape a little: careful satellite measurements show that the surface of the oceans, extended imaginarily under the continents at sea level, has small humps and hollows, slightly higher in the northern hemisphere near the pole and a little pear shaped. Because no geometrical name fits exactly, scientists gave the shape its own name, geoid, which simply means earth shaped. The geoid is defined as the shape that the mean sea level surface takes under the pull of gravity and the effect of rotation, and it is the reference from which all heights on the Earth are measured.

The student should remember three statements: the Earth is nearly a sphere; it is more exactly an oblate spheroid, flattened at the poles and bulging at the equator; and most exactly it is a geoid, a shape of its own. The equatorial bulge also explains why the peak of Chimborazo in Ecuador, though lower than Everest above sea level, is the point on the Earth farthest from its centre.

A Class 9 student is expected to know the main measurements of the Earth exactly, because later work on latitude, longitude, time and map scale depends on them. The figures below are the accepted values, rounded to whole kilometres.

The radius is half the diameter, so the equatorial radius is about 6,378 kilometres and the polar radius about 6,357 kilometres, a difference of 21 kilometres. The mean radius of 6,371 kilometres is the value used in most calculations. The circumference is the distance round the Earth; since it equals 2πr, the equatorial circumference is 2 × 3.14 × 6,378, about 40,077 kilometres. A useful memory aid is that the metre was originally defined as one ten millionth of the distance from the pole to the equator along a meridian, which makes the polar circumference almost exactly 40,000 kilometres.

From the circumference the student can work out the length of one degree of latitude or longitude at the equator: 40,077 divided by 360 gives about 111 kilometres. This figure appears again and again in the chapters on location and time. The surface area of a sphere is 4πr², which with the mean radius gives about 51 crore square kilometres; of this only 29 per cent is land and 71 per cent is water, most of it in the southern hemisphere, which is sometimes called the water hemisphere. The Earth's mass, about 6 × 10²⁴ kilograms, and its mean density of 5.5 grams per cubic centimetre tell us that the interior must be much denser than the surface rocks, which average only 2.7, pointing to a heavy iron core.

The first accurate measurement of the size of the Earth was made about 240 BC by Eratosthenes, a Greek scholar who was head of the great library at Alexandria in Egypt. His method is a beautiful example of how simple observation and geometry can answer a question that seems impossible, and it is worth learning step by step.

Eratosthenes knew that at the town of Syene (modern Aswan) in southern Egypt, at noon on the day of the summer solstice, the Sun stood exactly overhead: it shone straight down a deep well and a vertical stick cast no shadow. He also knew that at the same moment in Alexandria, almost due north of Syene, a vertical stick did cast a shadow. Measuring the angle between the stick and the ray of sunlight from the tip of its shadow, he found it to be about 7.2 degrees, which is one fiftieth of a full circle of 360 degrees.

He then reasoned as follows. The Sun is so far away that its rays reaching Syene and Alexandria are parallel. If the Earth were flat, both sticks would cast shadows of the same angle. The difference of 7.2 degrees must therefore be due to the curvature of the Earth; it is exactly the angle at the centre of the Earth between the two towns. Since 7.2 degrees is one fiftieth of the circle, the distance from Syene to Alexandria must be one fiftieth of the circumference of the Earth.

The distance between the two towns was known to be about 5,000 stadia, measured by professional pacers who walked the road. Multiplying by 50 gave a circumference of 2,50,000 stadia. The exact length of the stadion is uncertain, but taking it as about 157 metres the result is about 39,250 kilometres, within two per cent of the true value of 40,009 kilometres for the polar circumference. Even with a longer stadion the error is below 15 per cent, an astonishing result for the time.

The method has three requirements: two places on nearly the same meridian, a measurement of the Sun's angle at both at the same moment, and the ground distance between them. The same principle is used today with stars instead of the Sun and satellites instead of pacers. The importance of the experiment is not only its result but its demonstration that the size of the Earth can be found by reasoning from evidence, which is the method of all science.

To picture the Earth as a planet it helps to set it beside its two closest companions in the sky, the Moon and the Sun.

The Moon is the Earth's only natural satellite and the only other body that human beings have visited. Its diameter is about 3,476 kilometres, a little more than a quarter of the Earth's, so about fifty Moons would fit inside the Earth by volume. Its mass is about one eightieth of the Earth's, and its surface gravity is one sixth of ours, which is why astronauts on the Moon could bound in long leaps. It lies on average about 3,84,400 kilometres away, roughly thirty Earth diameters; light takes 1.3 seconds to cover the distance and a spacecraft about three days. The Moon has no atmosphere and no water on its surface, so its sky is black even in daytime and its temperature swings from about 120 degrees Celsius in sunlight to minus 150 in shadow. The Moon revolves round the Earth once in about 27.3 days and, because it rotates on its axis in the same time, always keeps the same face towards us. Its gravitational pull raises the tides in the oceans, a matter taken up in the chapter on movements.

The Sun is at the other extreme of scale. Its diameter of about 13,92,000 kilometres is 109 times the Earth's, and its volume about 13 lakh times, so 13 lakh Earths would be needed to fill it. Its mass is 3,30,000 times the Earth's. It is about 15 crore kilometres away, 400 times farther than the Moon; by a coincidence the Sun is also about 400 times wider than the Moon, so the two appear almost exactly the same size in our sky, which is why a total solar eclipse is possible when the Moon just covers the Sun's disc.

The table below sums up the comparison.

The lesson of these numbers is one of proportion. The Earth is a giant beside the Moon and a speck beside the Sun, and the Sun itself is an ordinary star among billions. Yet this middle sized planet at a middle distance is where the conditions for life came together.

Every planet holds itself together, holds its atmosphere and holds its satellites by gravity, the force of attraction that every mass exerts on every other. Newton showed that the force between two bodies grows with their masses and falls off with the square of the distance between them. Because the Earth is large its gravity is strong enough to give a falling body an acceleration of about 9.8 metres per second every second, written as g. This is why objects fall, why the atmosphere does not escape into space and why the Moon stays in orbit instead of flying off in a straight line.

The mass of the Earth cannot be weighed directly, but it can be calculated from g and the radius once the gravitational constant is known, a measurement first made by Henry Cavendish in 1798. The result is about 5.97 × 10²⁴ kilograms, nearly six thousand billion billion tonnes. Dividing this mass by the Earth's volume gives its mean density, about 5.5 grams per cubic centimetre, that is five and a half times the density of water. This is the highest of all the planets and it tells us something important: the rocks we can see at the surface have a density of only 2.7 to 3.0, so the interior must be far heavier. Scientists conclude that the Earth has a core of iron and nickel with a density above 10, surrounded by a rocky mantle, a structure discussed in the chapter on geomorphic processes.

Gravity also explains the Earth's shape. A body as massive as the Earth cannot hold itself up as a cube or a disc; gravity pulls every part towards the centre until the only stable shape, a sphere, is reached. This is why all large planets and stars are round while small asteroids, whose gravity is too weak, remain irregular lumps. The rotation of the Earth then modifies the sphere into the oblate spheroid described earlier.

Because of the equatorial bulge and the outward push of rotation, g is not the same everywhere. It is about 9.78 metres per second squared at the equator and 9.83 at the poles. A person weighing 60 kilograms at the pole would weigh about 300 grams less at the equator on a spring balance. Gravity also decreases with height above the surface, though very slowly: on the summit of Everest it is only about 0.3 per cent less than at sea level. Escape velocity, the speed needed to leave the Earth altogether, is about 11.2 kilometres per second, which is what rockets must reach to send satellites and probes into space.

Seen as a planet, the Earth is more than a ball of rock. It is made up of four interacting spheres, and geography is largely the study of how they act on one another.

The lithosphere, from the Greek lithos meaning stone, is the solid outer shell of the Earth: the crust and the uppermost rigid part of the mantle, about 100 kilometres thick, broken into the great plates on which the continents and ocean floors ride. It carries the mountains, plateaus and plains that later chapters describe. The continental crust, made largely of light granite type rocks, is 30 to 70 kilometres thick; the oceanic crust of dense basalt is only 5 to 10 kilometres.

The hydrosphere is all the water of the Earth: the oceans, seas, rivers, lakes, ground water, glaciers and ice caps. Water covers 71 per cent of the surface, and 97 per cent of it is salt water in the oceans; of the 3 per cent that is fresh, more than two thirds is locked in ice, leaving less than 1 per cent of all water as the rivers, lakes and ground water on which people depend. The Pacific alone is larger than all the land on the Earth put together.

The atmosphere is the envelope of gases held by gravity, densest at the surface and thinning upward until it merges into space at about 1,000 kilometres, though 99 per cent of its mass lies below 32 kilometres. It supplies oxygen, carries water vapour, moderates temperature and shields the surface from ultraviolet rays and meteoroids. The lowest layer, the troposphere, is where all weather occurs.

The biosphere is the zone of life: the narrow band, only about 20 kilometres from the deepest ocean trench to the highest point at which birds fly, where the other three spheres meet and living things exist. It is by far the thinnest of the spheres and the one that makes the Earth unique.

The spheres are bound together by constant exchange. The Sun heats the atmosphere and the oceans, driving winds and currents; water evaporates from the hydrosphere, falls as rain on the lithosphere, wears down the rocks and returns to the sea; plants of the biosphere take carbon dioxide from the air and release oxygen; volcanoes of the lithosphere add gases to the atmosphere. Because the parts work together the Earth is called a system. This idea is the thread that runs through the whole of Class 9 geography: the chapters on movements, weathering, hazards and resources are each about one part of the system and its links to the rest.

This chapter is examined through very short answer questions on facts and figures, short answer questions on proofs and definitions, and one or two longer descriptive questions. The following summary gathers the points most often asked.

Facts to recall exactly. The Sun is a star; the Earth is the third planet from the Sun and the fifth largest. The eight planets in order are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune; Jupiter is the largest and Mercury the smallest; Venus is the hottest. The inner four are terrestrial and the outer four Jovian. Pluto has been a dwarf planet since 2006. The Moon's diameter is 3,476 kilometres and its distance 3,84,400 kilometres; the Sun's diameter is 13,92,000 kilometres and its distance about 15 crore kilometres, which is one astronomical unit. Sunlight takes 8 minutes 20 seconds to reach us.

Figures of the Earth. Equatorial diameter 12,757 kilometres; polar diameter 12,714 kilometres; difference 43 kilometres. Equatorial circumference 40,077 kilometres; polar circumference 40,009 kilometres; difference 68 kilometres. Mean radius 6,371 kilometres. Surface area 51 crore square kilometres, of which 29 per cent is land and 71 per cent water. Mean density 5.5 grams per cubic centimetre. One degree of latitude is about 111 kilometres.

Definitions. A geoid is the Earth's own shape, an almost spherical body flattened at the poles and bulging at the equator, defined by mean sea level. An oblate spheroid is a sphere flattened at the poles. A light year is the distance light travels in one year. An astronomical unit is the mean Earth–Sun distance. A satellite is a body revolving round a planet. A lunar eclipse occurs when the Earth's shadow falls on the Moon.

Proofs of sphericity, any three of which may be asked with explanation: the ship disappearing hull first; the horizon widening with height; the circular horizon at sea; sunrise at different times in the east and west; the changing altitude of the Pole Star; the circular shadow in a lunar eclipse; circumnavigation; the Bedford Level experiment; photographs from space.

Reasoning questions. Why is the Earth flattened at the poles? Because of the outward effect of rotation at the equator. Why is the Earth called the blue planet? Because 71 per cent of its surface is water, which appears blue from space. Why is there life only on the Earth? Because of suitable distance from the Sun, liquid water, an atmosphere with oxygen and ozone, and moderate temperature. How did Eratosthenes measure the Earth? By comparing the Sun's noon angle at Syene and Alexandria, 7.2 degrees, one fiftieth of a circle, and multiplying the 5,000 stadia between them by 50. Practise writing each of these in four or five clear sentences with the figures included, because marks are given for exact numbers.