Chapter 1: Earth As a Planet
The sea looks flat. Stand on any open coast and the water runs out to a clean line where it meets the sky. Nothing about that line suggests a curve. Then a ship comes in. At first there is nothing at all. Then a thin mark stands up above the line, and it is only the mast. Wait a while and the deck appears beneath it, then the funnel, and last of all the hull, the largest part of the whole vessel and the part you would expect to see first. Wait longer still and the ship sits complete on the water. Nothing about the ship changed while you watched. It did not rise out of the sea and it did not grow. What changed was how much of the earth’s own surface stood between you and it. The ship was climbing into view over a curve so gentle that nobody standing on it has ever felt it. That curve is the first fact of this chapter. The second is stranger. The same planet that hides a ship’s hull also holds on to an ocean, an atmosphere and a temperature steady enough for life, which no other planet in the solar system has managed.
Diagram
Fig. 1.A1 Caption: The watcher on the shore sees the ship in parts, and always in the same order. Brief: Three small panels side by side of one observer on a shore with a gently curved sea surface; panel 1 shows only the mast above the water line, panel 2 adds the deck and funnel, panel 3 shows the full ship including the hull; a single arrow beneath the panels points from panel 1 to panel 3 to show the ship approaching, and the sight line from the observer’s eye is drawn as a straight line grazing the curved surface in each panel.
1.1 Earth In The Solar System
Earth is our home planet, and it is the third planet from the sun and the fifth-largest planet in the solar system. It is the only planet which supports life. Seen from space it looks blue, because 70 per cent of its total area is covered by water. For that reason it is also called the watery planet.
The Sun And Its Family
The family of the sun is called the Solar System. Its members are not scattered at random. Each one keeps to a certain distance from the sun, and that distance is what fixes its place in the family.
The chief members of the solar system fall into three groups. (a) The planets, of which the earth is one, each lying at its own distance from the sun. (b) The satellites, which revolve around the planets rather than sitting on their own in space. (c) Other celestial bodies called asteroids, comets and meteorites, which also belong to the sun’s family.
The planets, taken in order of their distance from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Counting outwards from the sun, the earth is the third name on that list. Its position in the order matters more than it first appears, and section 1.4 returns to it.
The whole family, and the way its members are grouped, is set out in Fig. 1.A2.
Flowchart
Fig. 1.A2 Caption: The solar system is the sun together with everything that keeps a fixed distance from it. Nodes: Solar System → the sun at the centre; Solar System → chief members → branch 1: planets → in order of distance from the sun: Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune; branch 2: satellites → revolve around the planets; branch 3: other celestial bodies → asteroids / comets / meteorites
1.2 The Shape Of The Earth
The earth is not a perfect sphere. It bulges slightly at the centre and is flattened at the poles, so a line drawn through it from side to side is not the same length as a line drawn through it from pole to pole.
People have not always thought of it as round at all. In the olden days the earth was considered to be a flat disc with steep edges, which fitted what anyone could see from the ground. Aryabhatta, the 5th century Indian astronomer and mathematician, believed in the round shape of the earth. He said that the earth was spherical in shape and rotated on its axis, the imaginary line through the earth about which it turns.
In the 6th century, the Greek philosopher Pythagoras said the same thing. Today we have convincing evidence to prove that the earth is spherical in shape, and that evidence is the subject of the rest of this section.
1.2.1 Proof That The Earth Is Spherical In Shape
No single observation settles the question on its own. What follows is nine separate lines of evidence, taken from ships, a canal, a star, a shadow, radio signals and photographs. Each one is tested the same way: what would we see if the earth were flat instead, and is that what we actually see?
Sighting Of A Ship
While standing at seashore, watching an approaching ship, we first see only its mast. Then, as it comes closer, we can see the deck in the middle, then its funnel, and finally the hull comes into full view. This can happen only when the earth’s surface is curved, as Fig. 1.1 shows. If the earth were flat, the entire ship would have been visible at once, growing steadily larger but never appearing in parts.
Diagram
Fig. 1.1 Caption: Sighting of a ship. Brief: Two stacked panels, flat earth above and spherical earth below, each with three ships at increasing distance from the observer; the observer’s line of sight is a dashed straight arrow running left; on the flat panel the whole of each ship is visible, on the curved panel the hulls of the farther ships are cut off by the curve.
The Bedford Level Experiment
An experiment was carried out in the Bedford Level canal area in England. Three poles of equal length were fixed at intervals of 5 km along the water, and it was ensured that the poles had the same height above the water in the canal. When the poles were viewed from one end of the canal, the middle pole looked slightly higher than the other two.
It could not have happened if the earth had a flat surface, because on a flat surface three poles of equal height would line up exactly, as Fig. 1.2 shows. The middle pole appeared higher because the water surface itself arches up between the two ends. A similar experiment was repeated at different places with the same result. This experiment demonstrated that the surface of the earth was curved.
Diagram
Fig. 1.2 Caption: Bedford level experiment. Brief: Two stacked panels, flat earth above and spherical earth below, each with three poles of equal length fixed 5 km apart with the spacing marked; a dashed sight line runs from one end along the tops of the poles, level on the flat panel and passing above the outer poles on the curved panel.
The Pole Star
The Pole Star can be seen at an angle of 90° at the North Pole. It lies in line with the axis of the earth, and its angle decreases as you travel towards the Equator, the line running right round the middle of the earth. At the Equator the angle is 0°, so the star sits on the horizon itself.
This steady fall in the angle can happen only in an arc of a circle, which is what Fig. 1.3 sets out. If the earth were flat, the Pole Star would have the same height for all latitudes, and latitude simply means how far north or south of the Equator a place lies.
Diagram
Fig. 1.3 Caption: Position of the Pole Star. Brief: Two panels side by side, a flat slab and a curved globe, each marked 45°N, 67.5°N and 90°N; parallel straight arrows labelled light from the Pole Star arrive from above; on the curved panel the angle between the arrow and the local horizon is drawn at each labelled place to show it shrinking towards the Equator.
Satellite Pictures
Jean Baptiste Delambre (1749–1822), the French astronomer, discovered the flattening of the earth at the poles. In the 20th century, when satellite pictures of earth were fed into computers, its spherical shape with the flattened poles was confirmed, as in Fig. 1.5. The conclusive evidence for the proof of the earth’s spherical shape is obtained from the photographs taken from space.
Diagram
Fig. 1.5 Caption: A satellite picture of the Earth. Brief: Source a photograph of the whole earth taken from space showing the full lit disc, with ocean, cloud and land visible and the outline of the disc unbroken.
The Lunar Eclipse
The shadow of earth on the surface of the moon is clearly visible from earth during a lunar eclipse. It appears as an arc of a circle. Since the earth is bigger than the moon, it is rarely oriented in the same position as the moon, yet the shadow is always circular. A flat disc would throw a circular shadow only when it happened to face the moon squarely, and a long thin shadow at other times.
The Circular Horizon
The view of the earth’s surface as seen from a height is circular. With increase in altitude the circular horizon also widens, which is why a passenger climbing in an aircraft sees steadily further. Had the earth’s surface been flat, the horizon would have been the same irrespective of altitude, as the two panels of Fig. 1.4 make clear.
In modern days, all radio waves that travel over the earth’s surface can be heard only within a certain horizon. Beyond this, the signal is not received. This is because of the spherical shape of the earth, which curves away from the travelling wave.
Diagram
Fig. 1.4(a) Caption: Spherical earth: the higher the altitude, the wider the circular horizon, and the horizon AB widens to CD as you move up from Y to X. Brief: A curved earth surface with two observer positions Y and X marked on a vertical line, the lower sight lines meeting the surface at A and B and the higher ones at C and D, with both horizons labelled and the widening emphasised.
Diagram
Fig. 1.4(b) Caption: Flat earth: whatever the altitude, the visible horizon is the same. Brief: A flat earth surface with observer positions Y and X on a vertical line, sight lines from both reaching the same edge points A, B and C, D so that the horizon width does not change with height.
Sunrise And Sunset
The sun rises from the east and sets in the west, and sunrise and sunset occur at different times at different places. This is because the earth rotates from west to east, so a place in the east is carried into the sunlight before a place further west. People in the east therefore see the sun earlier than those in the west.
Had the earth been flat, the sunrise and sunset would have occurred at the same time all over the world, because the whole surface would face the sun together. Fig. 1.6 shows why a curved surface cannot do this.
Diagram
Fig. 1.6 Caption: Sunrise and sunset occur at different times at different places. Brief: Two panels, a spherical earth on the left and a flat slab on the right, each lit by the sun from one side; on the sphere mark the lit half DAY and the shaded half NIGHT with the dividing line running across it, and on the slab show the whole upper surface lit at once.
The Earth As A Heavenly Body
The earth belongs to the Solar system. All planets in this system are spherical in shape, and this is because of their continuous rotation. So the earth, being a member of the Solar system, cannot be an exception.
This last proof works differently from the other eight. The first eight are observations of the earth itself, while this one reasons from the company the earth keeps.
1.3 The Earth As A Planet
Earth, as a planet, is a spherical object in space that moves around the Sun, which is a star, and receives light from it. Sunlight is borrowed light. The planets do not have light of their own, and what we see when we look at a planet in the night sky is sunlight that has bounced off it.
Nor do planets wander freely. They move around their stars in fixed paths called orbits, and each planet keeps to its own orbit at its own distance. That fixed path is what makes a planet’s position predictable year after year.
1.3.1 Earth An Oblate Spheroid
The earth is said to be spherical, but it is not a perfect sphere. Its diameter, which is the distance measured straight through the earth from one side of the surface to the other, varies at the Equator and at the Poles. Its diameter at the Equator is 12,756 km, while its Polar diameter is 12,714 km.
This difference in diameter is due to the centrifugal force of the earth’s rotation at a great speed. Centrifugal force is the outward push that acts on anything being spun, throwing material away from the line it spins about. Because the earth turns fastest at the Equator, which is furthest from the axis, the outward push is greatest there and least at the Poles.
The result is a bulge at the Equator and a compression at the Poles, as Fig. 1.A3 shows with the difference exaggerated. Thus the earth is said to be an Oblate Spheroid. The shape of the earth is also described as Geoid, which means earth shaped.
Diagram
Fig. 1.A3 Caption: The spin of the earth pushes the Equator outward and flattens the Poles. Brief: A cross-section of the earth with the axis drawn vertically and the direction of rotation shown by a curved arrow at the Equator; mark the equatorial diameter 12,756 km as a horizontal double-headed arrow and the polar diameter 12,714 km as a vertical one; add short outward arrows at the Equator for the centrifugal push, and state on the figure that the bulge is exaggerated for clarity.
The two diameters are the whole of the evidence for the bulge, so it is worth seeing how large the difference actually is. In the working below, both figures are diameters measured in kilometres, and the second step expresses the difference as a share of the larger of the two.
Forty-two kilometres sounds large on the ground, and it is roughly the width of a large city. Set against a body more than twelve thousand kilometres across, it is about a third of one per cent. A globe made to true scale would look perfectly round to the eye and would feel perfectly round to the hand. This is why the earth is still called spherical in ordinary speech, and why the flattening was not discovered by looking but by measuring.
1.4 Earth As The Home Of Humankind
Our Earth is the only planet that supports life. Unlike other planets, it is covered with green vegetation, enormous blue-green oceans containing over a million islands, and a large number of streams and rivers. It also carries huge land masses called continents, with mountains, ice caps and deserts on them.
Some form of life thrives virtually in every part of the earth, from the coldest part of the Poles to the warmest part of the Equator. Earth has certain features that make it habitable and thus a unique planet. Those features are set out one by one below, and none of them works alone.
1.4.1 Distance From The Sun
The earth is at an optimum distance from the sun, which means the distance that suits life best. Hence it is neither too hot nor too cold.
The distance between the earth and the sun is what makes life possible on the earth. If it was closer to the sun, any life present would get burnt from too much heat of the sun, while if it was farther from the sun life would freeze because of lack of heat. The earth’s position is therefore not merely a fact about where it sits, but a condition of its being alive at all.
1.4.2 Temperature
The earth is the third planet from the sun. It has an average temperature of 17°C, which is suitable for life to exist. That average is a balance struck between the heat arriving and the heat leaving, and it is more delicate than it looks. If the average temperature on the earth’s surface changes by only a few degrees, many species would perish due to extreme heat or cold.
Set that against the temperatures found elsewhere. The other two terrestrial planets, which are the solid rocky ones like our own, are Mercury and Venus, and both are very hot with maximum temperatures of more than 400°C. The remaining planets in the Solar system are very cold, with surface temperature below 0°C.
One of these planets breaks the pattern you would expect. Venus is the second planet from the sun, so Mercury lies closer to the heat, yet Venus is hotter than Mercury. This is because the atmosphere around Venus is mainly composed of carbon dioxide, and the carbon dioxide produces the Greenhouse Effect on the surface of Venus, which is the trapping of the sun’s radiation by gases present in the lower atmosphere. Thus the temperature on its surface remains very high. The earth’s atmosphere contains a very small quantity of carbon dioxide, and that small quantity is part of why the earth is habitable and Venus is not.
1.4.3 Atmosphere
The Atmosphere is the blanket of air surrounding the earth. It is a mixture of gases like nitrogen (78%), oxygen (21%), carbon dioxide, helium and argon, and the last three together make up less than 1% of the whole.
That thin mixture does four distinct jobs for life. (a) It receives heat from the sun by solar radiation and loses heat by earth’s radiation, which is the heat the warmed surface sends back outward, and in this way a balance is maintained. (b) It is made up of life supporting gases like nitrogen, oxygen and carbon dioxide, on which living things directly depend, while other gases include Helium and Argon. (c) Ozone present in the earth’s atmosphere absorbs the harmful ultraviolet rays of the sun before they reach the surface. (d) It also prevents loss of heat from the earth’s surface and helps to keep the earth warm, particularly after sunset when no fresh heat is arriving.
1.4.4 Water
Earth is a watery planet, with 70 per cent of the earth’s surface being covered by water. The distribution of that water is responsible for moderating the climate and surface condition of the earth, so where the water sits matters as much as how much of it there is.
Water from seas, rivers and lakes evaporates into the atmosphere, where it condenses and falls back as precipitation. Most of the water on land flows back to the oceans. Thus water moves in a continuous cycle, the hydrological cycle, as shown in Fig. 1.8. There is a proper balance between evaporation, condensation and precipitation, without which life would not be possible.
Diagram
Fig. 1.8 Caption: Water cycle — the movement of water between the atmosphere, hydrosphere and lithosphere. Brief: A landscape cross-section with mountain, river, vegetated slope and ocean; label evaporation from waterbodies, transpiration from plants, condensation, rain and groundwater moving towards the ocean, with arrows pointing upward for evaporation and transpiration, downward for rain, and along the slope and underground towards the sea, so the loop closes.
Water also steadies the temperature of the planet from hour to hour. Water can absorb an enormous amount of heat without causing much change in its own temperature. During the day, waterbodies rapidly absorb enormous amounts of heat, and thus the earth remains fairly cool.
At night the waterbodies release the vast amounts of heat that they absorbed during the day. This released heat, along with other atmospheric effects, keeps most of the surface from freezing at night. But for the tremendous amount of water on the earth, far greater day and night temperature variations would exist.
1.4.5 Solid Crust (Lithosphere)
Earth has a solid crust, called the Lithosphere, which is the layer of rocks on the surface of the earth. Due to the weathering of the rocks, which is the breaking down of rock where it stands, the surface of the earth has been formed.
That broken rock provides the soil layer essential for supporting plant life. Soil also provides various nutrients necessary for the growth of plants, which in turn support all forms of animal life directly or indirectly. The chain runs from bare rock to the animals at its far end, and every link in it depends on the one before.
1.4.6 Biosphere
The self-regulating and self-sustaining structural and functional unit of the biosphere is called the Ecosystem. This system depends upon the sun for its energy. A pond, a lake, a desert, a grassland, a meadow and forests are common examples of ecosystems.
There is an exchange of materials and energy within the ecosystem, as well as between adjoining ecosystems. They are all interconnected and hence interrelated. The vast network of all interconnected ecosystems constitutes the biosphere. The biosphere is thus that part of the earth’s terrestrial system, including air, land and water, in which life develops and where life processes in turn get transformed. It is the life zone of the earth.
Four things are worth holding on to about this life zone. (a) The biosphere is the narrow realm of contact and interaction between the atmosphere, the lithosphere and the hydrosphere, so it exists only where all three meet, as Fig. 1.9(a) shows. (b) It provides all the necessities for all the species living on earth, which are light, heat, water, food and habitats. (c) It is a thin layer of approximately 15 km from the deepest ocean trench to the highest mountain peak, which is a small band on a planet 12,756 km across. (d) At the ground level it extends to a depth of just 3 m below the surface, and in ocean waters about 200 m deep, where marine and freshwater life is found.
Within the biosphere, life is found chiefly in two forms, the plant kingdom and the animal kingdom. Fig. 1.9(b) shows how both are spread through the air, the ground and the water at once.
Diagram
Fig. 1.9(a) Caption: Biosphere. Brief: Three overlapping circles of equal size labelled lithosphere, atmosphere and hydrosphere, with the central region where all three overlap shaded and labelled biosphere.
Diagram
Fig. 1.9(b) Caption: Biosphere: the life zone. Brief: A single cross-section running from ocean to hillside, labelling birds in the air, vegetation and animals on the land, animals and microscopic organisms in the ground, dead plants and animals in the soil, and sea creatures and marine living organisms in the water; band the three realms as atmosphere, hydrosphere and lithosphere.
1.4.7 Life-Giving Cycles
Various life-giving cycles operate in nature, balancing the factors necessary for life. Each one takes a substance, moves it through living things, and returns it to where it started. Four of them are described here.
The Nutrition Cycle
The nutrition cycle represents a relationship between living and non-living things in our environment, as set out in Fig. 1.10. It shows the interdependence of all living things on earth. It operates through the food chain and other processes like photosynthesis, transpiration and respiration.
The living things in the cycle are divided into producers, consumers and decomposers. Plants are the producers of food, through the process of photosynthesis, by which green plants make their own food using sunlight, carbon dioxide and water. Energy has its origin from here.
Then there are the categories of consumers, the herbivores and the carnivores, who live on the producers. At the top of all is man, who can modify his own environment and create changes at the bottom or in the middle level of the food chain. All living beings undergo decomposition by the action of saprotrophs, or decomposers.
Diagram
Fig. 1.10 Caption: Nutrition cycle. Brief: A ring of arrows running from the sun to autotrophs (producers) and then on to heterotrophs (consumers), herbivores (plant eaters), carnivores (flesh eaters), saprotrophs (decomposers) and abiotics, and back to the producers, with every arrow pointing the same way round the ring.
The Carbon Cycle
Carbon is the basic raw material of all life, and all living things contain carbon. Fig. 1.11 traces its path. Atmospheric carbon dioxide is used by plants in the presence of sunlight and chlorophyll to make their own food by the process of photosynthesis, and during this process carbon dioxide is fixed as starch by the plants.
From the plants, carbon moves along the food chain through consumers at different levels. After the death of plants and animals, the carbon present in their bodies is decomposed and absorbed as food by saprophytic bacteria and fungi. When plants die and get buried in the soil, they undergo slow degradation and compaction, and this results in the formation of fossil fuels containing huge amounts of carbon.
Carbon returns to the air by three routes. During respiration, plants, animals and humans use atmospheric oxygen and release carbon dioxide back into the atmosphere. The burning of carbon containing fuels also results in the release of carbon dioxide into the atmosphere. Thus carbon taken by green plants from the environment through photosynthesis is returned to the environment through respiration, through decomposition and through the burning of fuels.
This cyclic movement of carbon from the atmosphere to the organisms, and from the organisms back to the atmosphere, is known as the carbon cycle. The carbon cycle helps in maintaining the carbon balance in nature. Any imbalance of carbon in the atmosphere heats up the earth and leads to Global Warming, so excessive use of gases that produce undue heat and global warming has to be avoided to maintain a healthy atmosphere.
Diagram
Fig. 1.11 Caption: Carbon cycle. Brief: A closed loop with carbon dioxide in the atmosphere at the top, arrows running down to photosynthesis on one side and up from respiration on the other, living organisms in the centre with decay feeding back to the atmosphere, and oxygen shown leaving photosynthesis and entering respiration.
The Nitrogen Cycle
The atmosphere is the largest reservoir of free nitrogen, holding approximately 78% of it. Plants take nitrogen in the form of nitrates from the soil, and it moves through the food chain to animals. Nitrogen is returned to the soil through manure, excreta and the earthly remains of plants and animals, and through micro-organisms. Fig. 1.A4 shows the path.
Flowchart
Fig. 1.A4 Caption: Nitrogen passes from the soil into plants, on into animals, and back to the soil again. Nodes: atmosphere, the largest reservoir of free nitrogen at about 78% → nitrates in the soil → taken up by plants → along the food chain to animals → manure, excreta and earthly remains of plants and animals, worked on by micro-organisms → back to nitrates in the soil
The Oxygen Cycle
Living things take in oxygen from the atmosphere. They use it to release energy from the food they eat. Oxygen is also used together with carbon, hydrogen and nitrogen to build new molecules in their bodies.
Oxygen is released back into the atmosphere by green plants during photosynthesis, and by plants and animals as part of carbon dioxide, so the loop closes, as Fig. 1.A5 shows. These cycles are nature’s mechanism to maintain balance in the ecosystem, and no one of them runs on its own.
Flowchart
Fig. 1.A5 Caption: Oxygen leaves the air with every breath and is put back by green plants. Nodes: oxygen in the atmosphere → taken in by living things → used to release energy from food; also used with carbon, hydrogen and nitrogen to build new molecules in the body → returned by two routes: released by green plants during photosynthesis, and given off by plants and animals as part of carbon dioxide → back to oxygen in the atmosphere
Aryabhatta And The Rig Veda
The Rig Veda contains verses suggesting that the earth was spherical in shape. Aryabhatta went further and proved that the earth revolved around the sun, challenging the prevalent belief of the earth being the centre of the universe. It should be noted that Indian society at that time did not prosecute those who challenged common beliefs, quite unlike the corresponding societies that existed in other parts of the world.
Aryabhatta, an Indian mathematician and astronomer during the Gupta Period, made calculations which are remarkably close to modern calculations, besides developing methods to observe celestial phenomena and bodies. One of his notable works, the Aryabhatiya, mentions that he was 23 years in 3600 Kaliyuga, which corresponds to 499 CE. He challenged a number of existing beliefs and provided proofs for them.
Three of his results are worth setting down. (a) He provided a detailed explanation of how the earth rotates on its axis, leading to night and day as well as the apparent movement of stars in the sky. (b) He estimated the earth’s circumference to be 39,968 km, which is a mere 0.2% less than the actual figure of 40,075 km. (c) He calculated the earth’s rotation as 23 hrs, 56 min and 4.1 sec, which is 0.01 seconds more than the actual value of 23 hrs 56 min 4.09 sec.
1.5 Summary
- Earth is the third planet from the sun and the fifth-largest planet in the solar system, and it is the only planet which supports life.
- Seen from space the earth looks blue, because 70 per cent of its total area is covered by water, and it is therefore called the watery planet.
- The family of the sun is called the Solar System, and its chief members fall into three groups.
- The planets, each keeping to its own distance from the sun.
- The satellites, which revolve around the planets.
- Other celestial bodies called asteroids, comets and meteorites.
- The planets in order of their distance from the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
- A planet has no light of its own. It moves around its star in a fixed path called an orbit and shines by the light it receives.
- The earth is not a perfect sphere. It bulges slightly at the centre and is flattened at the poles, though in the olden days it was considered to be a flat disc with steep edges.
- Aryabhatta, the 5th century Indian astronomer and mathematician, said the earth was spherical and rotated on its axis, and in the 6th century the Greek philosopher Pythagoras said the same thing.
- Nine lines of evidence prove that the earth is spherical in shape.
- A ship approaching the shore appears mast first and hull last, which can happen only over a curved surface.
- In the Bedford Level Experiment, three poles of equal height fixed 5 km apart showed the middle pole standing higher.
- The Pole Star stands at 90° at the North Pole and its angle falls to 0° at the Equator.
- Satellite pictures fed into computers, and photographs taken from space, confirm the shape with its flattened poles.
- The earth’s shadow on the moon during a lunar eclipse is always an arc of a circle.
- The horizon seen from a height is circular and widens with altitude, and radio waves travelling over the surface are received only within a certain horizon.
- Sunrise and sunset occur at different times at different places, because the earth rotates from west to east.
- Magellan’s ship Victoria completed a round-the-world voyage in 1522 and returned to Spain, where it had started.
- All planets of the Solar system are spherical because of their continuous rotation, and the earth is no exception.
- The earth’s diameter at the Equator is 12,756 km while its Polar diameter is 12,714 km.
- This difference is due to the centrifugal force of the earth’s rotation at a great speed, which forms a bulge at the Equator and a compression at the Poles. The earth is therefore called an Oblate Spheroid, and its shape is also described as a Geoid, meaning earth shaped.
- Certain features together make the earth habitable, and no one of them works alone.
- The earth is at an optimum distance from the sun, so it is neither too hot nor too cold.
- Its average temperature of 17°C is suitable for life, and a change of only a few degrees would destroy many species.
- The atmosphere balances heat gained and lost, supplies life supporting gases, absorbs harmful ultraviolet rays through ozone, and keeps the surface warm.
- Water covers 70 per cent of the surface, moderates the climate, and evens out day and night temperatures by absorbing and releasing heat.
- The solid crust, or lithosphere, weathers into the soil layer that supplies plants with nutrients and so supports animal life.
- The biosphere is the narrow realm where the atmosphere, the lithosphere and the hydrosphere meet, and it supplies light, heat, water, food and habitats.
- Venus is farther from the sun than Mercury, yet hotter, because its atmosphere is mainly carbon dioxide and produces the greenhouse effect on its surface.
- The biosphere is a thin layer of approximately 15 km from the deepest ocean trench to the highest mountain peak, reaching about 3 m below the ground and about 200 m into ocean waters, and life within it is found chiefly as the plant kingdom and the animal kingdom.
- The ecosystem is the self-regulating and self-sustaining unit of the biosphere, and it depends on the sun for its energy. The vast network of all interconnected ecosystems constitutes the biosphere.
- Four life-giving cycles keep the factors necessary for life in balance.
- The nutrition cycle links producers, consumers and decomposers through the food chain.
- The carbon cycle moves carbon from the atmosphere into living things and back through respiration, decomposition and the burning of fuels.
- The nitrogen cycle carries nitrogen from soil nitrates into plants and animals and returns it to the soil.
- The oxygen cycle takes oxygen from the air for respiration and returns it through photosynthesis.
- Any imbalance of carbon in the atmosphere heats up the earth and leads to global warming, which is why excessive use of gases that produce undue heat has to be avoided.
Flowchart
Fig. 1.A6 Caption: The chapter asks two questions about one planet — what shape it is, and why it is alive. Nodes: Earth as a planet → branch 1, its shape: nine lines of evidence (ship, Bedford Level poles, Pole Star, satellite pictures, lunar eclipse, circular horizon, sunrise and sunset, circumnavigation, other planets) → surface is curved → but the two diameters differ, 12,756 km against 12,714 km → centrifugal force of rotation → bulge at the Equator and compression at the Poles → Oblate Spheroid, also called Geoid; branch 2, its conditions for life: optimum distance from the sun → average temperature of 17°C → held steady by the atmosphere and by water covering 70% of the surface → lithosphere weathers to soil → atmosphere, lithosphere and hydrosphere meet in the biosphere → nutrition, carbon, nitrogen and oxygen cycles → balance maintained → back to conditions for life
What This Chapter Cannot Tell You
The nine proofs establish that the earth’s surface curves, but not one of them says by how much. The size of the flattening comes only from measuring the two diameters, and until those measurements were made the earth could honestly be called a sphere.
A single average temperature of 17°C is a figure for the whole planet at once. It hides the difference between the coldest part of the Poles and the warmest part of the Equator, and it hides the difference between day and night at any one place.
The cycles are drawn as neat closed loops, and a loop shows the route but not the rate. The chapter does not say how much carbon or nitrogen moves, or how long a single atom takes to complete the circuit, which is exactly what would be needed to judge how far the balance has already shifted.
1.6 Check Your Concepts
Study Fig. 1.9(a). Name the three realms shown in it, state what the region where all three overlap is called, and explain why life is found only in that region.
Aryabhatta estimated the earth’s circumference as 39,968 km, while the actual circumference given in this chapter is 40,075 km. Work out the difference between the two figures in kilometres, and state whether his estimate was greater or smaller than the actual value.
Set out, in the correct order, the steps by which bare rock on the earth’s crust comes to support animal life. Name the process that begins the chain.
The biosphere is about 15 km thick, while the earth’s diameter at the Equator is 12,756 km. What does this comparison tell you about the life zone, and what does the figure of 15 km fail to tell you about where living things are actually found?
What would happen if the average temperature of the earth increased by half a degree?
Trace the cyclical movement of carbon through the three realms of the earth.
How does the sighting of a ship from the seashore prove that the earth is round in shape?
Give a geographical reason for each of the following.
- Sunrise and sunset occur at different times at different places.
- The earth is not a perfect sphere.
- Venus is hotter than Mercury.
Draw a well labelled diagram to show that the horizon of the earth is circular in shape.
Describe the role of each of the following in making the earth a habitable planet.
- Atmosphere
- Water
- Temperature
- Distance from the Sun