Earth as a Planet
ICSE Class 9 · Geography · Chapter 1
What this chapter covers
- The Earth in the solar system
- Early ideas about the shape of the Earth
- Evidence that the Earth is round
- The true shape: an oblate spheroid, or geoid
- The size of the Earth
- Earth as the home of humankind
- A suitable range of temperature
- Water in liquid form
- An atmosphere that supports and protects life
- Other conditions that favour life
- The four spheres of the Earth
Key definitions
Circumnavigation is a journey that goes all the way around the Earth and returns to the starting point.
Gravity is the force with which the Earth pulls every object towards its centre.
Soil is the thin upper layer of the Earth's crust made of fine rock particles mixed with decayed plant and animal matter, air and water, in which plants grow.
Key points
- The Earth is the third planet from the Sun, the fifth largest planet and the largest of the four inner, rocky planets; it is about 150 million km from the Sun.
- The Earth rotates on its axis in about 24 hours and revolves around the Sun in about 365¼ days, with its axis tilted at about 23½°.
- Proofs that the Earth is round: the ship's mast appears first; the horizon is circular and widens with height; the Earth's shadow on the Moon is circular; sunrise is earlier at places further east; the Pole Star rises higher as one goes north; circumnavigation; photographs from space; other heavenly bodies are round.
- The Earth is an oblate spheroid, flattened at the poles and bulging at the Equator because of its rotation; its true shape is called the geoid.
- Equatorial diameter about 12,756 km, polar diameter about 12,714 km, equatorial circumference about 40,075 km, surface area about 510 million sq km.
- Eratosthenes estimated the circumference at about 40,000 km from the angle of the Sun's rays at two places 800 km apart.
- The Earth is the only planet known to support life, because it lies in the Sun's habitable zone and has all the conditions life needs together.
- Its average temperature of about 15 °C is kept steady by its distance from the Sun, the greenhouse effect of its atmosphere and its quick rotation.
- About 71 per cent of the surface is water, earning the name Blue Planet; only about 3 per cent of all water is fresh.
- The atmosphere supplies oxygen, carbon dioxide and nitrogen, and protects life through the ozone layer and by burning up meteors.
- Gravity, the magnetic field, fertile soil, a steady Sun and the cycle of day, night and seasons also favour life.
- The lithosphere, hydrosphere, atmosphere and biosphere are connected; life exists where they meet.
Questions answered
Define a planet. Name the inner planets and state the Earth's position among the planets.
A planet is a large, nearly spherical celestial body that revolves around a star in a fixed orbit, shines by reflected light and has cleared its orbit of comparable bodies. The inner planets are Mercury, Venus, Earth and Mars. The Earth is the third planet from the Sun and the fifth largest.
Why does the mast of an approaching ship become visible before its hull?
The surface of the sea is curved. When the ship is far away, the bulge of the water between the observer and the ship hides its lower part, so only the top of the mast is seen. As the ship comes nearer, more of it rises above the curve, until the hull also appears.
Explain how a lunar eclipse proves that the Earth is spherical.
During a lunar eclipse the Earth comes between the Sun and the Moon and casts its shadow on the Moon. The edge of the shadow is always circular. Only a sphere casts a circular shadow in every position, so the Earth must be spherical.
Give two more proofs of the roundness of the Earth, other than the ship and the eclipse.
Any two of these: the Sun rises earlier at places further east, which would not happen on a flat Earth; the height of the Pole Star above the horizon increases as one travels north; ships and aircraft can circumnavigate the Earth by moving steadily in one direction; photographs from space show a round Earth; the horizon is circular and widens with height.
What is meant by an oblate spheroid? Why is the Earth not a perfect sphere?
An oblate spheroid is a sphere slightly flattened at the poles and bulging at the equator. The Earth has this shape because of its rotation: points on the Equator move fastest, and the outward effect of the spin made the material of the early Earth bulge there. Its equatorial diameter (12,756 km) is about 42 km more than its polar diameter (12,714 km).
What is the geoid? Why is the term used instead of "sphere"?
The geoid is the true shape of the Earth, the shape that mean sea level would take if extended under the continents. The term is used because the Earth is neither a perfect sphere nor a perfect oblate spheroid; its shape is uneven and unique to itself.
Eratosthenes found that the Sun's rays differed by 7.2° between two cities 800 km apart. Show how he estimated the Earth's circumference.
7.2° is (7.2)/(360) = (1)/(50) of a full circle. The 800 km is therefore (1)/(50) of the circumference, and the circumference = 50 × 800 = 40,000 km.
Why is the Earth called the Blue Planet?
About 71 per cent of its surface is covered by water, so from space the Earth appears mostly blue. It is the only planet in the solar system with large amounts of liquid water on its surface.