Earth is the largest of the inner planets. Earth is known as the 'Watery Planet' or 'Blue Planet' due to presence of huge amount of water on it. Earth is the only known planet which provides sustenance or life on it. It has large quantity of oxygen which supports life.
Earth's Shape
Pythagoras, a Greek philosopher and mathematician, was among the first to suggest that the shape of Earth was shaped like a globe. If the earth were a flat disc, then the rising Sun would have been seen at all places at the same time. But this does not happen. Places in the east see the Sun earlier.
There are various theories that prove earth is a sphere. The earth is rarely oriented in the same position during successive eclipses but it always casts a circular shadow, thus proving that the earth is sphere. A sphere is the only body that will always cast a circular shadow. At the north pole, the polar star can always be observed at 90 degrees in the sky, since the star lies in the line with the axis of the earth. As one travel southwards, the angle of pole star decreases. At the equator the angle becomes zero degree, this observation also proves that the path of travel is an arc of circle.
Earth shape is defined in both ways as real world shape and geometrical shape. Real world shape is a practical example to define earth shape like an Ostrich egg or an Orange. Earth's shape is close to two geometrical shape, one for rotating earth and other for non-rotating earth. Geometrical shape are very difficult to create in real world as it require very high accuracy. If earth was not rotating then its shape will come close to the geometrical shape called Oblate Ellipsoid. As the earth rotate on its axis, so its shape with the bulge comes closer to a geometrical shape called Oblate Spheroid. Geoid is the name that humans have given to the shape of earth, geoid has no geometric equation.
Movement and their effect
There are two type of movements that the Earth does i,e Major Movements and Minor Movements.
Major Movements : This includes Rotation and Revolution of Earth.
- Rotation: Earth spins,west to east on its axis once in 24 hours approximately (23 hours 56minutes and 4.09 seconds). The Earth's axis is not vertical. It makes an angle of 23degree30' with the verical or 66degree30' with the plane of the Earth's orbit. The Earth's axis always remain pointed in the same direction i.e towards the Pole Star, As the Earth moves around the Sun. Thetilt of the Earth's axis is known as the inclination of the Earth's axis. Movements of the tides are mostly determined by the rotation of Earth.
- Revolution: It is the movement of the Earth around the Sun. It takes 365 days, 5 hours and 48 minutes to revolve around the Sun. Earth revolve around the Sun in anticlockwise direction due to the gravity of the Sun.
Minor Movements: These movements of Earth were given by Milankovitch and are called Milankovitch cycle. These are Orbital Eccentricity, Axial Tilt, Axial Precession.
- Orbital Eccentricity: The orbit of the earth wrt to the sun does not remain the same in every revolution. Gravity of the sun is not the only gravity acting on the earth, gravity of other planets, moon also act on Earth. Distance of the earth from the sun is fixed but distance of earth from other planet is not fixed because other planet has different time for revolution. Hence if the distance of the other planet from earth is changing therefore the gravitational effect on earth also changes and hence there is minor change in Earth's orbit. This change is primarily because of the gravitational effect of Saturn and Jupiter.
- Axial Tilt: Tilt of axis of the Earth (23.5degree) is not fixed. It varies from 22.1 degree to 24.5 degree. This variation takes around 41000 years. So for now the angle is 23.5 degree and it will increase upto 24.5 degree and after 41000 years it will again start from 22.1 degree. This will effect the climate as it varies Sun rays that bring climatic changes.
- Axial Precession: The Earth's rotation is stable because the rotation speed is very high. If the rotation speed decreases than the Earth will appear like a spin top. Earth's precession has period of about 26000 years because the tidal forces of the sun and the moon and the shape of the earth.
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