# Movements of the Earth

The Earth has three basic movements: Galactic movement, Rotation and Revolution.

### Galactic Movement

This is the movement of the Earth with the sun and the rest of the solar system in an orbit around the centre of the Milky Way Galaxy. This movement has little effect upon the changing environment of the Earth.

### Rotation

The Earth rotates or spins around its axis. The axis is an imaginary line passing through the centre of the Earth. Its two ends on the surface are called North Pole and South Pole. The axis of the earth makes an angle of 66½° with its orbital plane.

The earth receives light from the sun. Due to the spherical shape of the earth, only half of it gets light from the sun at a time. The portion facing the sun experiences day while the other half away from the sun experiences night. The circle that divides the day from night on the globe is called the circle of illumination.

The Earth completes a rotation in 24 hours. The Earth rotates in an eastward direction opposite to the apparent movement of the sun, moon and stars across the sky. Looking down on a globe from above the North Pole, the direction of rotation is counter-clockwise.

The velocity of rotation on the Earth varies depending on the distance of a given place from the Equator. The rotational velocity at the poles is nearly zero. The greatest velocity of rotation is found at the Equator where the distance traveled by a point in 24 hours is largest, the velocity is about 1700 km per hour.

### Revolution

Earth revolves around the sun in an elliptical orbit at an average distance from the sun of about 149,000,000 km. The path on which the Earth describes its motion is called Orbit.

Because of the elliptical shape of the orbit, the distance varies from time to time. About 3 January, the Earth is closest to the Sun and is said to be at Perihelion. Its distance then from the sun is approximately 147 million km. Around 4 July, the Earth is about 152 million km from the sun. It is then that the Earth has reached its furthest point from the sun and is said to be at Aphelion.

Five million km is insignificant in space and these varying distances from the Earth to the Sun do not materially affect the receipt of energy or Earth.

Speed of Revolution

The mean speed of the Earth is its orbit is 107,000 km per hour.

Period of Revolution

The period of time the Earth takes to make one revolution around the Sun determines the length of one year. Earth takes to complete one revolution of the Sun in 365 days and 6 hours.

Because of the difficulty of dealing with a fraction of a day, it has been decided that a year would have 365 days and that in every fourth year, called Leap Year, an extra day would be added in February.

### Seasons

A year is usually divided into summer, winter, spring and autumn seasons. Seasons change due to the change in the position of the earth around the sun.

On 21st June, the Northern Hemisphere is tilted towards the sun. The rays of the sun fall directly on the Tropic of Cancer. As a result, these areas receive more heat. The areas near the poles receive less heat as the rays of the sun are slanting. The North Pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months. Since a large portion of the Northern Hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21st June. At this time in the Southern Hemisphere all these conditions are reversed. It is winter season there. The nights are longer than the days. This position of the earth is called the Summer Solstice.

On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn, a larger portion of the Southern Hemisphere gets light. Therefore, it is summer in the Southern Hemisphere with longer days and shorter nights.

The reverse happens in the Northern Hemisphere. This position of the earth is called the Winter Solstice.

On 21st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun. So, the whole earth experiences equal days and equal nights. This is called an equinox.

On 23rd September, it is autumn season in the Northern Hemisphere and spring season in the Southern Hemisphere. The opposite is the case on 21st March, when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.