Friday, 18 July 2014 09:11


The earth has two main motions: (i) Rotation and (ii) Revolution.

The axis of the earth, which is an imaginary line, makes an angle of 66½° with its orbital plane. The plane formed by the orbit is known as the 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. This circle does not coincide with the axis as shown in figure below.

Rotation: The earth rotates around its axis. The axis is an imaginary line passing through the centre of the earth. The earth completes one rotation in 23 hours, 56 minutes, 4.09 seconds. The earth rotates from west to east.

Effects of the Rotation of the Earth

a) Causation of day and night

b) A difference of 1 hour between two meridians which are 15°apart.

c) Deflection of ocean currents and winds.

d) Rise and fall of tides every day

Revolution: It is earth's motion in its elliptical orbit around the sun. One revolution is completed in 365 1/4 days, resulting in one extra day every fourth year. The year, consisting of 366 days is called a "leap year" having 29 days in the month of February.

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 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 (23½° S), 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. Thus, we find that there are days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.