Motions of the Earth - Physical Geography, UPSC, IAS. | Geography (Prelims) by Valor Academy PDF Download

Motions of the Earth

CHAPTER 2

 MOTIONS OF THE EARTH

REVOLUTION OF THE EARTH

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 the ‘leap year’ having 29 days in the month of February. Effects of the Revolution of the Earth are as follows:

Change of seasons.
Variation in the length of day and night at different times of the year.
Shifting of wind belts.
The earth’s axis is tilled at an angle of 23.5° from a perpendicular to the orbital plane.
In other words, the earth’s axis makes an angle of 66.5° with the plane of the earth’s orbit.

During one half of the year, the Northern Hemisphere and during the other half of the year the Southern Hemisphere is inclined towards the sun. Every point in the Hemisphere facing the sun takes a longer time to go out of the sunlight, with the result that the days are longer and nights shorter.

The Hemisphere Pole which is inclined towards the sun, the will have no night and the other Pole will have no day.
It is only on the equator that the day and the night are always of equal length.

Revolution of the Earth and the Seasons

THE CYCLE OF SEASONS

On 21^st June - North Pole is inclined towards the sun, while the South Pole is away from it. The rays of the sun are falling vertically not on the equator but on the Tropic of Cancer. In Northern Hemisphere the days are longer, nights shorter. It receives more heat than it loses. Therefore, it is having the summer season.

In Comparison, the rays of the sun are slanting in the Southern Hemisphere. Here is winter season in the Southern Hemisphere.

December  – It is the Southern Hemisphere which is inclined towards the sun while the

Northern Hemisphere is away from it. The vertical rays of the sun are now falling vertically on the Tropic of Capricorn. The days are longer and the nights are shorter in the Southern Hemisphere .It is therefore summer in the Southern Hemisphere and winter in the Northern Hemisphere.

23^rd September and 21^st March – The rays of the sun falls vertically on the equator at noon. Both the Poles receive the rays of the sun. As a result , exactly one-half portion of each Hemisphere receives the sun’s rays, making day and night equal all over the world on these two days.

On 23^rd September it is the autumn season in the Northern Hemisphere and the Spring Season in the southern Hemisphere. The opposite is the case on 21^st March.

ROTATION OF THE EARTH

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 24 hours (23 hours, 56 minutes, 4.09 seconds to be exact).The earth rotates from west to east. Effects of the Rotation of the Earth are as follow:

Causation of day and Night.

A difference of 1 hour between two meridians which are 15°apart. Defection of ocean currents and winds.

Rise and fall of tides every day.