Do you know that the earth is tilted? Yes, we have seasons because of its tilt as it revolves around the sun. This tilt is called declination, is a Latin word which means the bending away. It is the angular distance between the sun north or south and the equator of the earth
The earth revolves & rotates
This declination angle keeps on changing with the revolution of the earth around the sun. Let us understand step by step through pictorial description that how this is responsible for the change in seasons?
The following diagram depicts the declination angle between the sun and the earth:
The earth is tilted
The earth’s equator is tilted by 23.45 degrees with respect to the earth’s orbit around the sun. As the earth revolves around the sun its angle of declination varies from 23.45 degrees north to 23.45 degrees south. This means that earth is always pointing to one side when it is revolving around the sun. This changing tilt give rise to the varying amount of the sunlight received by the earth, the reason behind the different seasons i.e. spring, summer, autumn and winter.
The earth takes 365.24 days to complete its revolution around the sun. As it orbits around the sun the amount of light each area receives varies in length. When the earth’s axis points towards sun then it is summer in that particular hemisphere. When northern axis points towards the sun then it is summer in the northern hemisphere and the winter in the southern hemisphere.
Case I: When the North Pole tilts towards the sun
Case II: When the North Pole tilts away from the sun
During autumn (after summer), the North Pole begins to move away from the sun. The days are getting shorter and when the sun comes in mid point it is called autumn equinox, which is around September 21.At this time both the day and night are 12 hours long.
During spring (after winters), the North Pole begins to move towards the sun. The sun move upwards in to the sky and the days starts getting longer. And when sun reaches the mid-point in the sky around March 21, the day and night becomes equal that is 12 hours each. It is called spring equinox.
During summers, the sun is almost overhead and the sun rays hits the earth’s surface at the steep angle and spread less, which causes earth to absorb more sun rays and the longer days makes the days hotter in the summer. The days are warmer and longer in the summer season.
During winters , the sun's rays reach the earth’s surface at the shallow angle causes rays to get refracted and less amount of the sun rays is absorbed by the earth and shorter days further minimizes the amount of energy absorbed by the earth during winters. There are long and cold nights in the winter season.
Declination angle calculation
The declination angle can be calculated as follows:
D = 23.45° × sin [(284+31)/365 × 360°]
= 23.45° × sin [(315/365) × 360°]
= 23.45° x sin [310.7°]
= 23.45° x -0.76
= -17.78° i.e. the North Pole is tilted 17.78° tilted away from the sun.
Similarly, on March 21-22(spring equinox) and September 21-22(autumn equinox), the declination is zero.
The three main reasons for the change in the seasons are:
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