The solstices and equinoxes are caused by the position of the Earth with respect to the Sun during fixed days. The seasons change on the Earth because of its revolution around the Sun.
Solstices and Equinoxes
- On March 21 and September 23, the sunrays are directly above the Equator. On these two days, the days and the nights are of equal duration, i.e. of twelve hours each. These are known as equinoxes.
- On June 21, the Northern Hemisphere is tilted towards the Sun. Because the direct rays of the Sun fall on the Tropic of Cancer, these areas receive more heat. The areas near the poles get the slanting rays of the Sun, and thus, they receive less heat from the Sun. Because the greater part of the Northern Hemisphere receives light from the Sun, it is summer in the places lying to the north of the Equator (Northern Hemisphere).
- On June 21, the day is longest and the night is the shortest in the Northern Hemisphere.
- The conditions are reversed in the Southern Hemisphere as it is tilted away from the Sun. The Sun’s rays do not reach the south Polar Regions beyond the Antarctic Circle. This marks the winter in the South Hemisphere. This is called the summer solstice. The imaginary line of a circle which divides the day and night on the globe is called the circle of illumination. The Earth revolves around the Sun in an elliptical orbit.
- On December 22, the Sun is directly above the Tropic of Capricorn. Because the South Pole is inclined towards the Sun, it is summer in the places lying south to the Equator (southern hemisphere).
- The conditions are reversed in the North Pole. Because it is tilted away from the Sun, it is winter. December 22 has the longest night and shortest day in the Northern Hemisphere. It is known as the winter solstice.
- On September 23, the Sun is vertically overhead at the Equator. The days and nights are of equal duration at the Equator. It is autumn in the Northern Hemisphere, and hence, it is called Autumnal Equinox. It is spring in the Southern Hemisphere at this time.
- On March 21, it is spring in the Northern Hemisphere, and hence, it is known as the Vernal Equinox.
Angle of Incidence
The angle of incidence is measured in degrees. It is calculated by subtracting the value of the latitude in degrees from 90°, as this is the maximum reached the Equator.