Seasonality refers to both the seasonal variation of the suns position above the horizon and the changing daylength. There are five reasons for seasons which are discussed here under:
- Earth’s revolution in its orbit around the sun: Earth completes its annual orbit in 365.24 days at a speed averaging 107,280 Km.hr in a counter clockwise direction when viewed from above the earth’s north pole.
- Earth’s rotation: It determines day length, causes the apparent deflection of moving bodies and produces tides. Earth’s rotation reduces the diurnal pattern of day and night. The dividing line between day and night is called the circle of illumination. Since this circle of illumination always bisect the equator, day length at the equator always evenly divided.
- Tilt of axis: Earth is tilted towards its axis. It makes an angle of 23.50 . This is also the reason for seasonality.
- Axial parallism: Throughout the revolution the earth’s axis maintain the same alignment relative to the plane of ecliptic . In each position earth is revolving with the axis oriented identically. This condition is known as parallelism.
- Spherical shape of earth: Earth curved surface presents a continuously varied angle to the incoming parallel rays of the sun. The latitudinal variation in the angle of solar rays results in an uneven global distribution of insolation.