Seasonality refers to both the seasonal variation of the sun’s position above the horizon and changing day lengths during the year.

Horizon refers to the apparent line at which earth and sky seems to be meet. Sun’s altitude refers to angle between horizon and sun. Solstice refers to sun standing still or sun reaching highest point.

Reasons for Seasons:

1. Earth’s Revolution in its orbit around the sun:

At an average distance from the sun of 150 million km earth completes its annual orbit in 365.24 days at a speed averaging 1,07,280 kmph in a counter clockwise direction when viewed from the above earth’s north pole.

2. Earth’s daily rotation on its axis:

Earth’s rotation determines day length, causes the apparent deflection of moving bodies and produces tides in relation to gravitational pull of sun, moon and earth.

Earth’s rotation produces diagonal pattern of day and night. The dividing line between day and night is called circle of Illumination since circle of Illumination always bisects the equator. Therefore, day length at equator is always evenly divided 12 hours of day and 12 hours of night. All other latitudes experiences uneven day lengths through the season except for 2 days a year on the equinoxes.

3. Tilt of Earth’s axis:                            

To understand earth’s tilt, imagine earth’s elliptical orbit about the sun as plane with the half of the sun and earth above the plane and half below. This flat surface is termed the plane of the ecliptic.

Now, imagine a perpendicular line passing through this plane from this earth’s axis is tilted 23.5 degree. It forms a 66.5 degree angle from the plane itself.

4. The unchanging orientation of earth’s axis:

Through out earth’s annual journey around the sun, earth’s axis maintains the same alignment relative to the plane of ecliptic and Polaris. In each position, Earth is revolving with the axis oriented identically or parallel to itself. This condition is known as Axial parallelism.

5. Spherical shape of  Earth:

Earth’s curved surface presents a continually varied angle to the incoming parallel rays of the sun. The latitudinal variation in the angle of the solar rays results in uneven global distribution of Insolation.

The combined effect of the above mentioned five physical factors is the Annual march of seasons on the earth.

December Solstice:

The circle of Illumination excludes the north pole region(Winter solstice in northern hemisphere) and includes the south pole region(Summer Solstice in southern hemisphere). The sub solar point is at 23.5 degree south latitude i.e. Tropic of Capricon.

The northern hemisphere is tilted away from the more direct rays of sunlight thereby creating a lower angle for incoming solar ray and more diffuse pattern of insolation. Thus causing winter season.