A year is 365 days long, because that’s about how long it takes for the earth to complete one full revolution around the sun. The earth rotates or spins around its own axis. It revolves around the sun in an elliptical orbit. One full revolution around the sun takes exactly 365 ¼ days.
Every fourth year is a leap year - the extra day in a leap year results by adding the 1/4th days or 6 hours each year over 4 years.
The earth’s axis is inclined at an angle of 23-½ degrees to the vertical. This tilt and the revolution is the reason for different seasons.
The northern hemisphere receives direct sunlight, for the longest duration on the June 21st. This position of the earth is called the Summer Solstice. In summers, the days are longer and the nights are shorter. The areas beyond the Arctic Circle have daylight for 6 months at a stretch.
The reverse happens in the southern hemisphere; with less sunlight, the days are shorter and the nights are longer. The northern hemisphere receives sunlight for the shortest duration December 22. This position of the earth is known as the Winter Solstice.
During its orbit, there are times when neither of the earth’s poles is inclined towards the sun. Thus, on two occasions - March 21 and September 23, sunlight falls directly on the equator. This position of the earth is called an Equinox. On these days, the day and night are of equal duration on earth.
On September 23, it is autumn season in the Northern Hemisphere and spring in the Southern Hemisphere. The opposite is the case on March 21, when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.