We have seen in earlier chapter that the sun heats up more when overhead. Similarly, the area of earth where the sun is overhead during the season, gets heated up more than other areas. Because of the axial tilt of the earth, there is an apparent movement of orbital plane in relation to the earth. We feel that the sun migrates from northern hemisphere to southern hemisphere and back in the period of a year. The midday sun is exactly overhead at all latitudes between tropics of Capricorn and cancer, at least once in a year. This area gets maximum heat and is called Torrid Zone.
The sun is never overhead in any place on earth beyond the Tropic of Cancer and Capricorn and the angle of sun's rays goes on decreasing towards the poles. At areas beyond Arctic Circle and Antarctic Circle, the sun appears just above the horizon and these areas are very cold due to this. These areas are called Frigid Zones. The areas between Torrid Zone and Frigid Zone have a moderate temperature and are called Temperate Zones.
Because of the constant inclination of the earth's axis in one direction, the Northern hemisphere faces the sun for about half of the year and faces away from the sun for next half of the year. The part which faces the sun gets more heat and light compared to the other part. Every point in this hemisphere remains in the sunlight for a longer period of time and hence we say that the days are longer in this hemisphere.
While northern hemisphere faces the sun, southern hemisphere will be facing away and will get less heat and light and will have shorter days and longer nights. This reverses itself in every 6 months. The hemisphere which faces the sun will be warm and is said to have summer season and the hemisphere which faces away from the sun will be cold and is said to be having winter. Hence you can say that summer in Northern hemisphere coincides with winter in the Southern hemisphere and vice versa.
There is a season of transition between the two when it is neither too warm nor too hot when the sun is almost around the equator. This season is called autumn or spring.
Spring is the season after winter and before summer. Days become longer and the weather gets warmer in the temperate zone because the Earth tilts towards the Sun. In many parts of the world, plants grow and flowers bloom.
Autumn is the season after summer and before winter. In the United States, this season is also called fall. In many places in the temperate zone, autumn is a time for harvesting most crops. Deciduous trees (trees that lose their leaves every year) lose their leaves, usually after turning yellow, red, or brown.
When it is autumn in the Northern Hemisphere, it is spring in the Southern Hemisphere and vice versa. On the Equator, autumn is very much like spring, with little difference in temperature or in weather.
When the sun reaches the northern-most point it will be overhead at the tropic of cancer, and when it reaches the southern most point it will be overhead the tropic of Capricorn. The day when the sun is overhead of Tropic of Cancer is called Summer Solstice(Northern Solstice) and the day it is overhead at the tropic of Capricorn it is called Winter Solstice(Southern Solstice ). The day of the solstice is either the "longest day of the year" or the "shortest day of the year" for any place on Earth depending on the hemisphere facing the sun and location of the place.
The Northern Solstice is also called June Solstice as it is usually on June 21. In the Northern Hemisphere the June solstice is called the Summer Solstice (and marks the longest day of the year), while in the Southern Hemisphere it is called the Winter Solstice (and marks the shortest day of the year).
The southern solstice is also called December Solstice and is usually on December 21. At the moment of the December solstice, the Sun is directly overhead some point on the Tropic of Capricorn; this is the furthest south that the subsolar point ever reaches. In the Southern Hemisphere, the December solstice is called the Summer Solstice (and marks the longest day of the year), while in the Northern Hemisphere it is called the Winter Solstice (and marks the shortest day of the year).
Similarly, the day when the sun is directly overhead the equator is called Equinox. There are two equinoxes each year. On these days, the nights are equal in length at all latitudes North and South. The word equinox comes from two Latin words meaning "equal" and "night". Around the day of the equinox, the length of the day and the length of the night are almost equal. They occur on or around March 21 and September 21.