Variations in the amount of sunshine received are the basis for the occurrence of different seasons. The apparent position of the sun during the earth’s revolution will be over the equator on March 21 and September 23. Hence the length of day and night will be equal during these days on both the hemispheres. These days are called Equinoxes.

The apparent position of the sun shifts from the equator to the northern hemisphere from March 21 to June 21. The sun will be vertically above the tropic of cancer on June 21. This day is known as Summer solstice. lt has the longest day in northern hemisphere and the longest night in southern hemisphere. From June 21 onward, the sun shifts from the tropic of cancer towards the equator and reaches vertically over the equator on September 23.As the sun is in the northern hemisphere from March to September, it will be summer in the northern hemisphere.

The sun continues its apparent movement from the equator to the southern atmosphere and reaches vertically above the Tropic of Capricorn on December 22. This day is known as Winter solstice. During the period from September to March, it will be winter in the northern atmosphere and summer in the southern atmosphere.