Solstices & Equinoxes
In the Northern Hemisphere the Sun shines vertically over the Tropic of Cancer (231/2ºN) on 21st June. This position is called the ‘Summer Solstice’, as the Sun seems to stand still (the word ‘Solstitium’ means ‘Sun standing still’). So, at this time of the year the Northern Hemisphere has summer with longer days and shorter nights, while the Southern Hemisphere, receiving slanting rays, has winter with shorter days and longer nights.
The word ‘Tropic’ stand for ‘turning point’. The Solstices are the turning points in the apparent movement of the Sun. While the Earth revolves round the Sun in the course of a year, the Sun seems to move between 231/2 ºN and 231/2 ºS latitudes. As the Sun actually does not move, this migration of the Sun is known as its ‘Apparent Annual Movement’. During this movement, the Sun appears to move up to the tropics and then turn back.
Hence it is named the tropic, the turning point. Once in a year the Sun shines directly over each tropic and twice a year over all the places lying between the tropics.
On December 22 the Southern Hemisphere leans most towards the Sun and Sun’s ray fall vertically on the Tropic of Capricorn (231/2ºS). Hence, at time of the year the Southern Hemisphere has summer with longer days and shorter days, while the Northern Hemisphere has winter with shorter days and longer days. This position is called the ‘Winter Solstice’.
On September 23, the Sun’s rays fall vertically on the Equator. As a result, on this day the Sun illuminates the Northern and Southern Hemispheres equally, making the days and nights of approximately equal length all over the Earth. Since the days and the nights are of equal length, the season is neither too hot nor too cold. After this date the position of the Earth, revolving round the Sun, is such that the Northern Hemisphere starts tilting away from the Sun and the Southern Hemisphere tilts gradually towards the Sun. So in the Northern Hemisphere the days start getting shorter and the nights longer. This position is called the ‘Autumnal Equinox’. Conversely, it is spring in the Southern Hemisphere.
Similarly, on March 21, the Sun’s rays again fall vertically on the Equator, making the days and nights of approximately equal length all over the Earth again, like the 23rd of September. This time, it is autumn in the south and spring in the north. This position is called the ‘Spring Equinox’. After this date the Northern Hemisphere starts leaning towards the Sun, so the days will start getting longer and the nights shorter.