Solstice and Equinox
The Earth’s polar axis does not coincide with vertical axis. Here vertical axis is assumed to be a line which is perpendicular to the orbital plane and passing through the centre of earth. The polar axis of Earth is tilted by 23.50 from the vertical axis. In other words the polar axis is inclined with 66.50 with the orbital plane. Orbital plane is the plane of orbit around the Sun and it is here assumed to be horizontal and passing through the centre of the Earth. During the revolution around the Sun, Earth attains four critical positions with respect to Sun. These are Solstice and Equinox.
On June 21, the Earth is so located in its orbit that the North Pole leans towards the sun and South Pole is out of view of Sun. On this day Tropic of Cancer in North Hemisphere receives vertical rays of Sun. This position of Earth on 21 June is called Summer Solstice. North hemisphere observes longest day and South Hemisphere has longest night on Summer Solstice. At this time the North Pole experiences continuous day while South Pole observes continuous night. During period of Summer solstice to Autumnal Equinox, the North Hemisphere is inclined towards the Sun and this is the reason why the North Hemisphere observes summer in this period. Since South Hemisphere is inclined away from the sun at this period of time the South Hemisphere observes winter.
On Dec 22, the Earth is in equivalent opposite position of its orbit as compared to June 21. On this day the North Pole is away from view of Sun and South Pole is inclined towards the Sun. At this time the Tropic of Capricorn receives vertical rays from the sun. This position of Earth is called Winter Solstice and it occurs on Dec 22. South Hemisphere has longest day and North Hemisphere has longest night. At this time North Pole observes continuous night and South Pole observes continuous day.
Equinoxes occur midway between the Solstices. During equinoxes Earth’s Polar axis is at 900 with the line drawn from Sun. In other words the equator receives vertical rays from Sun. The days and nights all over the Earth are equal on Equinoxes. Vernal Equinox occurs on March 21.
Autumnal Equinox occurs on Sept 23. Autumnal equinox occurs when the Earth is in equivalent opposite position of its orbit as compared to Vernal Equinox.
Amazing Facts about Solstice and Equinox
- At the time of Summer Solstice the sun is not visible from South Pole and there is continuous day at the North Pole.
- At the time of Winter Solstice the sun is not visible from North Pole and there is continuous day at South Pole.
- Due to the tilting of Earth’s polar axis there is continuous 6 months day and 6 months night at Poles.
- Due to this effect, the seasons in both Hemispheres are reverse to each other. For example when there will be summer in North Hemisphere, South Hemisphere will observe winter.
- Starting from Vernal Equinox up to Summer Solstice, the duration of day increases in North Hemisphere and decreases in South Hemisphere. The duration of day is maximum and length of night is minimum on Summer Solstice (June 21) in North Hemisphere. On the similar logic the duration of day is maximum and night minimum on Winter Solstice (Dec 22) in South Hemisphere
- At the equator the extent of day and night are equal throughout the year, but elsewhere it is possible only on equinoxes. In other words the extent of day and night will be same for 12 hours each only on two days (March 21 and Sept 23) anywhere on Earth.
- Beyond the Arctic and Antarctic circles longer than 24 hours day and night are experienced.
- The Earth’s rotation on its Polar axis is responsible for change of day and night. This statement is not true on Poles. The day and night changes on the poles due to effect of revolution around the sun and not by effect of rotation.
- Difference between the extents of day and night increases as we move towards pole and reduce as we move towards equator. At the equator there is no difference between the extents of day and night.
- Between periods of Vernal Equinox (March 21) to Summer Solstice (June 21), the North Hemisphere has spring and South Hemisphere has autumn season.
- Between Summer Solstice (June 21) to Autumnal Equinox (Sept 23) the North Hemisphere has summer season and South Hemisphere has winter season.
- During Autumnal Equinox to Winter Solstice (Dec 22) the North Hemisphere has autumn season and South Hemisphere has spring season.
- During Winter Solstice to Vernal Equinox, North Hemisphere has winter season and South Hemisphere has summer season.