What is Winter Solstice, which made December 21st the shortest day of the year?

In News:

  • December 21st is the day of Winter Solstice in the northern hemisphere, and it is the shortest day (day with the shortest day-time) of the year.
  • This year, the day-time of December 21st in Delhi was approximately 10 hours 19 minutes long.

About: Winter and Summer Solstices

  • The winter solstice, hiemal solstice or hibernal solstice occurs when one of the Earth’s poles has its maximum tilt away from the Sun.
  • Winter Solstice coincides with the shortest day and longest night of the year.
  • The summer Solstice coincides, on the other hand, with the longest day and the shortest night of the year.
  • The situation in the two hemispheres is always reversed.
  • In the Northern Hemisphere, the Winter Solstice occurs on December 21st. The same day in the Southern Hemisphere is known as the summer solstice.
  • Likewise, six months later, June 21st will be known as the Summer Solstice in the Northern Hemisphere. The same day in the Southern Hemisphere will be known as the Winter Solstice.
  • The Solstices along with their counterparts, the Equinoxes (Equal day and equal night), are used to define ‘seasons’ on a planet.

Reasons for its occurrence:

  • The duration of sunlight received varies on different days throughout the year as well as location on Earth’s surface because of various factors like:
    • Earth’s axis of rotation (tilted at an angle of 23.5° to its orbital plane)
    • Earth’s spin
    • Earth’s elliptical or slightly oval shaped orbit around the Sun

  • This tilt in the axis of rotation drives our planet’s seasons as Northern and Southern Hemisphere get unequal amounts of Sunlight over the course of the year.
  • From March to September, the Northern Hemisphere is tilted more towards the Sun, driving its spring and summer.
  • From September, the Northern Hemisphere is tilted away, so this results in the autumn and winter.
  • The situation is likewise reversed in the Southern Hemisphere.
  • On two days-which is known as the Solstices- Earth’s axis is tilted most closely towards the Sun.
  • The hemisphere tilted most towards the Sun sees the Summer Solstice and experiences the longest day.
  • The hemisphere tilted away sees the Winter Solstice and experiences the longest night.

Cultural Significance of Winter Solstice:

  • In Vedic tradition, the northern movement of Earth on the celestial sphere is implicitly acknowledged in the Surya Siddhanta, which outlines the Uttarayana.
  • Uttarayana is the period between Makar Sankranti and Karka Sankranti and Winter Solstice is the first day of Uttarayana.
  • Many cultures around the world celebrate the Winter Solstice in their own ways.
  • Many monuments have been built globally to align with the phenomena, like the Stonehenge of England.

About: Equinoxes

  • Equinoxes are the two days in a year when the Sun is exactly above the Equator.
  • The days and nights in this case are equal.
  • The vernal equinox or the spring equinox on March 21 marks the beginning of the spring in Northern hemisphere and the Autumnal equinox falls at September 23, marking the arrival of the autumn season.

The reverse is true for the Southern hemisphere. Science & Tech