December 21 is known as Winter Solstice or the shortest day of the year in northern hemisphere. This is a yearly phenomenon.
In the Southern Hemisphere, December 21 is Summer Solstice. In places like Australia, New Zealand and South Africa, December 21 is the year’s longest day.
This situation will be reversed after six months on June 21, 2021, when the Northern Hemisphere will see the Summer Solstice and the day will be the year’s longest. The Southern Hemisphere will see the year’s shortest day or longest night.
Reasons for variation in day length
The explanation lies in Earth’s tilt. Every planet in the Solar System is tilted relative to their orbits, all at different angles.
The Earth’s axis of rotation is tilted at an angle of 23.5° to its orbital plane. This tilt is combined with factors such as Earth’s spin and orbit, leading to variations in the duration of sunlight that any location on the planet receives on different days of the year.
The Northern Hemisphere spends half the year tilted in the direction of the Sun, getting direct sunlight during long summer days.
During the other half of the year, it tilts away from the Sun, and the days are shorter. Winter Solstice on December 21 is the day when the North Pole is most tilted away from the Sun.
The tilt is also responsible for the different seasons that we see on Earth. The side facing the Sun experiences day, which changes to night as Earth continues to spin on its axis.
On the Equator, day and night are equal. The closer one moves towards the poles, the more extreme the variation.
During summer in either hemisphere, that pole is tilted towards the Sun and the polar region receives 24 hours of daylight for months. Likewise, during winter, the region is in total darkness for months.
The Winter Solstice also influenced culture to the extent that ancient people built several architectural structures aligned to the phenomenon.
Some of these structures include the Stonehenge and Glastonbury (England), Chichen Itza (Mexico), Goseck Circle (Germany), and Temple of Karnak (Egypt).
Jewish people call the Winter Solstice ‘Tekufat Tevet’, which marks the start of winter. Ancient Egyptians celebrated the birth of Horus, the son of Isis for 12 days during mid-winter. In China, the day is celebrated by families coming together for a special meal.
In Iran and neighbouring Afghanistan, Uzbekistan, Tajikistan, Turkmenistan, Azerbaijan, and Armenia, the Winter Solstice is celebrated as Yalda or Shab-e-Yalda.
In the Southern Hemisphere, Peru celebrates the day with a festival called Inti Raymi, meaning “sun festival” in the Quechua language.
The Yule festival, which used to be celebrated in pre-Christian Scandinavian lands for 12 days, later became associated with Christmas as Yule-tide.