December 21, is Winter Solstice, the shortest day of the year in the Northern Hemisphere. In Delhi, making the day 10 hours, 19 minutes, and 3 seconds long.
December 22, will be one second longer, at 10:19:04, in Delhi.
In the Southern Hemisphere, conversely, December 21 is Summer Solstice in places like Australia, New Zealand and South Africa, therefore, December 21 is the year’s longest day. So, in Melbourne, marking a day that is 14:47:19 long.
This situation will be reversed six months from now — on June 21, 2021, the Northern Hemisphere will see the Summer Solstice when the day will be the year’s longest. And the Southern Hemisphere will see the year’s shortest day — or longest night.
What does ‘solstice’ mean?
The term ‘solstice’ derives from the Latin word ‘solstitium’, meaning ‘Sun standing still’.
On this day the Sun seems to stand still at the Tropic of Capricorn and then reverses its direction as it reaches its southernmost position as seen from the Earth. Some prefer the more teutonic term ‘sunturn’ to describe the event.
The winter solstice happens every year when the Sun reaches its most southerly declination of -23.5 degrees. In other words, it is when the North Pole is tilted farthest away from the Sun, delivering the fewest hours of sunlight of the year.
The Sun is directly overhead of the Tropic of Capricorn in the Southern Hemisphere during the December solstice and is closer to the horizon than at any other time in the year.
The day after the winter solstice marks the beginning of lengthening days, leading up to the summer solstice in June.
Why are the hours of daylight not the same every day?
- The explanation lies in Earth’s tilt. And it’s not just the Earth — 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 combined with factors such as Earth’s spin and orbit leads 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, 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 Earth’s tilt helps define some familiar imaginary lines, which are also key to determining when a Solstice occurs. These are latitudes, which are a measure of a location’s distance from the Equator.
- At latitudes of 23.5° (matching the tilt) are the Tropics of Cancer and Capricorn, north and south of the Equator. At 66.5° (or 90° minus 23.5°) are the Arctic and Antarctic Circles, to the north and south.
- It is at latitudes higher than 66.5° (in either direction) that days of constant darkness or light occur.
The Phenomenon of Seasons:
The phenomenon or change of seasons is caused chiefly by the revolution of the earth round the sun and the inclination of the earth’s axis at an angle of 66 1/2° to the plane of its orbit which constantly points to the same direction.
It can be understood from the diagram on the article which shows four positions of the earth during its revolution round the sun.
Celebrations associated with the Winter Solstice:
For centuries, this day has had a special place in several communities due to its astronomical significance, and is celebrated in many ways across the world.
- 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 (divine mother goddess) 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.
- The festival marks the last day of the Persian month of Azar, and is seen as the victory of light over darkness. It is also the birthday of the sun god Mithra, a pre-Islamic deity.
- Families celebrate Yalda late into the night with special foods such as ajeel nuts, pomegranates and watermelon, and recite works of the 14th century Sufi poet Hafiz Shirazi.
- In the Southern Hemisphere, where the Winter Solstice in June, Peru celebrates the day with a festival called Inti Raymi, meaning “sun festival” in the Quechua language.
- Before Peru’s colonisation by Spain, the Inca civilisation honoured the sun god Inti by fasting for three days, and celebrated on the fourth day with feasts and sacrifices. The festival was banned under Spanish rule, but was later revived in the 20th century and continues today, with mock sacrifices.
- In pre-Christian Europe, solstice was celebrated as the start of winter. People slaughtered their farm animals so they would not have to feed them. Wine created during the summer months was also ready for consumption.
- Hence, the solstice turned into an occasion for a feast, often a community one, before snow covered most of the land and people were forced to spend their time indoors.
- In Vedic tradition, the northern movement of the Earth on the celestial sphere is implicitly acknowledged in the Surya Siddhanta, which outlines the Uttarayana (the period between Makar Sankranti and Karka Sankranti). Hence, Winter Solstice is the first day of Uttarayana.
- The Yule festival, which used to be celebrated in pre-Christian Scandinavian lands for 12 days, later became associated with Christmas as Yule-tide.
- 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).
For many cultures around the world, the winter solstice (which falls on Dec. 21 this year) marks an important milestone.
It’s the shortest day of the year and the longest night of the year, and signals a powerful transition point between seasons that is impossible to ignore.
Because of this, it has been celebrated and revered in ancient civilizations, indigenous cultures, and various religions, all of which have their own rituals for taking advantage of the unique energy.
Earth’s axial tilt plays a much bigger role than its near-circular orbit in governing annual seasons.
Earth makes its closest annual approach of the sun about two weeks after the December solstice, during the Northern Hemisphere’s winter.
Earth is farthest from the sun about two weeks after the June solstice, during the Northern Hemisphere’s summer.