December 21, is Winter Solstice, the shortest day of the year in the Northern Hemisphere. In Delhi, the Sun rose at 7.10 am, and set at 5.29 pm, making the day 10 hours, 19 minutes, and 3 seconds long.

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.

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 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 Indian Culture

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 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).

Source:IE