Yesterday, December 21, was Winter Solstice, the shortest day of the year in the Northern Hemisphere. In the Southern Hemisphere, conversely, it was Summer Solstice, the year’s longest day.

Try this MCQ:

Q.On 21st June, the Sun

(a) Does not set below the horizon at the Arctic Circle

(b) Does not set below the horizon at Antarctic Circle

(c) Shines vertically overhead at noon on the Equator

(d) Shines vertically overhead at the Tropic of Capricorn

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 than any location on the planet receives on different days of the year.

Impact of the tilted axis

• 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.

Un-impacted regions

• 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.

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 Persian region, it 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.
• 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 Vedic tradition

• In Vedic tradition, the northern movement of the Earth on the celestial sphere is implicitly acknowledged in the Surya Siddhanta.
• It outlines the Uttarayana (the period between Makar Sankranti and Karka Sankranti). Hence, Winter Solstice is the first day of Uttarayana.

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