December Solstice 2021: Why Dec 21 Is Shortest Day Of The Year In Northern Hemisphere & Longest In Southern

The December solstice marks the shortest day of the year in the Northern Hemisphere and the longest day of the year in the Southern Hemisphere, and brings the astronomical winter and summer seasons.

December Solstice is the shortest day of the year in the Northern Hemisphere (Photo: Twitter/@NASA)

New Delhi: The December solstice marks the shortest day of the year in the Northern Hemisphere, and the longest day of the year in the Southern Hemisphere. Therefore, all regions in the Northern Hemisphere see daylength shorter than 12 hours, and all regions in the Southern Hemisphere see daylengths longer than 12 hours.

This year, the December Solstice, also known as the winter solstice, falls on Tuesday, December 21. The winter solstice can occur on December 20, 21, 22, or 23. 

On the day of the December solstice, Northern Hemisphere receives the most indirect sunlight, due to Earth's tilt on its axis, which is around 23.5 degree. For the same reason, the Southern Hemisphere receives the most direct sunlight. The December solstice is the shortest day of the year in the Northern Hemisphere because the Sun shines directly on the northern half of the Earth, and is the longest day in the Southern Hemisphere because the Sun shines indirectly on the southern half of our planet.

Since regions located north of the Equator receive indirect sunlight, the temperatures are cooler, while temperatures in regions south of the Equator are warmer. 

December Solstice Brings Astronomical Winter & Summer Seasons

Seasons change on Earth due to the planet's tilt, and its revolution around the Sun. Also, the December solstice brings the astronomical winter and summer seasons. This year, Earth's winter season for the Northern Hemisphere began on December 1, in meteorology.

However, Earth's astronomical winter and summer seasons commence later. The December solstice is said to bring the astronomical winter in the Northern Hemisphere, and the astronomical summer in the Southern Hemisphere. This year, the winter solstice will happen at 15:29 UTC (9:29 pm IST) on Tuesday, December 21, NASA said on its website.

The word Solstice is derived from the Latin words sol and sistere, which mean "Sun" and "to stand still", respectively. The subsolar point, which is the point on the Earth's surface directly beneath the Sun, slowly moves along a north-south axis over the course of a year. The subsolar point starts moving southward after reaching its northernmost point at the June solstice. At the December solstice, the subsolar point reaches the southernmost point in its journey around the Sun. After this, it starts moving northward again.

The earliest sunset in the Northern Hemisphere, and the earliest sunrise in the Southern Hemisphere, happen before the December solstice. This is because the true solar noon, which is the time of the day at which the Sun reaches its highest point in the sky, comes nearly 10 minutes earlier by the clock than it does at the December solstice, according to earthsky.org.

After the winter solstice in the Northern Hemisphere, the days will get longer and the nights will get shorter in this half of the Earth until the summer solstice on June 21, 2022. The summer solstice will mark the beginning of the astronomical summer and winter seasons in the Northern and Southern Hemispheres, respectively.