Why December 21 is the longest night of the year

The Winter Solstice is a significant astronomical event in many cultures, marking the official middle of winter in many temperate regions, while in other places it's considered the start of winter. It also marks the longest night of the year, from the sunset on Thursday, December 21 to sunrise on Friday, December 22.

The Winter Solstice, technically, is a specific moment on the Earth's orbit around the sun. It is when Earth's tilt on its orbit causes either the Northern or the Southern hemisphere to get the least amount of sunlight in a given year. This year, the Northern hemisphere reaches this point on Friday, December 22, at 9:27am Bangladesh Standard Time.

The Earth's tilt of 23.5 degrees on its orbit is the cause of all of this. In winter, this results in the planet "leaning away" from the sun, thus the Northern hemisphere gets less sunlight and cooler temperatures. Less sunlight means more dark hours, and that's how the night of the Winter Solstice becomes the longest night of the year.

The longest night this year starts with sunset on December 21 at 5:16pm in Dhaka, and ends with sunrise on December 22 at 6:37am. The longest night of the year in Dhaka, as a result, lasts for 13 hours and 21 minutes.

Notably, the origins of the word "solstice" explains a lot about how early human cultures perceived this event. It comes from the Latin word "solstitium", meaning "sun stands still". The Winter Solstice is the point when the days stop shortening and start lengthening, beginning the long journey from winter to summer.

In the sky, this is seen as the point when the sun reaches the southernmost point over the course of the year. After this, the sun path starts swinging back northwards until June 21, the Summer Solstice, when it starts moving back south again.