Why is December 21 important to science?

December 21 is the winter solstice. This is the first day of winter in the Northern Hemisphere, and the shortest day of the year. The earth is tilted approximately 23.5 degrees on its axis. On two days of the year, the equator of the earth is closer to the sun due to this tilt. These days are June 21 and December 21. On December 21, the sun appears to be close to the horizon, resulting in the shortest day of the year. From December 21 onward, days get longer and longer.

In the Southern Hemisphere it is the day with most sunlight.