December 21 is winter solstice here in the Northern Hemisphere. Winter solstice is the longest night of the year. But why is this night longer than other nights?

To understand solstice, we need to review how Earth moves through the solar system. First, the Earth rotates, which means it spins and gives us day and night. If we imagined the Earth was a top, the line that goes through the center like the handle of a top is the axis. Now, Earth’s axis does not go straight up and down like a top, but it’s also not sideways like Uranus. It is at a slight angle, about 23 degrees. As the Earth revolves around the sun, the angle means that sometimes the Northern Hemisphere is closer to the sun and sometimes the Southern Hemisphere is closer to the sun. This tilted revolution is what gives us seasons.

When the Northern Hemisphere is closest to the sun, the days are longer and the weather is warmer. We call this summer. But while the Northern Hemisphere is closest, the Southern Hemisphere is tilted away from the sun. The Southern Hemisphere then has shorter days and cooler weather. The point when the Northern Hemisphere is at the very closest is usually around June 20 or 21 and this is the longest day of the year for the Northern Hemisphere, which we call the summer solstice. The Southern Hemisphere is at the farthest on that day and has the shortest day of the year, which is their winter solstice.

Then the Earth continues to revolve, reaching the equinox where both North and South are about equal distance and the equator has exactly equal amounts of day and night. This happens around September 20 and March 20. Then the revolution continues and the Northern Hemisphere continues to move farther away, reaching the farthest point from the sun around December 21, which is the longest night of the year for the Northern Hemisphere. Meanwhile, the Southern Hemisphere has the longest day of their year.

Many civilizations used this time of year to celebrate light and community in the Northern Hemisphere, reminding each other that more light will come and things will begin to grow again.