Today, December 21st at 5:03 PM CST marks the arrival of the Winter Solstice, most commonly referred to as the shortest day of the year for us living on the Northern Hemisphere. We’ve been losing daylight since June 21st. You remember that day, right? We called it the Summer Solstice, or longest day of the year. There’s that word again...Solstice. So what is a Solstice anyway?
Literally Solstice comes to us from Latin where Sol is the sun and Sistere means to stand still. What it refers to is the oscillation that the sun makes through the year between its northern-most point to its southern-most point in the sky and back again. If you observe either the sunrise or sunset over a year’s time, you will find that the sun will rise or set at progressively different points on the horizon. At one point, it will stop its progression to the north or south and start a progression in the opposite direction. The point at which the sun stops, or stands still, only to reverse its course is called the Solstice. Its farthest point north is the Summer Solstice where the sun is high in the sky and the days are long and warm. The Winter Solstice would then be the sun’s farthest point south, characterized by its low path across the sky to the south. This low journey above the horizon is also a short trip, accounting for short days, long nights, and progressively colder average temperatures.
There are a few celestial motions involved that explain these apparent solar wanderings. These motions are rather simple once you construct the picture. Refer to the image below. The consequences of these motions account for the change in seasons and the varying types of weather we incur through the year.
Earth spins like a top, alternating between day and night, and making a complete spin in 24 hours. During the day, land masses will readily absorb heat from the sun and radiate much of that heat back into space at night. If days are longer than night, we build up a resource of heat, gaining more in the day than losing during the night. As days lose time to night, approaching the Winter Solstice, land masses radiate more heat than they gain.
Now this top that we live on leans in one direction while it spins. It leans at a 23.5 degree angle from the vertical. All the while that it’s spinning daily at this angle, the earth also makes a near circular trip around the sun, completing this trip in 365 days. At one point in this circle, the Northern Hemisphere is leaning toward the sun at 23.5 degrees. Halfway around the circle from that point, it will be leaning away from the sun at that same 23.5 degrees. This latter position is the Winter Solstice. Leaning away from the sun exposes us to low incidences of sunshine within a short timespan and a longer nighttime period of losing what little heat we have gained.
On the other hand, the Winter Solstice provides a point for optimism. After the 21st of this month, the daytime starts to increase in length and the night becomes shorter.