It's late November, and you find yourself driving (or walking, bicycling, subway-ing, Segway-ing
...) home from work immersed in twilight. By the time you get home to see your loved ones, have dinner or do anything else, night has already fallen. We, in the northern hemisphere at least, are nearing the darkest time of the year. This is a bummer for many, and debilitating
Why do the seasons change this way? It seems like a simple question, but do you really know for sure? To answer, we have to know a bit about how the earth's orbit works. You probably know that the Earth is tilted in regards to the sun. How does this produce seasons though?
First off, the Earth makes a complete circle around the Sun every 365.246 days
. The path itself is elliptical, but it does repeat with high precision, despite gradually slowing down. The earth, on this path however, does not sit rotating exactly vertically relative to the sun. Instead, it is tilted by about 23.4 degrees from vertical, rotating like a top that is not standing up straight. The tilt of the Earth causes the sun to shine more directly on whichever part is angled towards the sun and to shine only weakly on the part tilted away:
If the Earth were rotating vertically, we would have no seasons! The average day would be the same over the entire Earth at all times of the year, every year. It would be as though every day were late September or late March. This might not seem so dramatic, but consider that most of the plants on Earth are specifically adapted to have life cycles that vary with the time of year. Many animals, too, are adapted this way. Mammals hybernate and mate by the seasons; humans plant and grow and harvest by them.
Some planets are actually tilted at much stranger angles to the sun. Uranus rotates north to south, instead of east to west, and the planet has incredibly harsh 20 year long winters and summers!
Venus rotates in the opposite direction to Earth, but almost vertically. Seasonal variation is nearly non-existent.
A few other facts about the Earth's orbit complicate slightly our seasons. First, they are shifted slightly
by the elliptical nature of the earth's orbit. So close, though, is our orbit to a circle, that this only changes the amount of sunlight by about 7%
, making northern seasons slightly milder than southern seasons.
Over the course of about 41,000 years, the tilt angle of the Earth goes through a cycle of changing by a few degrees
. (The full range is roughly 21 to 25 degrees). Think of the top again; this is like the top wobbling. Warmer summers of longer days and colder winters of shorter days occur at one extreme and milder seasons with less variation in daylight hours at the other. This cycle is so gradual and small that it is almost undetectable.
The orbit of the earth is a very complicated thing. So is human brain chemistry. But, if you are feeling a bit cramped for time and down, the simple answer might be the former influencing the latter. Fortunately, in three weeks or so, the days lengthen once more!