Today is the Winter Solstice, commonly referred to as the shortest day of the year, although astronomers are more inclined to think of it as the longest night of the year, the time that (in theory) we get to spend the most time enjoying the night sky. In practice, this is rarely the case because of weather. Pretty much by definition, when the day is short and the night long it’s cold outside. And here in the Pacific Northwest there’s a very good chance that it’s also cloudy and/or wet.
But what is the solstice, and why does it happen? Why does it happen at this time, and if it’s defined as “the shortest day of the year”, why do scientists say that it happened at a much more specific time (about 2:20PM today on the west coast)? How and why it happens is pretty easy to understand if you know that Earth’s axis of rotation is tilted (about 23.5 degrees) from perpendicular to the plane of our orbit around the Sun. The above diagram should help with this concept. Because of this tilt, the incident angle of light from the Sun at noon on each day changes throughout the year. In the northern hemisphere we are tilted toward the Sun in the Summer and away from it in the Winter (I’m using the terms for the seasons in their commonly understood meanings, not the literal definitions), which accounts for the changing length of the day, and for the amount of local heat we get from the Sun during each day. In the southern hemisphere it is the opposite.
But we can be more precise about when we switch from shrinking days to growing days. In the above diagram the axis of Earth’s rotation is shown as little lines at the top and bottom of each image of Earth. Another set of lines emanating from the Sun show the axis of Earth’s orbit around the Sun. While these lines are never parallel, if you extend the lines far enough, the rotation axis and orbit axis lines do intersect twice per year – at the Winter and Summer solstices. At the Winter Solstice the line “below” (south of) Earth intersect the axis of orbit, and at the Summer Solstice the northern axis line intersects the orbit axis. These two points are 180 degrees apart. Halfway between these points (on either side) we have the equinoxes, one in Spring (Vernal Equinox) and one in Fall (Autumnal Equinox). On these days the length of night and day are roughly equal. But the precise moment of the solstice intersections is not tied to the schedule of Earth’s rotation, so the precise moment of a solstice or equinox can happen any time. They are not even always on the same calendar date each year.
We can be grateful for this tilt in Earth’s axis of rotation because without it our weather would be very different. At the very least, it would be more boring – each location on Earth would have pretty much the same weather all the time (as is partially true of locations near the equator even with the tilt). But it would probably be worse than boring, with much more extreme temperatures at both ends of the scale – hotter near the equator and colder at the poles.
You might say that Earth HAS to be tilted. That is, if it weren’t tilted we would probably not be here to observe it. But exactly how it became tilted is uncertain. Certainly, in Earth’s early days it was struck repeatedly by other objects, some of which were large enough to radically change everything about the planetary dynamics. In fact, it has been theorized that our Moon is really a piece of Earth that got knocked off in just such a collision.
Most of the planets in our solar system have a significant tilt in their axis of rotation (they all rotate while simultaneously orbiting the Sun). Uranus has the most extreme tilt at over 82 degrees, so it’s axis of rotation is almost perpendicular to its orbital plane, which just seems weird!