The Longest Night of the Year

For those that don't know, today is the winter solstice, the longest night (and shortest day, obviously) of the year. For those in the Southern Hemisphere, of course, it's the summer solstice -- the official beginning of summer. What many people don't know is why we have a solstice at all. Earth's axis of rotation is tilted by 23.5 degrees relative to the plane of its orbit (called the ecliptic). Yeah, okay, most of us did know that. But contrary to what a lot of people believe, the Earth's axis does not wobble back and forth over the course of a year.* Relative to the stars, the axis always points in the same direction -- coincidentally, very near the star called Polaris, or the "North Star." As we orbit around the Sun, however, the effect of this is that when we are on the "Polaris side" of the Sun, our axis is pointing away from the Sun, while when we are on the side of the Sun opposite form Polaris, our axis is pointing towards the Sun. A simple diagram makes this clear:

The labels are for the Northern Hemisphere, and this is not even close to drawn to scale, of course. So, while it's true that the Northern Hemisphere is pointed towards the Sun in northern summer and away from it in northern winter, the axis itself hasn't (really) moved. Solstices are a result of our position in our orbit, not because of any wobble of the Earth's axis. we will be in the exact position of the winter solstice at precisely 11:35 A.M. MST (18:35 UT).

The seasons result from the fact that during winter the rays of energy from the Sun strike the surface of the Earth at a steep angle. Because of this, the same amount of energy is spread over a much larger area, so the energy per square foot shining on the Earth is much less. Seasons have nothing (much) to do with the length of the day. After all, at the poles, we have six months of continuous daylight, yet they are still the coldest places on Earth! Here's a diagram which shows what's happening:

The area on the ground at B is much larger than the area on the ground at A, but each area is receiving the same amount of sunlight. As a result, it is colder at B than at A. This is also why it's colder in Boston than in Florida!

Now don't you feel smart?

*It does wobble, but very, very slowly. This is called precession, and is the same thing that makes a top wobble as it spins. Precession will make our axis point towards Vega instead of Polaris when I'm a great grandfather -- in about 23,000 years or so. The effect over the course of a year is too small to be noticed. The great pyramids are aligned to the North Star, but it was not Polaris.