In this composite image, the sun has reached its northernmost point in Earth's sky, marking a season change and the first solstice of the year 2004.
It's the solstice again, which is an astronomer's favorite time of year. That's because it's one of the few occasions where we have anything semi-practical to say to anyone.
"Hey, Adam, you're an astronomer. What's this whole solstice thing about?"
Well, I'm glad you asked.
Let's start with why we have a solstice at all. The cool thing about this question is that it's really asking why we have seasons at all. There's a great documentary made years back where new graduates from Harvard are asked to explain the cause of Earth's seasons. Lots of them get the answer dead wrong. So if you don't know, you're in good company. Remarkably, Earth has seasons for the same reason that a good quarterback can throw a tight spiral. It's all about the stability of spinning things.
Earth is a big rock spinning in space, and the axis of Earth's spin is tilted compared to its orbit. If you imagine Earth's orbit drawn on a giant sheet of paper, then its spin axis would be tilted about 23 degrees to the paper.
The important point here is that spinning things don't like to change their orientation in space (meaning the orientation of their spin axis). It's a law of nature — and of a good pass in football.
As Earth swings around in its orbit, the basic orientation of its spin — meaning its tilt — doesn't change. That means the North Pole is always pointed at the same point on the sky (toward the North Star). In December, Earth's tilt angles the day side of the Northern Hemisphere away from the sun. Six months later, in June, when Earth has swung through half its orbit, the spin axis still points toward the North Star. That means the day side of the Northern Hemisphere in June must be tilted toward the sun (the opposite of December).
So in December, the orientation of the spin axis and the sun means the sun at noon will be relatively lower in the sky. This means that, for those in the Northern Hemisphere, the sunlight hits the ground at a shallow angle. (This is also why the days are shorter, since the sun spends less time in the sky.) In June, the effect is reversed. The sun is closer to overhead at noon, hits us more directly and also spends more time above the horizon (i.e., a longer day).
So the solstice and seasons have nothing to do with how close Earth is to the sun. It's all about the stable tilt of its spin.
There's a helpful image, courtesy of NASA, here.
People often ask why the solstice is not the time when the sun sets earliest. The exact answer to that question can be a bit complicated. A simple way to look at it, though, is to understand that our clocks are built to keep track of days that are fixed at exactly 24 hours long. But "sun time," meaning time measured by the sun, has variations in it across the year. In particular, the solstice/sunset problem comes because sun's daily motion in the sky has two parts that get added together: The first comes from the spin of the planet, and the other comes from Earth's orbit around the sun. It's the orbit part that really matters for the question of sunsets and the solstice.
Earth's orbit is not quite a perfect circle. It moves around the sun following an ellipse, which is like a squashed circle (it's only squashed by a tiny bit — a fraction of a percent — but that is enough to throw things off).
The important point about the elliptical orbit is that Earth moves faster when it's closer to the sun (which happens in early January). Six months later, when the planet reaches its farthest point from the sun, it's traveling slowest in its orbit. That variation in orbital speed is why the earliest sunsets occur before the shortest day.
Sometimes people will also ask if we are the only planet with seasons and solstices.
The answer, again, is it all depends on the tilt between a planet's spin and its orbit. Mars has pretty strong seasons because its tilt is a lot like ours.
There is, however, something new to this rather ancient story of planets and seasons. Over the past 20 years, we've been discovering "exoplanets" orbiting stars other than our sun. And among the things we've found are planets that probably don't even have days.
How is that possible?
If a planet orbits really close to its star, then gravity can slow the planet's spin until its year and its day get synchronized. The process is called tidal locking. On this kind of planet, the year and the day have the same length. That means one half of the planet is always in sunlight, and the other half is always in night. (The moon is tidally locked to Earth. That's why we only get to see one side of it.)
So, you know how people say, "It's always happy hour somewhere." Well, on these planets, it's always happy hour in the same place — forever. Viewed from the day side of these planets, the sun is locked into its position. It never moves.
As crazy as it seems, some of these planets may even have life on them. But it would be kind of sad for astronomers on those worlds. Without seasons and solstices, why would anyone ask them to explain anything?
Adam Frank is a co-founder of the 13.7 blog, an astrophysics professor at the University of Rochester, a book author and a self-described "evangelist of science." You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4