Solsticizing

Today is the summer solstice (that’s today as I’m writing this, not today as I’m posting it), the first day of summer. All yesterday morning, I listened to the weatherman on our local TV station describe the beginning of summer as, “the moment the Sun passes over the equator.”

Now, I know that meteorologists today have graduate degrees in the subject, and they know all kinds of things that I don’t. I couldn’t begin to predict the weather as well as they do (at least, not without some darts and a dartboard marked “Sunny,” “Cloudy,” and “Rainy”). But I also know that he was wrong about that sun-over-the-equator thing.

Let’s start at the beginning. You all know why the weather is colder in the winter and warmer in the summer, right? I mean, Earth is actually closer to the Sun during winter (in the Northern Hemisphere) than during summer, so that’s not it. This picture shows what it is.

Yes, this an actual photo of Earth taken from space.

It shows how light from the Sun strikes Earth during summer in the Northern Hemisphere. Actually, it shows how radiation from the Sun strikes Earth during summer in the Northern Hemisphere. That radiation includes light, heat, and a bunch of other stuff. Anyway, because the Northern Hemisphere is tilted toward the Sun, the radiation falls on it fairly directly.

For comparison, this next picture shows how radiation from the Sun strikes Earth during winter in the Northern Hemisphere.

OK, I added the arrows. But it's still a photo of Earth.

Because the Northern Hemisphere is tilted away from the Sun, the same amount of radiation is spread out over a larger area. Here’s what I mean:

June (top) and December (bottom)

The hole in that giant, green space wall thing changes position, but it doesn’t change in size. It’s letting the same amount of light/heat/etc. through in both pictures. But that light/heat/etc.—the key factor being heat—is spread over a larger area in the second picture, the one taken in December. The same amount of heat spread out over a larger area means less heat at any given point, which means it’s colder. It’s winter, and it’s colder. Even in the Caribbean, which is, for some reason, where I showed the radiation falling.

At this point, I hear you saying, “Dude, that’s one of the three things I remember from grade-school science classes. I don’t need you to explain that to me.” Maybe not, but you should be congratulated for correctly hyphenating grade-school when you speak.

OK, you knew that already, So think about this. All through the spring—

No, wait. I want to clarify something first. The pictures above make it look as though Earth wobbles back and forth. It doesn’t, really; I just changed points of view from the winter to the summer pictures. A more accurate representation would be this:

This may not be exactly to scale.

See how Earth is always tilted the same way. but sometimes the bottom half is tilted toward the Sun and sometimes the top half is?

So anyway, all through the spring, the North Pole is tilting more and more toward the Sun. At some point it tilts as much as it’s going to, and starts tilting away again, heading back toward winter. That moment, when it has tilted as much as it’s going to and is about to start tilting back, is the Summer Solstice.

When your part of the world is tilted toward the Sun, the Sun appears higher in the sky. You’ve undoubtedly noticed that at noon in June, the Sun is much higher in the sky than it is at noon in December (assuming you live in the Northern Hemisphere and not too close to the equator). The word solstice comes from the idea that the Sun’s path has stopped moving upward, and pauses for a moment before starting its downward movement. Solstice is actually an Old French word, and it comes from the Latin word solstitium, which means “point at which the sun seems to stand still,” and which in turn comes from sol, “sun,” and sistere, “to come to a stop” or “to make stand still.”

Oddly enough, the word assist is also derived from sistere, the idea being that when you assist someone, you stand by ready to help. But I digress.

The summer solstice can fall on June 20, 21, or 22, depending on the year. Basically, this is because our calendar lines up imperfectly with realty. Since 1972, the solstice has been bouncing around between the 20th and the 21st, and it will continue to do so through 2202. June 22, 2203, will be the first June 22 solstice since June 22, 1971.

This year, by which I mean 2010, the summer solstice occurred at 7:28 A.M. EDT on June 21st. At that moment, in whatever part of the world the Sun was highest in the sky (western Europe or just off its coast, I’m guessing), the Sun was directly over the Tropic of Cancer. That’s the definition of the Tropic of Cancer—the latitude at which the Sun’s rays are perpendicular to Earth’s surface at the summer solstice:

The Tropic of Capricorn is the same thing, only for the Winter Solstice, which is just like the Summer Solstice only six months later and upside-down.

So our intrepid weatherman should have been saying, “the moment when the Sun pauses over the Tropic of Cancer.” Better yet, WFSB should hire me to write scripts for them.

By the way, the Summer Solstice corresponds to the first day of summer here in the United States and Canada. But that’s really an arbitrary decision, and doesn’t necessarily hold true in other places. For more on that, check out this article by master explainer Cecil Adams.

Let me leave you with this thought. We all think fondly of the summer months, especially the ones from our childhood, when there was no school, and the days grew longer and longer, and we could play outside until our bedtimes, right? We all know that the days get longer as summer progresses, right? No, that’s wrong. The longest day of the year, the 24-hour period with the most daylight in it, is the summer solstice, which is—as I just said—the first day of summer. By the time school ends (or at least within a week or so of school ending, depending on the school district), the days are already starting to grow shorter.

Didn’t know that, did you?