When the stars align

by Julian Phillips, Centerfold Editor
photo contributed by Julian Phillips

If you were up before 6:30 a.m. on June 10, you might have noticed that something was a little off, besides the fact that you were up before 6:30 a.m. That’s right, you witnessed a partial solar eclipse! If you didn’t see it, first read The Roar and then cry; the next eclipse to cover more than half the sun will come on April 8, 2024, anyway. But what causes solar eclipses? The science is more complicated than you might think.

We all know that the moon orbits around the Earth, which orbits around the sun, and most of us know that the moon takes about a month to orbit the Earth. Theoretically, then, some place on Earth should have an eclipse every month or so, when the three bodies align and the moon covers the sun.

However, the moon’s orbit has a 5 degree tilt compared to the Earth’s orbit, so every time there is an alignment, there’s only a slight chance that the moon will be in the right place to cast its shadow. It could be too high above the Earth, or too low, but only around twice a year is it in the right place to cause a noticeable shadow.

Speaking of shadows, the moon — just like every celestial body in the solar system — has two main ones: the umbra and the penumbra. This morning, we were in the penumbra of the moon, the lighter and diffuse shadow that results in a partial solar eclipse. As you go farther away from the moon and towards the Earth, the penumbra grows larger.

On May 1, 2079, when Boston has its next total solar eclipse, we’ll be in the umbra, the darker shadow that grows smaller as it heads towards Earth. The umbra is around 267 square kilometers on the surface of the Earth.

There’s one more shadow-eclipse pair, and if you happen to be in Northern Greenland today, you’ll see it. Since the moon’s orbit isn’t perfectly circular, there are eclipses at times when it’s farthest away from Earth — the apogee — or closest — the perigee. The moon is two days past its apogee today, so the umbra will have to be longer in order to reach Earth.

After a certain point of traveling towards Earth, the umbra becomes so small that it inverts like an hourglass and continues on, growing bigger. It’s now the antumbra, and it results in an annular eclipse, one where the moon is too small to fully cover the sun, so there’s a ring of light around it.

One more type of eclipse you might have heard of is a lunar eclipse. This is almost exactly the same as a solar eclipse, except this time the Earth is in front of the moon and casts its shadow, making the moon appear red. These are much more common, as the Earth has a larger shadow.

There’s so much more to say, but I’ll leave you with the most interesting fact of all: you know how the sun and the moon are the same size in the sky, allowing total eclipses to occur? That’s a complete coincidence: the sun is 400 times larger and farther away from the Earth than the moon. So on May 1, 2079, thank probability — or your choice deity — for bringing us this delight.