There is no dark side of the moon, really. Matter of fact, it’s all dark. The only thing that makes it look light is the sun.
–Gerry O’Driscoll, doorman at Abbey Road Studios
The Solar System moon we know best is of course our own Moon, and its name in English gets passed on to other satellites. Sometimes people will name it in its Latin form (Luna, from which we obtain “lunar”), but I’ll stick to “Moon” for simplicity: the capital “M” will refer only to our satellite, while the small “m” is the general word. There are many moons, but only one Moon. We know the Moon well enough that I won’t try to summarize all we know about it in one post; expect several revisits on coming Moondays. For now, let’s pass over all the fun stuff we’ve discovered this century and just think about the Moon as it appears in the sky: its phases and its eclipses, in honor of Friday night’s total lunar eclipse.
Just like the previous three moons I’ve written about (Io, Europa, and Enceladus), our Moon is tidally-locked to Earth: it presents the same face to us all the time. Many people say “the dark side of the Moon” when they really mean “the far side”. There is no “dark side of the Moon”, because as the Moon orbits Earth, the Sun rises and sets, so it does have days and nights – though Earth will always appear in the same place in its sky. (On the far side, of course, the Earth will never appear at all, since that side never turns to face us.)
Earth orbits the Sun in a plane called the ecliptic, which as you can guess derives from “eclipse”. (Earth’s equator is tilted 23.5° from the ecliptic, which is why we experience seasons.) The Moon orbits Earth, but its plane of orbit is about 5.2° degrees from the ecliptic, so it is slightly inclined compared to our path around the Sun. That may not sound like much, but it’s enough: it means the Moon doesn’t pass directly between us and the Sun on every orbit, and it doesn’t pass directly behind Earth every time either. (Quick and fun vocabulary lesson: when Earth, Moon, and Sun lie in a straight line, that configuration is known as a syzygy. That word is not possible to play in Scrabble without a blank tile.)
Even when the Moon is between Earth and the Sun, it’s far enough away that its shadow misses us most of the time: that’s what we call a “new Moon” (or more technically the dark of the Moon). When Earth is between the Moon and the Sun, but not directly between, the Sun illuminates the entire Moon, and it appears full to us. The other phases – quarter Moon, crescent, and gibbous – account for the other relative positions of Earth and Moon in their mutual dance around the Sun.
The length of time for the Moon to go through all its phases is one lunation: that’s about 29.5 days, which is the basic length of one month in most calendar systems. Since Earth is boogying around the Sun while the Moon orbits, a lunation isn’t the same length of time as the length of orbit (called the sidereal period), which is 27.3 days. This discrepancy combined with the inclination of the Moon’s orbit means that there won’t be a perfect alignment between Earth and Moon except during special times, about twice per year.
When the Moon passes directly between the Earth and Sun, the shadow of the Moon falls onto Earth and we experience a solar eclipse. In one of those odd coincidences, the Moon and Sun appear to be nearly the same size in the sky, even though the Sun is obviously a lot bigger. (Lest you think some kind of cosmic perfection is present, recall that Earth’s orbit is elliptical, so the Sun appears to change size slightly over a year. Similarly, the Moon’s orbit is elliptical too, so it also changes size in the sky; that means Sun and Moon only rarely appear precisely the same size.) During a solar eclipse, part of the Sun may be blocked (a partial eclipse), the whole thing may be blocked (a total eclipse), or all but a thin ring of the Sun may be blocked (an annular eclipse). Since the Moon has to be on the same side of Earth as the Sun, solar eclipses can only occur during the new Moon phase.
In the same way, lunar eclipses (like last weekend’s) only happen during certain full Moons, when the Earth is between the Moon and the Sun. In this case, the Earth’s shadow falls on the Moon, but Earth is a lot bigger, relatively speaking, so its shadow also is larger. Earth also has an atmosphere: as light passes through the air around the disc of Earth, it’s like a big sunset, with attendant red light. Thus, a total lunar eclipse isn’t completely dark: the Moon turns a lovely red-orange shade at totality. (Well, I find it lovely; obviously many others don’t, including the prophet Joel. He described doom using eclipses as his metaphor: “the Sun shall be turned into darkness and the Moon into blood”.)
When you put all this together, you can begin to see why eclipses aren’t every month and why not every place on Earth is able to see them. Last weekend’s lunar eclipse was happening just after sunrise for me, so even though I was technically awake, there was nothing for me to see, since the Moon had already set. Similarly, there was a solar eclipse visible primarily in Antarctica about two weeks prior, but most of us, not being penguins, didn’t see that one.
(The words that begin this post are spoken at the end of the song “Eclipse”, the final cut on the classic Pink Floyd album Dark Side of the Moon. That album is my favorite music to play while grading exams.)