Today, a total solar eclipse will pass over the United States, from Oregon to Georgia, cutting a path that will dim the Sun an appreciable amount for everyone in the Continental United States. It’s something that hasn’t happened in 38 years, since an eclipse in 1979. It’s going to be an event shared by an entire generation of Americans, thanks to media coverage (and notwithstanding the concerns about which glasses are safe).
The interesting thing is that it hasn’t been 38 years since the last eclipse… just 38 years since the last eclipse to pass over the continental US. Eclipses actually happen every 5 or 6 months. The last solar eclipse actually happened in February of this year… but it was an annular eclipse, not a total eclipse, and it went over far-southern Chile and Argentina, the South Atlantic, and ended shortly after reaching East Africa. The last TOTAL eclipse was in March 2016, and probably hit some small islands in Indonesia before spending most of its time in the North Pacific. The next total eclipse will be in July 2019 and will entirely be in the South Pacific, never hitting land at all (Seriously, here’s a full list from Fred “Mr. Eclipse” Espenak). We know this math pretty exactly because this sort of thing has been studied for millennia*, and we know the orbits of the Earth and the Moon very precisely.
So what makes an eclipse an eclipse? A lunar eclipse happens when Moon passes into the Earth’s shadow.
This doesn’t happen EVERY time the Moon goes behind the Earth (a full moon), because the orbit of the moon is tilted by 5 degrees. Try this: get some kind of ring: a hair tie, a hula hoop, or a bike tire, and say that’s the orbit of the Earth. Then get another one and tilt it slightly. The two only really intersect at two points now that they’re tilted.
5 degrees isn’t much, but it’s enough to keep the Moon from ending up directly behind the Earth during most full moons (or directly in front during most new moons).
(You can now think of your double ring as moving – but NOT spinning – around the Sun. The two points of intersection will only point at the Sun twice a year)
What this means is that we get lunar eclipses every 5 or 6 months (or two near-miss partial eclipses in back-to-back months). Because the Earth is bigger than the Moon, the Moon can fit entirely within the Earth’s shadow, resulting in a total lunar eclipse. Because the entirety of the Moon is in the Earth’s shadow, everyone on the planet can see it. Lunar eclipses only happen during what would otherwise be a full moon, when the moon is ‘behind’ the Earth and we would usually see only the fully lit part.
Solar eclipses get more interesting: For a solar eclipse, the Moon has to be between the Earth and the Sun. That means it happens during a new moon, because we’d usually only be seeing the back (unlit) side of the moon. However, because the Moon is smaller than the Earth, the Earth can’t fit entirely within the Moon’s shadow… so only a small part will actually see totality. That will ALSO happen every 5 or 6 months. Given that it’s still the same points on our hoop/ring/tire model that have to be lined up, solar eclipses will happen around the same time as lunar eclipses – either 2 weeks earlier or 2 weeks later (the time between a full moon and a new moon). Sure enough, there was a partial lunar eclipse two weeks before August 21, on August 7.
The other interesting thing going on is that the Moon is slowly receding from the Earth; it’s spiralling outwards. Millions of years ago, the Moon was larger in the sky, eclipses covered more of the Earth’s surface when they happened, they lasted somewhat longer (today, the maximum will be about 2 minutes, 30 seconds), and the eclipses would be more a little more frequent because the angles involved wouldn’t have to be quite as exact.
Conversely, millions of years from NOW, the Moon will have receded so far from the Earth that it can never completely cover the Sun as seen by any point on the Earth’s surface. All we’ll see will be annular eclipses, where a bright ring (annulus) of sunlight still remains encircling the moon.
We happen to live in a brief lucky time where we can see BOTH kinds. The Earth’s orbit around the Sun and the Moon’s orbit around the Earth are both elliptical, which in a practical sense means sometimes the Earth is farther from the Sun (such that the Sun is smaller in the sky) and sometimes it’s bigger. The same goes for the Moon. That means, some times (like tomorrow), the Moon is large enough to completely cover the Sun. Other times, it’s not quite big enough and we get an annular eclipse.
But some day, there will be no more total eclipses. Perhaps by then we’ll be watching them from the Moon, where we’ll still get to see the Sun blocked out by the Earth every six months or so during what we currently see as lunar eclipses.
* One of the more ridiculous parts of the Tintin story “Prisoners of the Sun” was the idea that Tintin knew about an impending solar eclipse and used it to awe the terrified Maya-like natives out of executing him. In reality, the ancient Mayans (and many other civilizations) had eclipses all figured out (at least mathematically) millennia ago, long before western civilization. They would have been MORE likely than Tintin to have known it was coming.