Plenty of amazing things happen up in the sky—and sometimes, we’re even lucky enough to be able to see them from right here on Earth. Take, for example, the total solar eclipse that occurred in 2017: Somewhat miraculously, it was visible across the entire United States. But the Sun isn’t the only celestial body that can undergo an eclipse; the Moon can, too. And if you’ve ever wondered how solar and lunar eclipses are different, there’s a lot of fascinating stuff to unpack about it all.
The short version is that solar eclipses happen when the Moon passes between the Earth and the Sun, while lunar eclipses occur when the Earth passes between the Moon and the Sun. In each case, the three celestial bodies align with each other in a phenomenon astronomers refer to as syzygy (from the Latin syzygia, meaning “conjunction,” which itself comes from the Ancient Greek σύζυγος, or “yoked together”).
There’s a lot more to it than that, though, of course. (There always is, isn’t there?) Here’s what you need to know about the differences between solar and lunar eclipses.
Fun in the Sun
As you’re probably aware, the Earth orbits the Sun, and the Moon orbits the Earth. When the path of the Moon’s orbit takes it between the Earth and the Sun, it spends a portion of time on its journey blocking the light of the Sun from reaching the Earth, casting its shadow on the Earth as it goes. This phenomenon is what we call a solar eclipse. Solar eclipses can only appear during a new moon (although they don’t appear during every new moon); usually, there are about two of them per year, although there might be more, depending on how things literally align in a specific year.
But there isn’t one kind of solar eclipse; indeed, there are several. The most dramatic is the total eclipse, which occurs when the Moon completely covers the Sun—something which can only happen as a result of what Joe Rao at Space.com calls “a happy accident of nature.” The Sun, you see, is much, much larger than the moon—864,000 miles in diameter, as opposed to the Moon’s 2,160-mile diameter. But because the Moon is so much closer to the Earth than the Sun is, sometimes, a sort of forced perspective trick emerges: The disc of the Moon appears to us from the Earth to be about the same size as the Sun, and it’s perfectly aligned with the Sun and the Earth, thereby allowing it to cover up the Sun completely.
Interestingly, a total solar eclipse will only appear to be total to viewers on Earth if those viewers happen to be standing within what’s called the path of totality. The path of totality is the relatively narrow band of the Earth where the Moon’s umbra—the “dark core” of its shadow, as Time and Date describes it—hits the Earth’s surface. Outside of the path of totality, the eclipse will appear only as a partial eclipse. The eclipse will still be referred to as total, however; the terminology kicks in if a solar eclipse reaches totality anywhere on Earth, even it’s not total everywhere on Earth.
If an eclipse doesn’t ever reach totality from Earth, though, it’s called a partial solar eclipse. Partial solar eclipses when the Sun, Moon, and Earth aren’t perfectly aligned, which, in turn, results in the Moon casting not its umbra, but its penumbra—the lighter, outer part of its shadow—on the Earth. During a partial solar eclipse, instead of appearing to be blocked out by the Moon, the Sun will instead appear to have a slice carved out of it. This “slice” is the portion of the Moon covering up the Sun.
Then there are annular solar eclipses. These eclipses are functionally similar to total eclipses—that is, during them, the Moon passes completely in front of the Sun, rather than only partially—but there’s one big difference: Due to where they’re at in their orbits, the Moon’s disc appears to be much smaller than the Sun’s to us here on Earth. Accordingly, the Moon is unable to completely cover the Sun, leaving a ring of light visible from around the Moon’s edges. This effect is sometimes called the “ring of fire” when viewed from Earth.
By the Light of the Moon
Lunar eclipses are similar to solar eclipses in that they occur when the Earth, Moon, and Sun line up together during their orbits—but they do so in a different way. Instead of the Moon passing between the Earth and the Sun, lunar eclipses see the Earth positioned between the Moon and the Sun. They can only occur during the full moon, but, like solar eclipses, they don’t happen every full moon. There are usually two per year, but each year varies, so there can be more if the conditions are right. For example, in 2020, a whopping four lunar eclipses are on the calendar.
Like solar eclipses, there are a few different kinds of lunar eclipses, two of which even operate according to similar principals as two types of solar eclipses. Total lunar eclipses occur when the Moon, Sun, and Earth are all perfectly aligned, which in turn results in the Earth’s umbra falling on the Moon and completely covering it. Interestingly, though, a Moon in total lunar eclipse won’t appear to disappear; instead, it can look like it’s turned red due to the sunlight filtered through the atmosphere during the event. Meanwhile, partial lunar eclipses occur when only part of the Earth’s umbra falls on the Moon, thus leaving the Moon with a slice seemingly carved out of it when viewed from the Earth.
However, there’s also a third kind of lunar eclipse. Called a penumbral lunar eclipse, this kind of eclipse occurs when only the Earth’s penumbra falls across the Moon. Unlike in the other two types of lunar eclipse, the Earth’s umbra isn’t involved; it doesn’t fall on the Moon even the slightest bit. Penumbral lunar eclipses are also difficult to see, simply because their effects are much subtler than other kinds of eclipses. As Space.com puts it, parts of the Moon can appear “smudged” or “shaded” and with fuzzy edges during a penumbral lunar eclipse, but otherwise, you might not witness much change at all.
How to Safely View Solar and Lunar Eclipses
Besides the actual mechanics of how solar and lunar eclipses work, the other big difference between the two concerns how to view them safely from Earth. Here’s the thing with solar eclipses: You should never, ever look directly at a solar eclipse, unless you have access to eclipse glasses or a hand-held solar viewer equipped with special solar filters. (Regular sunglasses or camera lenses won’t cut it!) If you look at a solar eclipse without the aid of these solar filters, which are regulated according to a worldwide standard, you risk burning your retinas or otherwise damaging your eyes.
If you don’t have eclipse glasses or a solar viewer, though, you can make a pinhole projector with which to view a solar eclipse using just a few simple items you probably already have lying around your home. The simplest version involves just poking a hole in a paper plate, while more elaborate ones consist of cardboard boxes with a few other bells and whistles. These homemade devices allow you to watch a projection of a solar eclipse rather than the eclipse itself—so, even though you won’t be to look directly at the Sun with them, you’ll still be able to see what the Sun is up to with their help.
Lunar eclipses, however, are safe to view all on their own; they won’t damage your eyes the way solar eclipses will. All you need to do is find out when the eclipse will occur and where it will be best visible, then turn your eyes to the sky at the appropriate time and place.
Of course, not all eclipses are visible everywhere on Earth, whether they’re solar or lunar in nature. For example, the penumbral lunar eclipse that occurred in January of 2020 wasn’t visible to most of North America due to its darkest phase occurring when the Moon had dipped below the horizon and out of sight. However, Europe, Africa, and western Asia were able to view it with no trouble at all. But these days, it’s easy to find not only when specific celestial events will occur, but also where you’ll be able to view them from. If you’re either willing to wait until one reaches or willing to travel to get to one, you should eventually be able to view one—from eclipses to meteor showers.