How to watch tonight’s total lunar eclipse

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Astronomy News

It’s a big Universe. Here’s a thing about it.

Good news: There’s a total lunar eclipse tonight!

Bad News: The Earth is more or less turning its back on it.

Worse news: There won’t be another total lunar eclipse until 2025.

But some better news: It’s a good chance for you to see Uranus.

OK, details:

A lunar eclipse is when the Moon passes into the shadow of the Earth. Or, stated in a way that’s maybe easier to visualize, it’s when the Earth is directly between the Sun and the Moon. The Sun gets blocked, so the Moon gets dark.

It takes hours for the Moon to pass through the shadow as it orbits the Earth, so it’s a slow event. Still, you can only see it if you’re on the part of the Earth facing the Moon when it happens.

In this case, it happens when the Moon is over the Pacific Ocean, which sucks for most people (though congrats to Hawaii, Alaska, and extreme northeastern Asia, where you get to see the whole thing). For the US, the farther west you are the better; folks in the Pacific time zone and parts of Mountain Time will also see the whole thing.

Sky and Telescope has a great table breaking down the timing (you can also get some good local info at TimeandDate). For me, in Colorado, the real show doesn’t start until after 2:00 a.m., and totality doesn’t start until after 3:00. Ouch.

To be clear: This happens tonight, Monday night/Tuesday morning. So technically it’s 2:00 a.m. Tuesday morning for me, but for Hawaii, say, it happens after 11:00 p.m. Monday night. Time zones are weird.

You’ll see the Sky and Telescopl table has lots of events listed, like penumbral phase, partial phase, totality, and all that. What does all that mean?

First, watch my Crash Course Astronomy episode on eclipses. It’ll help prep you.

I also have a longish overview of lunar eclipses on the old blog, from 2019.

The best way to think of this is to pretend you’re standing on the Moon. You’ll see the Sun in the sky, and then the Earth will start to move over the Sun, blocking it. At first only a little is blocked, so the illumination lighting up the rocks and craters around you is about what it normally is. But as the Earth blocks more of the Sun you’ll see the light dimming, until the Earth completely blocks the Sun. It will then be dark all around you.

From Earth, we see the Moon dimming slowly as the above events happen. At first it’s not noticeable, but over an hour or so the Moon will start to dim noticeably.

Here’s where things get fun. The whole Moon doesn’t get into Earth’s shadow all at once. I made the diagram above to show this. If you’re on the side of the Moon labeled point A (technically the west side of the Moon, or you can think of it as the leading side, the side facing into its orbital motion), you see the Sun totally blocked — the solid line connecting point A to the Earth and Sun shows the Sun completely behind the Earth. That happens before someone on the other side (point B, or the trailing side) sees the Sun gone — the dashed line shows that some of the Sun or so can still be seen (note, the sizes are not to scale; the diagram is representative, not accurate). So one side of the Moon gets completely dark before the other side does.

As long as the Sun is still shining on a little on part of the Moon it’s still lit there, even if less than usual. We call that the penumbral eclipse — the Moon is getting darker, but not completely dark. Once some part of the Moon is completely dark, like point A, we say the Moon is partially eclipsed. Eventually the Earth blocks the Sun for every part of the Moon we can see, and we say the Moon is totally eclipsed.

A cool part is that, because the Earth is round, it casts a curved shadow onto the Moon’s surface, like someone has taken a bite out of it.

Totality is neat. Imagine again you’re on the Moon. The Earth has completely blocked the Sun! As I said above, it’s dark where you are… except that’s not completely (or totally, haha) true. The Earth has an atmosphere, and the Sun will still be shining through that. The air bends light toward you, so the Earth will look like a dark circle with a bright ring round it. But the ring is red! Bluer light gets absorbed and scattered by the air, letting only the redder light through — this is the same reason the Sun and Moon can look red when they’re on the horizon.

So there’s still light illuminating the Moon during totality, but it’s red light, and from the Earth the Moon itself can look red! The color depends on a lot of things, including air quality and where the Moon is in our sky. I’ve seen it blood red, orange, even brown. But it’s much dimmer than usual and can actually be hard to find in the sky!

Eventually the Moon’s orbital motion starts to move out of the shadow, and the process is reversed. The leading side starts to see the Sun so it gets lit up while the other side is still dark, so that’s a partial eclipse again. Then more and more gets lit until it’s out of the deepest part of the shadow. On the trailing side the Sun is still partially blocked, so that’s a penumbral eclipse again.

The penumbral parts are barely noticeable, but once it gets deep enough into the shadow the Moon gets visibly dimmer. That’s what all those timings in the Sky and Telescope table are telling you.

Lunar eclipses are slow and safe to watch. I usually go outside or just look out a window every 15 minutes or so to see how it’s progressing. Using binoculars is a great way to see it, honestly, because they give a nice overview. A small telescope can be cool, too; I prefer using my small spotting ‘scope to watch.

If you do have binoculars or a telescope, the eclipse is a great chance to see Uranus! The distant planet is just around the limit of naked eye visibility, but it’s pretty easy to spot with a little optical aid. When the eclipse is total Uranus will be about 2° to the northwest of the Moon, depending on your location (parallax changes their relative position). They get closer toward the end of the eclipse, but as the Moon gets brighter Uranus will be harder to spot. I suggest using some planetarium software like Stellarium or Sky Safari to chart the planet’s location. There are many stars around the same brightness nearby, so it’s easy to be confused. Uranus does distinguish itself by being weirdly greenish, so that helps. Through a small ‘scope you might be able to see it’s a disk, too, and not a point like stars are.

As it happens there won’t be another total lunar eclipse until 2025; there will be a partial one in 2023 and again in 2024, but only a tiny part of the Moon will get dark. Those will be interesting to see, but lack the real fun of a total eclipse. So watch this one if you can! And if you can’t (due to clouds, time zones, whatever) the Virtual Telescope will be streaming it live. Do a web search and you’ll likely find lots of folks livestreaming it, too.

Enjoy!

Et alia

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