Bad astronomy | Guide to viewing the lunar eclipse of November 18, 2021

From Thursday evening / Friday morning, the sky will offer a spectacle: a lunar eclipse, when the Moon enters and leaves the Earth’s shadow. It strongly favors viewers in North America, but will be visible in parts of Central and South America and East Asia.

It’s always fun, but this one is unusual in a lot of ways. On the one hand, the Moon will be near the apogee, the farthest point in its elliptical orbit around the Earth, so it will appear slightly smaller than average and also move more slowly, so the eclipse lasts a little longer than usual. It will actually be the longest partial lunar eclipse of the century.

But more importantly, it is almost corn not enough a total eclipse; very very close but not exactly. About 97% of the Moon will be in Earth’s deep shadow, so even at the maximum eclipse, a tiny bit near its south pole will still be lit by the Sun. It should look pretty cool, so it’s worth checking out.

I get this question often, so right off the bat: it’s perfectly safe to observe a lunar eclipse with the naked eye, with binoculars (which I highly recommend), or even a telescope. It’s a solar eclipse which can be dangerous to see, when the Moon blocks the Sun. But a lunar eclipse occurs when the Earth blocks the Sun as seen from the Moon, so it’s all good to watch.

A lunar eclipse occurs when the Sun, Earth, and Moon align, with the Earth between the Sun and the Moon. Earth’s shadow is still out there in space, but the Moon’s orbit is tilted slightly relative to Earth around the Sun (around 5 °) and the Moon is small (around 0.5 ° in our sky) so it is usually missing, going “above” or “below”. But sometimes the Moon is in the right place at the right time and passes through the Earth’s shadow.

I explain how it all works in “Crash Course Astronomy: Eclipses”, so give him a watch for a general explanation of the mechanics of an eclipse and what you will see in general.

Earth actually casts two shadows, essentially a narrow cone nestled inside a larger cone. If you cast them on the sky, the outer shadow, called the twilight, is about 4 degrees in diameter, and the inside, the shadow, is about 2 ° wide.

If you are on the Moon when it is in the semi-darkness, you see the Earth partially blocking the Sun, so the shadow is not very dark. But if you are on the side of the Moon in the Earth’s shadow, the Sun is completely blocked off, so it’s dark.

The Moon enters the penumbra shadow (this moment is called P1) at 6:02 am UTC Friday morning (1:02 am Eastern US time; note that for the mountains and the Pacific it is 11:02 pm and 10:02 pm thursday evening, respectively). You won’t really notice it because that part of the shadow isn’t very dark.

He begins to step into the shadow of the shadow (U1) at 07:18 UTC, and you will see that the western part (to the left of the Moon for northern hemisphere observers) of the Moon begins to darken, like a line crossing it. Over the next few minutes, as the Moon sinks deeper into the shadows, it will seem like a mouthful is taken out, bigger and bigger.

The maximum of the eclipse is at 09:02 UTC (04:02 EST)*. Everything except the southern tip will be dark, but even that part will not be very bright.

Many times when the Moon slides into the darkest shadow of Earth, it appears brownish, then near the maximum of the eclipse, it turns a deep blood red. In this case, the southern tip may appear orange or yellowish. It should be fascinating to see!

Why is it turning red? From the Moon, you see the solid Earth blocking the Sun. But the Earth’s atmosphere is always lit by the Sun and can appear red for the same reason that sunrises and sunsets are red, and from the Moon you basically see all sunrises and sunsets on Earth simultaneously! So, this indirect lighting can make the Moon quite red during an eclipse, which is weird and quite charming.

After the maximum eclipse, the Moon will begin to emerge from the Earth’s shadow (U4), leaving it entirely at 10:47 UTC. It leaves the half-light shadow (P4) at 12:03 UTC, and at that time, the eclipse is over.

It should look a lot like the April 2014 eclipse, seen in this beautiful time-lapse by my friend Adam Block:

Who can see it?

On this map, any area in white sees all. If you’re just past the eastern edge (like, say, Cuba or the east coast of the United States), the Moon sets around sunrise, just before the eclipse has fully ended. For most of South America, the Moon still partially sets in shadow. The western strip of Africa sees the Moon set as it enters the semi-darkness, so they won’t see much. Everyone in the dark region will not see anything; their part of the Earth will be opposite the Moon when that happens. For people in shaded areas to the right of the Moon mounted near the end of the eclipse.

It turns out that the Moon will be in the constellation Taurus during this eclipse, and very close to both the Hyades star cluster (which forms the V-shape of the bull’s head and horns) and even close to the lovely Pleiades cluster, which looks like a mini-version of the Little Dipper. When the Moon is darker it should be obvious, so if you want to try your hand at astrophotography this would be the best time. I think we’ll see some pretty cool photos. Bonus: the protoplanet Ceres is right in the middle of the Hyades at the time, so a few deep photos should capture it. Consult a sky map to find it.

If you want more info I recommend Sky and Telescope editorial staff and that of the time and date also. You can always just search on it; tons of sites describe what’s going to happen and how to watch it.

Keep in mind that it can be cold outside, and it does happen overnight, so dress warmly. And since it’s a slow event, taking place over several hours, you can go out for a few minutes every now and then to check the progress and then reenter. This is how I do it.

Good luck, and may you have clear skies and a great viewing!


* CORRECTION: I originally wrote that 09:02 UTC was 02:02 Eastern, but it is 04:02.



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