What’s Up: Partial Lunar Eclipse

Nov 22, 2021 | Daily Space, Earth, Moon, Sky Watching, The Sun

What’s Up: Partial Lunar Eclipse
IMAGE: During a lunar eclipse, 2 shadows are cast. The dark inner part of Earth’s shadow is called the umbra. The 2nd shadow is called the penumbra. During the November 18-19, 2021, partial lunar eclipse, the moon will be 97% covered by Earth’s dark umbral shadow. CREDIT: NASA.

This week in What’s Up is a partial lunar eclipse. Starting November 19 at 08:58 UTC, the Moon will begin to skirt the edge of the Earth’s umbra, or shadow, as the Earth goes in between the Moon and the Sun. Peak eclipse will happen at 09:02 UTC. Nearly the entire United States, including Hawaii, will be able to see this eclipse. In addition, Central America down to Guatemala will be able to see the full eclipse. Parts of the United Kingdom will as well but only in the few minutes before the Moon sets below the horizon.

But I don’t want to get your hopes up as partial lunar eclipses are not that visually striking, especially compared to a total solar eclipse. At best you will be able to see the Moon get a little darker and redder at full eclipse and not much else. The Moon gets red because the light that hits it has passed through Earth’s atmosphere, which scatters out blue wavelengths through a process called Rayleigh scattering, leaving red wavelengths only. This is also why the Sun appears red when at the horizon. 

One thing to look for in this partial eclipse is the Pleiades star cluster, which will be positioned very close to the Moon during the eclipse.

IMAGE: For us in North America, the partial lunar eclipse will take place overnight on November 18, and in the early morning on November 19, 2021. The moon will be high in North American skies, shifting toward the west. In this illustration, the times are in EST. Mid-eclipse will be at about 4 a.m. EST, 3 a.m. CST, 1 a.m. PST, etc. Precise time of mid-eclipse is 9:03 UTC. But watch the creeping shadow on the moon’s disk for an hour or so before that time, too! In this illustration, the white disks represent partially eclipsed moons. The maroon disk represents the moon at greatest eclipse, 97% covered by the Earth’s dark umbral shadow. Watch for the dipper-shaped Pleiades star cluster near the eclipsed moon. CREDIT: John Jardine Goss via EarthSky

You can try viewing this event in a telescope, but again, there isn’t much to see – at best a mild darkening compared to its usual brightness. It is safe to view a lunar eclipse in a telescope without any special filters, though you may want to attach a neutral density filter to your eyepiece to cut down on the Moon’s brightness. These are sometimes included with a telescope but are inexpensive to purchase separately. They are often marketed as “moon” filters because that is what they are mainly used for.  It can get somewhat annoying, like looking right at a bright light for a few minutes and a neutral density filter makes it more comfortable.

So-called variable moon filters consist of two moon filters attached together with a ring in the middle so they can rotate freely. These allow you to change the amount of light transmission easily, to adjust for the brightness of different moon phases. You may want to get one of these if the Moon is one of your main interests in the sky as it is more flexible than a fixed transmission Moon filter. You can get a variable moon filter and help support our efforts to put science into your brain by purchasing one using the Amazon affiliate link in the show notes.

The Moon has many different features of all shapes and sizes that it rewards coming back again and again as different as different portions of its surface are illuminated. Counterintuitively, the worst time to view the Moon is during the full moon phase. This is because the entire disk is evenly illuminated. You want some parts to be dark, so that the contrast between light and dark allows you to see features better. 

More Information

Deep partial lunar eclipse November 18-19: Great for North America (EarthSky)

PDF: Partial Lunar Eclipse of 2021 Nov 19 (NASA)


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