Milwaukee Astronomical Society


Beginner's Guide

Solar System - Lunar Eclipses


Observing Lunar Eclipses

Total Solar Eclipse series by John Asztalos, MAS image.

Lunar eclipse diagram. Wikipedia Commons. Lunar eclipses occur when the moon passes into the shadow of the Earth. This only can happen at the time of the full moon which is when the moon is opposite the sun.

As you can see from the lunar eclipse diagram at the right (not to scale!), there are two distinct parts of Earth's shadow: the umbra (the dark shadow) and the penumbra (light shadow). But there are three kinds of lunar eclipses: penumbral, partial, and total.

Total: A total lunar eclipse happens when the moon passes entirely into the umbra (the dark shadow). A total eclipse will also have a partial eclipse phase as well as a penumbral phase.

Partial: A partial lunar eclipse happens when the moon passes partially into the umbra (the dark shadow). A partial eclipse will also have a penumbral phase.

Penumbral: A penumbral lunar eclipse happens when the moon passes totally or partially into the penumbra (the light shadow). Lunar eclipses which are strictly penumbral eclipses can be viewed as non-events. There is only a very slight dimming of the moon which most will not even notice.

Full "Blood" Moon showing background stars. Gabe Shaughnessy. MAS image. The great feature of a total lunar eclipse is the "blood moon" which describes the orangish-red color the moon takes on when the moon moves entirely into the umbra. This is not immediately logical! When the Earth's shadow is moving across the face of the moon it looks like the moon will be totally obscured and therefore almost essentially invisible. But this doesn't happen because Earth has a substantial atmosphere. So the scattered light of our atmosphere still illuminates the moon. Just like seeing a sunset, the scattered light is orange to red.

One opportunity a total lunar eclipse offers is the chance to actually see stars in close proximity. When the moon is full you might be able to one extremely bright star, about 1st magnitude or brighter. So the sky is essentially washed out. The dark background during the total phase is almost surreal.

Though it should go without saying, viewing a lunar eclipse is entirely safe! The confusion for the general public comes from all the dire warnings given to solar eclipses which is entirely appropriate. The public just hears the word eclipse and don't look.


Upcoming Total Lunar Eclipses

Compared to solar eclipses, lunar eclipses happen far more frequently. And the visibility is much easier. To see a total solar eclipse you have to be within the line of totality which is on average just 70 miles wide. With a lunar eclipse, if you can see the moon in the sky you'll see the event! Here are some upcoming total lunar eclipses:

May 26, 2021: This eclipse is only partially visible from North America. The best views will be from western North America; and the eclipse will also be entirely visible from Hawaii. The Moon will enter the penumbra at 3:46 A.M. CDT and umbra at 4:45 A.M. CDT. It will leave the umbra at 7:53 A.M. CDT and penumbra at 8:51 A.M. CDT. So from Milwaukee the moon will set before the eclipse is over.

May 15, 2022: This eclipse is visible from North America, except in northwestern regions. The Moon will enter the penumbra at 8:31 P.M. CDT on May 15 and leave it at 1:52 A.M. CDT on May 16.

November 8, 2022: This eclipse is visible from North America, although the Moon will be setting during the eclipse for observers in eastern regions. The Moon will enter the penumbra at 2:01 A.M. CST on November 8 and leave it at 7:58 A.M. CST.