Watch and Measure a Total Lunar Eclipse

A lunar eclipse happens when the full moon passes into Earth’s shadow.

The Moon passes through two parts of Earth’s shadow during a lunar eclipse. You can tell which part of the shadow the Moon is passing through based on the way the Moon looks from our perspective on Earth.

You have probably noticed that some shadows on the ground are darker than others, depending on how much outside light enters the shadow. The same is true for Earth’s shadow.

The outer part of Earth's cone-shaped shadow is called the penumbra. The penumbra is less dark than the inner part of the shadow. The inner part of the shadow, known as the umbra, is much darker because Earth blocks more sunlight from entering the umbra.

As the Moon enters the penumbra, it will dim very slightly. Because the penumbra is not fully dark, you may only notice some dim shading (if anything at all).

When the Moon moves into the darker shadow, the umbra, it is very noticeable! Some say that during this part of the eclipse, the Moon looks as if it has had a bite taken out of it. That “bite” gets bigger and bigger as the Moon moves deeper into Earth's shadow.

The total lunar eclipse officially begins once the Moon is completely inside the umbra. At this point, the Moon will look reddish-orange.

Even though a full moon occurs about every 29 days, lunar eclipses are much more rare. This is because the Moon’s orbit is tilted, so most full moons happen when the Moon is above or below Earth’s shadow. Sometimes, the Moon only moves part way into the edge of the shadow. We call this a partial lunar eclipse.

In the next steps you'll be watching and measuring a total lunar eclipse as it happens. To do that, you'll first need to know when you can watch the next total lunar eclipse. Here's how:

A total lunar eclipse map for May 26, 2021 shows (circled in red) where you can find the times of the greatest eclipse (top) and the start and end of the total lunar eclipse (middle right).

Credit: NASA

1. Visit NASA’s lunar eclipse website to find an upcoming total lunar eclipse.
2. Look for "Total" eclipses in the Eclipse Type column and click on the date for one of your choosing. This will pull up the eclipse data sheet, which will tell you when and where you can view that total lunar eclipse. Look at the map to make sure the eclipse will be visible where you live. If you live in an area that’s white on the map, you’ll be able to see the entire eclipse. If you live in a shaded area that’s between the white area on the map and the line marked U3, you should be able to see at least some of the total lunar eclipse before the Moon sets or rises. If you live in the dark shaded area, or between the dark area and the U3 line, you’ll have to look for another eclipse that’s visible in your area.
3. Next, use the eclipse evaluation worksheet or a blank piece of paper to record the times of the start of the total lunar eclipse (U2), the greatest eclipse, and the end of the total eclipse (U3). They’re listed in Universal Time (UT).
4. Convert UT to your local time zone using this online calculator. Note: Once you convert to your local time, the date when you can see the eclipse may be different from what you clicked on originally. Look carefully so you’re not trying to view an eclipse that has already happened!

You can observe and classify a lunar eclipse based on its color and brightness!

Go outside to look at the Moon at the times you found in Step 2. You can use a telescope or binoculars, but just viewing with your eyes works great. If it’s cloudy, keep checking throughout the total eclipse. Sometimes a short break in the clouds will let you see the eclipsed Moon!

Using the scale below as a guide, rate the brightness and color of the eclipse and record it on the Eclipse Evaluation Worksheet.

Note: This scale is known as the Danjon Scale of Lunar Eclipse Brightness and it's used to evaluate the appearance and brightness of the Moon during total lunar eclipses.

Danjon Scale of Lunar Eclipse Brightness

A lot of things can affect the appearance of the Moon during a total lunar eclipse. Clouds, dust, ash, chemical droplets and organic material in the atmosphere can change how much light is refracted, or bent, into Earth’s shadow, giving the Moon a reddish color during a lunar eclipse.

Are there significant wildfires around the world that might be causing the color you see? Have there been any major volcanic eruptions on Earth in the past year or two? Are there seasonal activities near you, such as crop harvests, that might be contributing to material in the air?

Discuss your observations with friends and family who also saw the eclipse. Then, keep your records and compare them to the next total lunar eclipse you observe.

Check out some of these NASA sites to learn more about the Moon and eclipses, and try out some of the Moon projects below.

• Facts & Figures: NASA Science - Earth's Moon