Lunar Eclipse: Definition & Types

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  • 0:02 What's an Eclipse?
  • 0:32 The Lunar Eclipse,…
  • 2:24 The Total Lunar Eclipse
  • 3:58 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will teach you about the lunar eclipse. You'll learn the difference between a penumbral, partial, and total lunar eclipse, and see what totality means with respect to a total lunar eclipse.

What's an Eclipse?

Every now and then, the moon, earth, and sun will all lie in a straight line. In some of these instances, Earth's shadow will fall on the moon. In other cases, the moon's shadow will fall on Earth. Either one of these events is known as an eclipse, an event that obscures the light of the moon when it's in Earth's shadow, or the light of the sun when the moon is between it and the earth. This lesson will focus on the lunar eclipse, what it means, and how it occurs.

The Lunar Eclipse, Umbra, & Penumbra

A lunar eclipse is the darkening of the moon as it passes through Earth's shadow. This event will occur when the moon, sun, and earth are in a straight line and when the earth is right in between the sun and moon during this time. In such a scenario, the moon is in its full phase as it moves through Earth's shadow.

The earth actually has two parts to its shadow. The earth's umbra is the darkest portion of Earth's shadow, or its total shadow. The earth's penumbra is the partially-shaded portion of the earth's shadow. It may help you to know that umbra comes from the Latin for 'shade,' and the prefix 'pen-' implies 'almost' or 'nearly.' So, a penumbra is nearly a total shadow, but not quite.

The lunar eclipses that are easy to miss are known as penumbral eclipses, where the earth will only stop some of the light coming from the sun. This means none of the lunar surface will be completely blocked out, so the moon will be dimmer than we'd expect, but not totally blocked out. These eclipses are nothing to write home about, so you can be forgiven for not noticing them.

But it shouldn't come as a big shock then that most of us notice a lunar eclipse when the moon is partially or totally in Earth's umbra. If the moon passes through only part of the umbra, you get a partial lunar eclipse. If the moon is entirely within the umbra, you get a total lunar eclipse.

During a lunar eclipse, the period during which the moon is completely inside of Earth's umbra is known as totality. Since the moon moves through Earth's shadow at about one kilometer per second, the totality can last for approximately one hour and 45 minutes.

The Total Lunar Eclipse

But don't let the name 'total lunar eclipse' mislead you. When the moon is totally eclipsed, you can still see it. What a head scratcher of a notion, huh? The reason this occurs can be explained relatively easily.

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