What is a Lunar Eclipse? - Definition, Facts & Frequency

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  • 0:00 Lunar Eclipse Background
  • 0:19 Lunar Eclipse Definition
  • 1:33 Lunar Eclipse Frequency
  • 1:54 Lunar Eclipse Facts
  • 3:08 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Lybrand
In this lesson, you will learn what a lunar eclipse is and how frequently lunar eclipses occur. You will also learn facts related to lunar eclipses, which are studied by scientists all over the world.

Lunar Eclipse Background

Have you ever observed the marvel of a lunar eclipse? Witnessing a lunar eclipse is quite an exciting experience that has been pondered by cultures throughout history. In this lesson, you'll learn the scientific reasons why lunar eclipses occur.

Lunar Eclipse Definition

In our solar system, an eclipse happens when a celestial object, such as a planet or moon, passes between another planet or moon and its light source, the sun. The moon is in constant orbit around Earth, leading to changes in how much of the moon is lit by the sun or how much of the moon we can see from Earth. A lunar eclipse occurs when Earth crosses between the moon and the sun, which casts a shadow of Earth onto the moon.

A lunar eclipse can only happen during a full moon and when the moon passes through Earth's shadow. Earth has two types of shadows referred to as the umbra and the penumbra.

The shadowing of the moon by Earth can lead to a partial lunar eclipse, where only a portion of the moon is darkened by Earth's penumbra shadow. A total lunar eclipse results when the sun, Earth, and moon are perfectly aligned, resulting in a complete darkening and/or reddening of the moon because no direct sunlight reflects off of the moon's surface. During a total lunar eclipse, the moon is in Earth's umbra shadow because all direct light from the sun is blocked.

Lunar Eclipse Frequency

Lunar eclipses generally last from a few minutes to a few hours. The length of time the eclipse is visible depends on where on the planet the eclipse is being viewed and on the type of eclipse that is happening. Partial lunar eclipses occur at least twice a year, while total lunar eclipses are rarer.

Lunar Eclipse Facts

The reddening of the moon, or the blood moon phenomenon, is due to light from the sun being scattered by Earth's atmosphere. The light traveling from the sun to the edges of Earth is forced to pass through thick layers of Earth's atmosphere. The shorter wavelengths of light, such as blue and purple, are scattered by the dense, thick atmosphere, allowing only the longer wavelengths of light, such as red, to reach the moon.

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