Sunday, April 02, 2017

What is an Eclipse?

e-clipse (i-╦łklips) noun. 1. a. The total or partial obscuring of one celestial body by another. b. The passing into the shadow of a celestial body. Compare OCCULTATION, TRANSIT. [Middle English, from Anglo-French, from Latin eclipsis, from Greek ekleipsis, from ekleipein to omit, fail, suffer eclipse, from ex- + leipein to leave]

An eclipse happens when one heavenly body, such as a moon or planet, moves into the shadow of another heavenly body. There are two types of eclipses as seen from Earth: an eclipse of the moon and an eclipse of the sun.

An eclipse of the moon...

The moon moves in an orbit around Earth, and at the same time, Earth orbits the sun. Sometimes Earth moves between the sun and the moon. When this happens, Earth blocks the sunlight that normally is reflected by the moon. Instead of light hitting the moon’s surface, Earth's shadow falls on it. This is an eclipse of the moon—a lunar eclipse. A lunar eclipse can happen only when the moon is full.

A lunar eclipse can be seen from Earth at night. There are two types of lunar eclipses: total lunar eclipses and partial lunar eclipses.

A total lunar eclipse happens when the moon and the sun are on exact opposite sides of Earth. Although the moon is in Earth's shadow, some sunlight reaches the moon. The sunlight passes through Earth's atmosphere, which causes Earth’s atmosphere to filter out most of the blue light. This makes the moon appear red to people watching from Earth.

A partial lunar eclipse happens when only a part of the moon enters Earth's shadow. In a partial eclipse, Earth's shadow appears very dark on the side of the moon facing Earth. What people see from Earth during a partial lunar eclipse depends on how the sun, Earth and moon are lined up.

A lunar eclipse usually lasts for a few hours. At least two partial lunar eclipses happen every year, but total lunar eclipses are rare. It is safe to look at a lunar eclipse.

A lunar eclipse. Image credit: NASA

An eclipse of the sun...

Sometimes when the moon orbits Earth, it moves between the sun and Earth. When this happens, the moon blocks the light of the sun from reaching Earth. This causes an eclipse of the sun, or solar eclipse. During a solar eclipse, the moon casts a shadow onto Earth.

There are three types of solar eclipses. The first is a total solar eclipse. A total solar eclipse is only visible from a small area on Earth. The people who see the total eclipse are in the center of the moon’s shadow when it hits Earth. The sky becomes very dark, as if it were night. For a total eclipse to take place, the sun, moon and Earth must be in a direct line.

The second type of solar eclipse is a partial solar eclipse. This happens when the sun, moon and Earth are not exactly lined up. The sun appears to have a dark shadow on only a small part of its surface.

The third type is an annular (pronounced “ANN-you-ler”) solar eclipse. An annular eclipse happens when the moon is farthest from Earth. Because the moon is farther away from Earth, it seems smaller. It does not block the entire view of the sun. The moon in front of the sun looks like a dark disk on top of a larger sun-colored disk. This creates what looks like a ring around the moon.

A solar eclipse. Image credit: NASA

During a solar eclipse, the moon casts two shadows on Earth. The first shadow is called the umbra (pronounced “UM-bruh”). This shadow gets smaller as it reaches Earth. It is the dark center of the moon’s shadow. The second shadow is called the penumbra (pronounced “pe-NUM-bruh”). The penumbra gets larger as it reaches Earth. People standing in the penumbra will see a partial eclipse. People standing in the umbra will see a total eclipse.

The umbra and penumbra. Image credit: NASA

Solar eclipses happen approximately once every 18 months. Unlike lunar eclipses, solar eclipses only last for a few minutes.

You should never look directly at the sun. It can permanently damage your eyes. You must use proper safety equipment to look at any type of solar eclipse. More on this later.


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