How does a lunar eclipse occur

By Max. D Gray. Updated: January 16, 2017
How does a lunar eclipse occur

A Moon eclipse or lunar eclipse occurs when the planet Earth crosses between the Sun and the Moon, creating a perfectly straight line between all three. This astronomical phenomenon occurs more often than solar eclipses, but is still a much-anticipated natural phenomenon that many people find amazing to watch. To learn more about this type of eclipse and find out what conditions are needed for it to happen, check out this OneHowTo article about how a lunar eclipse occurs.

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What is a lunar eclipse?

According to Wikipedia, a lunar eclipse, or eclipse of the Moon, is "an astronomical event that occurs when the Earth is between the Sun and the Moon, causing the latter to enter the umbra of the Earth and consequently fall into darkness. For the eclipse to happen, the three celestial bodies, the Earth, the Sun and the Moon must be exactly aligned or very close to being aligned, so that the Earth blocks the sunlight that reaches the Moon."

How does a lunar eclipse occur - What is a lunar eclipse?

When does a lunar eclipse occur?

The shadow of the Moon is divided into two regions: the "umbra" which is the dark cone-shaped inner portion, and the "penumbra" which is the outer shadow. Lunar eclipses can thus only occur at full moon for the Moon to penetrate the cone of shadow cast by the Earth in space when the three bodies are aligned.

You might therefore be under the impression that there should be an eclipse every month with every full moon, but this is not the case because the Moon's orbit is on a slight angle to the ecliptic plane (the curved line where the Sun "passes" around the Earth, in its "apparent motion" as seen from the Earth).

Types of lunar eclipse

  • Partial eclipse: occurs when only part of the Moon enters the umbra and is hidden.
  • Total eclipse: occurs when the Moon completely enters the umbral area in the cone of the Earth's shadow. A special kind of total eclipse is the total-centre, in which the Moon passes right through the middle of the Earth's umbra.
  • Penumbral eclipse: the Moon enters the cone of the Earth's penumbra, i.e. it only crosses the outermost regions of the Earth's shadow. In these cases the Moon hardly darkens - so little that it is in fact very difficult to see that an eclipse is happening.

How can you see a lunar eclipse?

Unlike solar eclipses that can only be seen from a part of the Earth, a lunar eclipse can be seen from anywhere on Earth during the night. Furthermore, lunar eclipses last for several hours, while solar eclipses can only be seen for a few minutes. It's also worth noting that we can admire lunar eclipses directly with binoculars or a telescope; whereas solar eclipses should always be viewed with eye protection.

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