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What is a Penumbral Lunar Eclipse?

A penumbral lunar eclipse takes place when the Moon moves through the faint, outer part of the Earth's shadow. This type of eclipse is often mistaken for a normal full Moon.

Illustration image
A full Moon, or a penumbral lunar eclipse?
A penumbral lunar eclipse can be mistaken for a normal Full Moon (pictured here).
©iStockphoto.com/Hydromet

The Moon shines because its surface reflects the Sun rays. A lunar eclipse happens when the Earth comes between the Sun and the Moon and blocks sunlight from directly reaching the Moon.

Imperfect Alignment

A penumbral lunar eclipse occurs when the Sun, Earth and Moon align in an almost straight line. When this happens, the Earth blocks some of the Sun's light from directly reaching the Moon's surface, and covers a small part of the Moon with the outer part of its shadow, also known as the penumbra. The rest recieves direct sunlight and can be as bright as a full Moon.

Because of this, it is often hard to differentiate between a normal full Moon and a penumbral eclipse of the Moon.

Conditions for a Penumbral Lunar Eclipse

Two celestial events must happen at the same time for a penumbral lunar eclipse to occur:

  • the Moon should be a full Moon, and
  • the Sun, Earth and Moon must be imperfectly aligned in a straight line.

The reason we do not see a lunar eclipse every full Moon night is because of the inclination of the Moon's orbital path. The plane of the Moon's orbit around the Earth is inclined at an angle of 5° to the Earth's orbital plane (ecliptic) around the Sun. The points where the two orbital planes meet are called lunar nodes. Lunar eclipses can only take place when a full Moon occurs near one of the lunar nodes.

How to see a Penumbral Lunar Eclipse?

Unlike solar eclipses, which can only be seen along a narrow path on Earth, eclipses of the Moon can be observed all across the night-side of Earth when the eclipse happens.

About one-thirds of all lunar eclipses are penumbral. They are however easy to miss because when they happen, the Moon looks no different than a full Moon.

Topics: Astronomy, Eclipses, Moon, Earth, Sun

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  6. Penumbral Lunar Eclipses
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