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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's 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 part of the Moon with the outer part of its shadow, also known as the penumbra. The rest receives the same amount of sunlight as usual and is 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.

Two Conditions

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?

Lunar Eclipse Penumbral
Sun, Earth, and Moon are not perfectly aligned during a penumbral eclipse. (Not to scale)
Sun, Earth, and Moon are not perfectly aligned during a penumbral eclipse. The Moon misses the Earth's umbra and travels through the much fainter penumbra. (Not to scale)

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-third of all lunar eclipses are penumbral. They are, however, easy to miss because when they happen, the eclipsed Moon tends to look very similar to a full Moon. In fact, it is impossible to observe the start and end of a penumbral lunar eclipse, even with telescopes.

Only penumbral eclipses where a large portion of the Moon is in the Earth's penumbral shadow may be detectable to observers on Earth. Trained eyes can usually see penumbral eclipses with a penumbral magnitude that is more than 0.60.

Topics: Astronomy, Eclipses, Moon, Earth, Sun

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