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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 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 the 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 all or part of the Moon with the outer part of its shadow, also known as the penumbra. Since the penumbra is much fainter than the dark core of the Earth's shadow, the umbra, a penumbral eclipse of the Moon is often difficult to tell apart from a normal Full Moon.

2 Conditions

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

  • It must be around Full Moon.
  • The Sun, Earth, and Moon must be imperfectly aligned.

Not Every Full Moon Night

The reason we do not see a lunar eclipse every Full Moon night has to do with 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 around the Sun, the ecliptic. The points where the 2 orbital planes meet are called lunar nodes. Lunar eclipses can only take place when a Full Moon occurs near a lunar node.

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 1 in 3 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.

However, penumbral eclipses that involve the darker central portion of the Earth's penumbral shadow are normally visible to the naked eye. Trained observers can usually see penumbral eclipses with a penumbral magnitude greater than 0.60.

Topics: Astronomy, Moon, Earth, Sun, Eclipses

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