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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, the penumbra. This type of eclipse is not as dramatic as other types of lunar eclipses and is often mistaken for a regular Full Moon.

A Full Moon against a dark sky.

A penumbral lunar eclipse can be mistaken for a regular Full Moon.

©iStockphoto.com/CochiseVista

Why Do Lunar Eclipses Happen?

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 some or all of the Sun's light from reaching the Moon.

Imperfect Alignment

A penumbral lunar eclipse occurs when the Sun, Earth, and the Moon are imperfectly aligned. 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.

Illustration of lunar nodes with Sun, Earth, and Moon
Illustration of lunar nodes with Sun, Earth, and Moon

Lunar nodes are the locations where the Moon crosses the Earth's orbital plane.

Penumbral lunar eclipses can be partial or full. During a partial penumbral eclipse, only part of the Earth's penumbra covers the Moon's surface. This kind of eclipse is almost impossible to see. During a full penumbral lunar eclipse, on the other hand, the Earth's penumbra covers the entire face of the Moon. Very keen observers may then see the Moon turn a shade darker during the maximum of the eclipse.

Two Conditions

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

  • The Moon must be in the Full Moon phase.
  • The Sun, Earth, and Moon must be nearly aligned, but not as closely aligned as during a partial eclipse.

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 two orbital planes meet are called lunar nodes. Lunar eclipses can only take place when a Full Moon occurs near a node.

Penumbral lunar eclipse illustration with positions of Sun, Earth, and Moon in space
Penumbral lunar eclipse illustration with positions of Sun, Earth, and Moon in space

Sun, Earth, and Moon are not perfectly aligned during a penumbral eclipse. (Not to scale)

How to See a Penumbral Lunar Eclipse

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

About one in three of all lunar eclipses are penumbral. It is impossible to observe the start and end of a penumbral lunar eclipse, even with telescopes.

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

Upcoming 5 Penumbral Lunar Eclipses

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Topics: Eclipses, Moon, Sun, Astronomy