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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.

Next Penumbral Lunar Eclipse: Fri, Jun 5, 2020 … See animation

A Full Moon against a dark sky.

A normal Full Moon, or a penumbral lunar eclipse?

©iStockphoto.com/CochiseVista

Penumbral Lunar Eclipses in 2020

There will be a penumbral lunar eclipse darkening the Strawberry Moon on June 5/6, the Buck Moon on July 4/5, and the Beaver Moon on November 29/30, depending on the time zone. Use the links below to see maps, animations, and to find local eclipse times in your location.

Penumbral Lunar Eclipses 2020—2029

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

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

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.

How to See a Penumbral Lunar Eclipse

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)

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

More details about upcoming Eclipses

Topics: Eclipses, Moon, Sun, Astronomy