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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.

A Full Moon against a dark sky.

A normal Full Moon, or a penumbral lunar eclipse?

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

©iStockphoto.com/CochiseVista

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 image
Lunar nodes are the locations where the Moon crosses the Earth's orbital plane.
A lunar eclipse only happens when the Moon passes a lunar node during the Full Moon phase, when the Earth is positioned between the Moon and the Sun.

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

Illustration image
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 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

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Lunar Eclipses

  1. When Is the Next Lunar Eclipse?
  2. Total Lunar Eclipse
  3. Why Does the Moon Turn Red?
  4. Partial Lunar Eclipse
  5. Penumbral Lunar Eclipse
  6. Can I See a Lunar Eclipse?
  7. Blood Moon - Total Lunar Eclipse
  8. Magnitude of Eclipses

Eclipses

The Moon

  1. What Is a Supermoon?
  2. The Moon Illusion
  3. The Moon Phases
  4. The Moon's Effect on Tides
  5. What Is a Micro Moon?
  6. How Can Full Moon Be in the Daytime?
  7. Is a Blue Moon Blue?
  8. The Moon's Orbit
  9. The Far Side of the Moon
  10. What Is a Black Moon?
  11. What Are Moonbows?
  12. Full Moon Names
  13. Taking pictures of the Moon

Moon index

Solar Eclipses

  1. When Is the Next Solar Eclipse?
  2. Different Types of Eclipses
  3. What Are Solar Eclipses?
  4. How Often Do Solar Eclipses Occur?
  5. Total Solar Eclipses
  6. Partial Solar Eclipses
  7. Annular Solar Eclipses
  8. Hybrid Solar Eclipses
  9. Solar Eclipses in History
  10. Solar Eclipse Myths
  11. Magnitude of Eclipses

Eclipses

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