Lunar eclipse
A lunar eclipse occurs when the Moon passes through Earth's shadow.
A lunar eclipse, also called a blood moon, is an astronomical event that occurs when the Moon orbits through Earth's shadow. Lunar eclipses occur during eclipse season, when the Moon's orbital plane is approximately in line with Earth and the Sun. The type and length of a lunar eclipse depend on the Moon's proximity to the lunar node. Unlike solar eclipses, lunar eclipses can be observed from anywhere on the night side of Earth and often last for an hour or longer. They are safe to observe without eye protection.
- type
- Astronomical event
- frequency
- Approximately one-third of all lunar eclipses are penumbral; of those, only 3% are total penumbral eclipses
- central_eclipse_frequency
- 59.6% of all total lunar eclipses
- cause
- Moon passing through Earth's umbra; sunlight refracted through Earth's atmosphere
Lore & Background
Lunar eclipses cause the Moon to appear orange or red because sunlight reaching the Moon must first pass through Earth's atmosphere, where Rayleigh scattering removes short-wavelength colors such as violet and blue. The resulting reddish coloration is similar to the effect that causes sunsets and sunrises to appear red. The amount of dust or clouds in the atmosphere affects the depth of the red color; volcanic eruptions can expel large quantities of dust, making the Moon appear a deeper red.
Reader's Guide
Lunar eclipses are classified into several types. A penumbral lunar eclipse occurs when the Moon is partially inside Earth's penumbra, causing a subtle dimming. A partial lunar eclipse occurs when the Moon lies partially inside the umbra, producing a distinct dark region. A total lunar eclipse occurs when the Moon's near side entirely passes into Earth's umbral shadow; the Moon does not completely darken because of refraction of sunlight by Earth's atmosphere. A central lunar eclipse occurs when the Moon passes near the center of Earth's shadow, contacting the antisolar point; this type occurs in 59.6% of all total lunar eclipses. A selenelion, or horizontal eclipse, occurs when both the Sun and an eclipsed Moon can be observed at the same time just before sunset or after sunrise, due to atmospheric refraction. The Danjon scale rates the darkness of lunar eclipses from L=0 (very dark) to L=4 (very bright copper-red or orange). In culture, the symbol for a lunar eclipse is U+1F776. Ancient Greeks used the Earth's shadow observed during a lunar eclipse as evidence that the Earth was round.
Did You Know?
- Lunar eclipses are safe to observe without eye protection.
- The Moon's average orbital speed is about 1.03 km/s (2,300 mph).
- If Earth had no atmosphere, the Moon would be completely dark during a total lunar eclipse.
- A selenelion occurs during every total lunar eclipse, but is an experience of the observer, not a separate planetary event.
Frequently Asked Questions
What is a lunar eclipse?
A lunar eclipse happens when the full Moon slides into Earth's shadow. This alignment only becomes possible during eclipse season, when the Moon's orbital path crosses close to the Sun-Earth line.
Why does a lunar eclipse look red?
The reddish glow, often nicknamed a blood moon, comes from sunlight that bends through Earth's atmosphere and gets refracted onto the Moon's surface. Shorter blue wavelengths scatter away while the longer red wavelengths pass through, tinting the shadow.
Is it safe to watch a lunar eclipse with the naked eye?
Yes—unlike a solar eclipse, a lunar eclipse poses zero risk to your eyes. You can observe the entire event from anywhere on Earth's night side without any filters or special gear.
How common are total lunar eclipses?
Roughly one-third of all lunar eclipses are only penumbral, and just 3% of those are total penumbral events. Among the total eclipses that do occur, about 59.6% are central, meaning the Moon passes near the very center of Earth's shadow.
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