Lunar Eclipses
When the Moon passes through Earth's shadow.
Overview
A lunar eclipse occurs when Earth lies between the Sun and the Moon and the Moon passes through Earth's shadow. Unlike solar eclipses they are entirely safe to watch, last for hours, and are visible from the whole night side of Earth at once.
- Lunar eclipses are visible to everyone on the night side of Earth, not from a narrow path.
- The Moon turns red rather than going dark because Earth's atmosphere refracts sunlight into the shadow.
- The red colour is, in effect, the light of every sunrise and sunset on Earth at once.
- They always occur at full Moon, and they are completely safe to observe unaided.
The geometry
At full Moon the Sun, Earth and Moon are roughly aligned with Earth in the middle. When that alignment is close enough — the Moon near an orbital node — the Moon enters Earth's shadow.
That shadow has two parts. The penumbra is the region where Earth blocks part of the Sun's disc, producing only a subtle dimming that is often hard to notice. The umbra is where Earth blocks the Sun entirely, and it is entry into the umbra that produces a visibly darkened, reddened Moon.
Why the Moon turns red
Inside the umbra no direct sunlight reaches the Moon, so a naive expectation is that it should vanish. It does not, because Earth's atmosphere refracts sunlight around the planet's limb and into the shadow. Along that long grazing path through the atmosphere, shorter blue wavelengths are scattered out and longer red ones survive — the same physics that reddens a sunset.
The light illuminating a totally eclipsed Moon is therefore the combined light of every sunrise and sunset occurring on Earth at that moment. The depth of colour varies with atmospheric conditions: major volcanic eruptions loading the stratosphere with aerosols have produced notably darker eclipses.
Types and duration
- Penumbral: the Moon passes only through the outer shadow. The dimming is slight and easily missed.
- Partial: part of the Moon enters the umbra, producing a distinct dark bite with a curved edge.
- Total: the entire Moon enters the umbra and takes on the characteristic red colour. Totality can last well over an hour.
- Because Earth's umbra at the Moon's distance is several times wider than the Moon, lunar eclipses last far longer than solar ones and are visible from an entire hemisphere rather than a narrow track.
Observing them
No equipment or protection is needed — a lunar eclipse is simply the Moon, and it is dimmer than usual. Binoculars enhance the colour, and the curved edge of Earth's shadow crossing the lunar surface is clearly visible with the unaided eye.
Aristotle used exactly that curved shadow as an argument that Earth is a sphere: only a sphere casts a circular shadow from every direction, whatever its orientation. It remains one of the most direct observations anyone can make of the shape of the planet they are standing on.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- Why does the Moon turn red during a total lunar eclipse?
- Because Earth's atmosphere refracts sunlight around the planet into the shadow. That light travels a long grazing path through the atmosphere, which scatters out shorter blue wavelengths and lets longer red ones through — the same reason sunsets are red. The eclipsed Moon is lit by the combined light of every sunrise and sunset on Earth at that moment.
- Are lunar eclipses safe to look at?
- Completely. You are looking at the Moon, which is dimmer than usual during an eclipse. No filters or protection are needed, and the event is comfortable to watch with the unaided eye, binoculars or a telescope for its full duration.
- Why are lunar eclipses seen by more people than solar eclipses?
- Because Earth's shadow at the Moon's distance is several times wider than the Moon, so the whole night side of Earth sees the same eclipse simultaneously. A total solar eclipse, by contrast, casts a shadow only a few hundred kilometres wide that races across the surface, so only observers within that narrow path see totality.
- Why isn't there a lunar eclipse every full Moon?
- Because the Moon's orbit is tilted about five degrees from Earth's orbital plane, so at most full Moons it passes above or below Earth's shadow. An eclipse requires the full Moon to occur near one of the nodes where the two planes intersect, which happens during eclipse seasons roughly every six months.