Lunar Eclipse: How It Happens and the 3 Types
A lunar eclipse happens when the Earth passes between the Sun and the Moon, so the Moon moves into Earth’s shadow. It can only occur at full Moon. There are three types: total, partial and penumbral. During a total eclipse the Moon turns red because Earth’s atmosphere bends red sunlight onto it.
What is the definition of a lunar eclipse?
A lunar eclipse is the dimming of the Moon when it passes through the shadow cast by the Earth. It needs three bodies in a straight line, in this order: Sun, Earth, Moon. Only the night side of the Earth sees it, so anyone on that side with a clear sky can watch, and no eye protection is needed.
How does a lunar eclipse happen?
A lunar eclipse follows a short sequence:
- The Earth blocks sunlight and casts a long shadow into space, on the side facing away from the Sun.
- The Moon orbits the Earth once about every 29.5 days, and at full Moon it is on the opposite side from the Sun.
- If the Moon’s path takes it through the shadow, the eclipse happens.
- The Moon moves on, leaves the shadow and returns to full brightness.
The shadow has two parts. The umbra is the dark, central cone where the Sun is completely blocked, and the penumbra is the fainter outer region where the Sun is only partly blocked.
What are the types of lunar eclipse?
| Type | What happens | What you see |
|---|---|---|
| Total lunar eclipse | The whole Moon passes through the umbra | The Moon turns deep orange or red (a "blood moon") |
| Partial lunar eclipse | Only part of the Moon enters the umbra | A dark curved "bite" covers part of the Moon |
| Penumbral lunar eclipse | The Moon passes through the penumbra only | A subtle dimming, hard to notice |
A total eclipse can have totality (the full red phase) lasting up to about 1 hour 47 minutes. The whole event, from the first touch of the shadow to the last, takes several hours.
Why is the Moon red during a lunar eclipse?
Even in the middle of the umbra the Moon does not go completely dark. Sunlight passes through the atmosphere around the edge of the Earth. The air scatters away the blue light, in the same way that it makes a sunset look red, and bends the remaining red and orange light inwards, onto the Moon.
In effect, the Moon is lit by all the sunrises and sunsets around the Earth at once. The exact shade depends on the amount of dust and cloud in the atmosphere. After a big volcanic eruption the Moon can look much darker.
Why isn't there a lunar eclipse every month?
If the Moon’s orbit were in the same plane as the Earth’s orbit round the Sun, there would be an eclipse at every full Moon. But the Moon’s orbit is tilted by about 5°. Most months, the full Moon passes above or below the Earth’s shadow and nothing happens.
An eclipse is only possible when a full Moon is near one of the two points where the Moon’s orbit crosses the Earth’s orbital plane, called nodes. Counting partial and penumbral events, this happens about two to four times a year, although not every one is total. To see how tilt shapes the sky, try Seasons and Earth's tilt.
Lunar eclipse vs solar eclipse
| Lunar eclipse | Solar eclipse | |
|---|---|---|
| Order | Sun, Earth, Moon | Sun, Moon, Earth |
| Moon phase | Full Moon | New Moon |
| What is blocked | Sunlight reaching the Moon | Sunlight reaching the Earth |
| Who can see it | Everyone on the night side | Only a narrow strip of Earth |
| Safe to look at | Yes | No, needs eclipse glasses |
| Duration of totality | Up to about 1 h 47 min | Up to about 7 min 30 s |
Worked example: how long is the Moon in the shadow?
The Earth’s umbra at the Moon’s distance is about 9,200 km across, roughly 2.6 times the Moon’s width of 3,474 km. The Moon moves through the shadow at about 1 km/s. Totality lasts while the whole Moon is inside the umbra, so the Moon can travel about 9,200 − 3,474 = 5,700 km during it. At 1 km/s, that is 5,700 s, or about 1.6 hours, which matches the longest totalities of about 1 h 47 min.
Common mistakes
- Saying that the Moon turns red because the Earth’s shadow is red. The shadow is dark, and the red light comes from the atmosphere.
- Mixing up lunar and solar eclipses. In a lunar eclipse the Earth is in the middle.
- Thinking that lunar eclipses are dangerous to the eyes. They are not.
- Forgetting that it can only happen at full Moon.
Frequently asked questions
What is a lunar eclipse?
A lunar eclipse is an event in which the Moon moves into the Earth’s shadow, so it dims or turns red. It happens when the Sun, Earth and Moon are in a straight line, with the Earth in the middle, and always at full Moon.
How many types of lunar eclipse are there?
There are three: total (the whole Moon is in the umbra), partial (part of it is) and penumbral (only the faint outer shadow). Total eclipses are the most dramatic.
Why is the Moon red during a lunar eclipse?
Earth’s atmosphere scatters blue light away and bends red light round the edge of the planet onto the Moon. So the Moon is lit only by red, sunset-coloured light, giving it a copper or blood-red look.
Is it safe to look at a lunar eclipse?
Yes. The Moon is only reflected light, and it is not brighter than normal during the eclipse. You can watch with the naked eye, binoculars or a telescope. A solar eclipse is different and needs proper eye protection.
Try it yourself
Open Gravity & Orbits, set the Sun, Earth and Moon in motion and pause when the Moon is directly opposite the Sun. Then think about why there is not an eclipse every time this happens, and what the Moon’s tilted orbit does.
See it for yourself, in 3D and free.
Try it in the Gravity & Orbits