The geometry
Earth casts a long shadow into space, pointing directly away from the Sun. Like any shadow cast by a broad light source, it has two parts:
- The penumbra, a faint outer zone where only part of the Sun is hidden from view.
- The umbra, a dark inner cone where Earth blocks the Sun completely.
At the Moon’s distance, the umbra is roughly two and a half to nearly three times as wide as the Moon. A total lunar eclipse happens when a full Moon lines up closely enough with Earth and the Sun to pass entirely inside it. Because the Moon’s orbit is tilted about 5 degrees to Earth’s orbit, this alignment does not happen at most full Moons; see types of eclipses for the role of eclipse seasons.
The phases of a total lunar eclipse
A total lunar eclipse unfolds in a fixed sequence, described by contact points:
| Contact | Phase | What it looks like |
|---|---|---|
| P1 | Penumbral eclipse begins | Nothing visible at first |
| U1 | Partial eclipse begins | A dark bite appears on one edge |
| U2 | Totality begins | The last bright sliver disappears |
| Greatest eclipse | Middle of totality | Usually the darkest, reddest moment |
| U3 | Totality ends | A bright edge reappears |
| U4 | Partial eclipse ends | The bite slides off |
| P4 | Penumbral eclipse ends | The Moon is back to full brightness |
The partial phases before and after totality each typically last about an hour. Including the faint penumbral stages, the whole event can stretch over five hours or more.
How long totality lasts
The length of totality depends on how close the Moon passes to the center of the umbra. A Moon that crosses near the center can stay in totality for well over an hour and a half, while one that only just fits inside the umbra near its edge may be total for just a few minutes. Totality cannot exceed about 1 hour 47 minutes.
Totality is longer when the Moon is near its farthest from Earth, because it then moves more slowly along its orbit and takes longer to cross the shadow.
The color during totality
Earth’s atmosphere bends sunlight into the umbra, and filters out most of its blue light on the way. That reddened light gives the eclipsed Moon its glow, which is why a total lunar eclipse is popularly called a blood moon (see what is a blood moon). The shade ranges from bright orange to almost black depending on dust, clouds and volcanic aerosols in the atmosphere. The details are in why is the moon red.
The Moon is often not evenly colored. The side closer to the center of the umbra is usually darker, while the side near the umbra’s edge can stay brighter, sometimes with a bluish or whitish tint.
How often it happens
Total lunar eclipses occur worldwide a little less than once a year on average, but irregularly. Some years have two, and runs of several years can pass with none. Since each one is visible from about half the planet, you are likely to have a chance to see one every few years from wherever you live, if the weather cooperates. The eclipse calendar lists upcoming eclipses and where they are visible.
Watching a total lunar eclipse
No eye protection is needed: the eclipsed Moon is completely safe to look at, with or without binoculars or a telescope. During totality, the darkened sky makes fainter stars visible around the Moon, which is a rare sight on a full Moon night. For planning tips, time zone conversion and photography advice, see how to view a lunar eclipse.