Lunar Eclipses: How They Happen

A simulation of a total lunar eclipse, using Stellarium open source software. During totality, the Moon can take on a reddish-brown color. Carolyn Collins Petersen

The Essential Lunar Eclipse

Lunar eclipses are amazing celestial events to watch. They occur when Earth passes directly between the Sun and the Moon. This means that lunar eclipses can only happen during a full Moon at certain points in the Moon's orbit. During the event, which takes several hours, Earth blocks direct sunlight from reaching the lunar surface, although the Moon can still be seen in the faint reflected light. People often wonder why they can still see the Moon during the eclipse. That's because some of the Sun's light can still reach the Moon's surface during an eclipse because of the bending of the light around Earth.

In some eclipses, light from the Sun can actually be bent by Earth's atmosphere, casting the Moon in a reddish/brown or orangish color. Other eclipses block a percentage of the Sun's rays, making the Moon appear darker. Some are a combination of the two phenomena.

Eclipses happen as a result of the Moon's orbit around Earth, and the pair's orbit around the Sun. When all three happen to line up, then an eclipse can occur. The same orbital mechanics are responsible for the phases of the Moon. These are the different shapes the Moon appears to take on throughout a month. 

The Parts of Lunar Eclipses

Earth itself is casting a shadow, broken up into two distinct parts: the umbra is the portion of the shadow that does not contain any direct radiation from the Sun. The focus of the umbra is the point at which all three celestial bodies' shadows are properly aligned. Even so, the eclipse does not completely darken the Moon. Light from the Sun can actually be refracted through the Earth's atmosphere and find its way to the Moon. That refraction separates sunlight into individual colors. The more directly aligned Earth, Moon and Sun are the more reddish the Moon appears in an eclipse.

When the Moon is fully contained within the umbra, the Moon is said to be in total eclipse. This event can last nearly two hours, while the Moon can be in at least partial eclipse for nearly four hours.

The penumbra is the region of space where Earth is only partially blocking the light from the Sun. As the Moon moves from outside the shadow toward the umbra, the Moon starts to appear darker. 

Normally the Moon will lie only partially in the penumbra area (known as a penumbral eclipse), but occasionally, the Moon will find itself completely in the penumbra. These events, called total penumbral eclipses, are rare. They can immediately precede or follow a partial eclipse, where the Moon is partially in each of the umbral and penumbral regions.

The Danjon Scale of Lunar Eclipse Brightness

To classify what kind of lunar eclipse is occurring in a given event, astronomers use the Danjon scale. Essentially an L value is determined based solely on the appearance of the Moon. Using only the naked eye, the observer estimates into which category the eclipse falls:

  • L=0, This represents the darkest eclipse and is probably what most people imagine when they think of a lunar eclipse.
  • L=1, While still very dark, there is a grey or brown hue to the Moon. However, details of the Moon are still difficult to identify.
  • L=2, During this type of eclipse the Moon will appear dark red, possibly with a slight hint of orange. The Moon still appears very dark at this value.
  • L=3, The Moon is now brick-red and noticeably lighter than the previous value. Also, the edges can appear lighter, possibly with a yellowish hue.
  • L=4, The Moon now appears bright red or orange, while the edge of the Moon appears almost bluish

The Danjon scale is highly subjective and different people observing the same eclipse can arrive at different L values. So, it's not very precise, but typically it yields a fairly good idea of what kind of eclipse you are observing.

When Is the Next Lunar Eclipse?

There are always at least two lunar eclipses per year. However, these are sometimes penumbral eclipses which can be difficult to see because the Moon simply appears slightly darker. And given atmospheric conditions, no noticeable difference may be apparent.

Total and partial eclipses are the rarer types. Typically there are anywhere from zero to three total or partial eclipses each year. To determine when the next eclipse will occur, NASA has put together a handy online tool, which tells the date and time of the next lunar eclipse for any location on Earth. Since lunar eclipses don't involve looking directly at the Sun, they're much safer to watch.  For many eclipse watchers who are also photographers, eclipses give great opportunities for some spectacular images. 

Edited and updated by Carolyn Collins Petersen.