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Science: Mechanics of Lunar Eclipses



A lunar eclipse happens at a Full Moon, when the Moon's tilted orbit brings it into the Earth's shadow, which can then be seen cast onto the Moon. While not as spectacular as a total solar eclipse, a lunar eclipse is much easier to see; and a total lunar eclipse is an amazing and beautiful sight.

This page attempts to explain how lunar eclipses work, and the different types of lunar eclipse.


Shadow Zones

A lunar eclipse is very different to a solar eclipse in terms of how the effects we see are created, because of our different point of view: in a solar eclipse, we stand at a particular point within the shadow of the Moon, and experience the effects of the shadow at that point; but in a lunar eclipse, we witness the whole of the Earth's shadow falling upon the Moon.

For that reason, the types of lunar eclipses don't correspond exactly to the types of solar eclipses. In addition, the Earth's shadow is much larger than the Moon's -- because the Earth is larger -- so it becomes possible for the whole Moon to be totally eclipsed, as this diagram shows (bear in mind that the scale is exaggerated; the Earth's shadow doesn't really cover a huge part of the Moon's orbit):

The shadow cast by the Earth has two parts:

  • In the penumbra, the light from the Sun is partly blocked by the Earth, but not completely. An observer standing on the Moon within the Earth's penumbra would see part of the Sun obscured; that is, they would see a partial solar eclipse. From Earth, when the Moon passes through the penumbra we see it dimming due to the reduced light, although in practice this can be hard to see with the eye.
  • In the umbra, the light from the Sun is completely blocked by the Earth. Our lunar observer would see a total solar eclipse; we see the Moon darkened, but glowing a dull red from light scattered by the Earth's atmosphere.

As seen from the Earth, the penumbra and umbra form 2 concentric circles, through which the Moon passes during an eclipse. The type of eclipse seen depends on how close the Moon passes to the center of the shadow, as shown here:

The diagrams below illustrate how this works during the different types of a lunar eclipse. As you can see above, a total eclipse is always preceeded and followed by penumbral and partial stages; so the desriptions below of the penumbral and partial eclipse apply to these stages of a total eclipse, too.


Penumbral Eclipse

In a penumbral eclipse, the Full Moon enters the Earth's penumbral shadow. The light from the Earth is partially blocked, and the Moon grows dimmer.

In principle, a penumbral eclipse can be a partial penumbral eclipse (with only part of the Moon in the penumbra) or a total penumbral eclipse, where the entire Moon is in the penumbra; however, most penumbral eclipses are partial, since the penumbral shadow of the Earth is only about as wide as the Moon, so it's rare for the Moon to fit entirely within the penumbra without entering the umbra (and hence making a partial umbral eclipse). Once in a while, though, it happens -- about 1.2% of all lunar eclipses are total penumbral eclipses.

Most penumbral eclipses are pretty uninteresting, since the Moon is still quite brightly lit, except in the most advanced stages. Still, in a deep penumbral eclipse, sharp-eyed observers should see a subtle but distinct shading across the Moon at maximum eclipse. This will be quite obvious in a total penumbral eclipse.


Partial Eclipse

In a partial lunar eclipse, part of the Moon is within the Earth's umbral shadow. From the Earth, we see the Moon partially in shadow, almost as if it wasn't full.

In the later stages of a partial eclipse, as the Moon darkens, red coloration may become visible on the shadowed side of the Moon.


Total Eclipse

A total lunar eclipse is when the Moon is completely shadowed by the Earth. The Moon passes through the Earth's umbra, and no direct light can reach it from the Sun. However, the Earth's atmosphere refracts -- or bends -- light, at the same time filtering it, so that it illuminates the Moon with a dark red colour. Depending on the prevailing condition of the Earth's atmosphere, in terms of cloud cover and dust from volcanic eruptions, the actual colour of the Moon at totality can vary from near black (particularly at mid-totality), to rust, brick red, or bright copper-red or even orange.

As with a solar eclipse, the distance between the Earth and the Moon depends on the position of the Moon within its elliptical orbit; however, due to the large size of the Earth's umbra, the only effect of this is upon the size of the umbra where the Moon passes through it, and therefore upon the duration of the total eclipse.

Now go on to read about the interlocking cycles that dictate when eclipses occur, and why we don't get an eclipse at every Full Moon.

Copyright (C) 1995-2006 Ian Cameron Smith.
visits since 18Aug05. Last modified: Sat May 3 11:44:31 PDT 2008 ($Revision: 1.9 $)