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