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.
