The Solar System

36 PART 1^ |^ EXPLORING THE SKY

To sun

Visual

During a total lunar eclipse,

the moon takes a number

of hours to move through

Earth’s shadow.

(Not to scale)

Orbit of

moon

Umbra Penumbra

Motion of moon

A cross section of

Earth’s shadow shows

the umbra and penumbra.

Sunlight scattered from Earth’s

atmosphere bathes the totally

eclipsed moon in a coppery glow.

see a total lunar eclipse (■ Figure 3-3). As you watch the eclipse
begin, the moon fi rst moves into the penumbra and dims
slightly; the deeper it moves into the penumbra, the more it
dims. Eventually, the moon reaches the umbra, and you see the
umbral shadow fi rst darken part, then all, of the moon.
When the moon is totally eclipsed, it does not disappear
completely. Although it receives no direct sunlight, the moon in
the umbra does receive some sunlight that is refracted (bent)
through Earth’s atmosphere. If you were on the moon during
totality, you would not see any part of the sun because it would
be entirely hidden behind Earth. However, you would see Earth’s
atmosphere illuminated from behind by the sun. Th e red glow
from this ring of “sunsets” and “sunrises” illuminates the moon
during totality and makes it glow coppery red, as shown in
■ Figure 3-4.
How dim the totally eclipsed moon becomes depends on a
number of things. If Earth’s atmosphere is especially cloudy in
the regions that would bend light into the umbra, the moon will
be darker than usual. An unusual amount of dust in Earth’s
atmosphere (from volcanic eruptions, for instance) also causes a
dark eclipse. Also, total lunar eclipses tend to be darkest when
the moon’s orbit carries it through the center of the umbra.

Th e exact timing of a

lunar eclipse depends on

where the moon crosses

Earth’s shadow. If it crosses

through the center of the

umbra, the eclipse will

have maximum length.

For such an eclipse, the

moon spends about an

hour crossing the penum-

bra and then another hour

entering the darker umbra. Totality

can last as long as 1 hour 45 minutes,

followed by the emergence of the

moon into the penumbra, plus another hour as it

emerges into full sunlight. A total lunar eclipse can

take nearly six hours from start to fi nish.

Partial and Penumbral Lunar

Eclipses

Because the moon’s orbit is inclined by a bit over 5°

to the plane of Earth’s orbit, the moon does not

always pass through the center of the umbra. If the moon passes a

bit too far north or south, it may only partially enter the umbra,

and you see a partial lunar eclipse. Th e part of the moon that

Visual

■ Figure 3-3

In this diagram of a total lunar eclipse, the moon passes from right to left
through Earth’s shadow. A multiple-exposure photograph shows the moon
passing through the umbra of Earth’s shadow. A longer exposure was used to
record the moon while it was totally eclipsed. The moon’s path appears curved
in the photo because of photographic effects. (© 1982 Dr. Jack B. Marling)

■ Figure 3-4

During a total lunar eclipse, the moon turns coppery red. In this photo, the

moon is darkest toward the lower right, the direction toward the center of

the umbra. The edge of the moon at upper left is brighter because it is near

the edge of the umbra. (Celestron International)