CHAPTER 3 | CYCLES OF THE MOON 33
■ Figure 3-1
In this sequence of lunar phases, the moon cycles through its phases from crescent to full to crescent. You see the same face of the moon,
the same mountains, craters, and plains, but the changing direction of sunlight produces the lunar phases.
Lunar Eclipses
In cultures all around the world, the sky is a symbol of order
and power, and the moon is the regular counter of the passing days.
So it is not surprising that people are startled and sometimes worried
when they see the moon grow dark and angry-red. Such events are
neither mysterious nor frightening once you understand how they
arise. To begin, you can think about Earth’s shadow.
Earth’s Shadow
Th e orbit of the moon is tipped only a few degrees from the plane
of Earth’s orbit around the sun, and Earth’s shadow points directly
away from the sun also in the plane of Earth’s orbit. A lunar eclipse
can occur at full moon if the moon’s path carries it through the
shadow of Earth. Th at is unusual because most full moons pass
north or south of Earth’s shadow and there is no eclipse. If the moon
enters Earth’s shadow, sunlight is cut off , and the moon grows dim.
Th e shadow consists of two parts (■ Figure 3-2). Th e umbra
is the region of total shadow. If you were drifting in your spacesuit
3-2 in the umbra of Earth’s shadow, the sun would be completely hid-
den behind Earth, and you would not be able to see any part of
the sun’s bright disk. If you drifted into the penumbra, however,
you would see part of the sun peeking around the edge of Earth,
so you would be in partial shadow. In the penumbra, sunlight is
dimmed but not extinguished.
Th e umbra of Earth’s shadow is over three times longer than
the distance to the moon and points directly away from the sun.
A giant screen placed in the shadow at the average distance of the
moon would reveal a dark umbral shadow about 2.5 times the
diameter of the moon. Th e faint outer edges of the penumbra
would mark a circle about 4.6 times the diameter of the moon.
Consequently, when the moon’s orbit carries it through the umbra,
it has plenty of room to become completely immersed in shadow.Total Lunar Eclipses
Once or twice a year, the orbit of the moon carries it through the
umbra of Earth’s shadow; and, if no part of the moon remains
outside the umbra in the partial sunlight of the penumbra, you■ Figure 3-2
The shadows cast by a map tack resemble the shadows of Earth and the moon. The umbra is the region of total shadow; the penumbra is the region of partial shadow.
UmbraPenumbraLight source Screen closeto tack Screen farfrom tack