For the next few nights, the moon will be a narrow crescent
in the western sky after sunset.
New moon, when the moon is between the Earth and the sun,
was on Nov. 16, and the moon is now moving towards first quarter on Nov. 24.
The moon is mostly backlit by the sun at present, so that
only a thin crescent is in full sunlight. If you look closely at this crescent,
you should be able to see the rest of the moon, the full disk, illuminated only
by the ghostly light reflected onto it by the Earth. This is known as earthshine or
earthlight.
The most obvious feature on the sunlit crescent is a dark
grey oval towards the top, the Mare Crisium or "Sea of Crises," 355 miles (570
km) in diameter.
The large open plains on the moon were named "seas" by the
early astronomers, who didn't know that there was no open water on the moon.
This "sea" is actually a huge scar resulting from the impact of an asteroid
billions of years ago in the moon's history. If you look at it with a small
telescope, you'll see that it bears the scars of many more recent falls by
smaller asteroids.
An interesting thing about the Mare Crisium is that it
changes shape noticeably depending on the libration of the moon. Because of its
elliptical orbit, the moon doesn't quite keep the same face always turned
towards the Earth. As it speeds up and slows down in its orbit, it shows more,
or less, of the area close to its edge, as seen from Earth.
The Mare Crisium is actually a circular basin, but appears
to us as an oval because of its location "around the bend" of the moon's
surface. As the moon rotates toward and away from us, this changes Crisium's
proportions from month to month. At this time this month the moon's libration
is close to zero.
If you have a telescope, scan the moon's terminator, the
boundary between sunlight and shadow. About half way from the Mare Crisium to
the moon's south pole is the large crater Petavius. This crater is 110 miles in
diameter (177 km) and has a prominent central peak. It also has a beautiful
rille on its floor 50 miles (80 km) long. This channel, probably a collapsed
lava tube, winds from one crater wall to the central peak, and then back to the
crater wall.
If you're looking at the crescent moon with binoculars or a
telescope, be sure to check out the part of the moon in shadow, lit by
earthshine. Most of the familiar features visible at full moon are also visible
in ghostlike form on the earthlit moon. Look especially for the bright crater
Tycho, with its striking system of bright rays radiating from it over the
moon's surface. The earthlit part of the moon is sometimes called "the old moon
in the new moon's arms."
This
article was provided to SPACE.com by Starry Night Education, the
leader in space science curriculum solutions.
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