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.?