Online Astronomy eText: The Sky
Phases of the Moon Link for sharing this page on Facebook
(introduction, to be greatly expanded; but also see The Motion of the Moon)
      As previously discussed, the Moon moves to the east among the stars once every 27.3 days, along a path which is tilted by about 23 1/2 degrees, on the average, relative to the Celestial Equator. It also moves relative to the Sun once to the east every 29.5 days, because its eastward motion (about 13 degrees per day) is much faster than the Sun's eastward motion (about 1 degree per day). This relative motion changes the portion of the Moon which we see lit, but does not change the portion turned toward us, which is always the same (compare the seas shown in the crescent and new moon photos below).

Crescent Moons
      A brief examination of the images below might give the impression that they are identical, because the crescents have the same shape, the portion of the Moon lit by earthlight looks about the same (the same side always being turned toward us), and the Moon is in the same place, just to the east of the Pleiades, a star cluster in Taurus. But they are not at all the same, as discussed in the caption between them. (Much, much more to follow)
An image of the waning crescent Moon and Pleiades rising on the eastern horizon
Above (Vincent Jacques, apod060629) and below (Jerry Lodriguss, apod050414), two views of the Moon and the Pleiades. What is the difference between these two photos? In each case, the Moon is on the eastern side of the Pleiades and is a relatively thin crescent; but in the first image the Moon is lit on the right, and in the second image it is lit on the left. This difference means that the earlier image (below) was taken in the evening, when the Moon was a waxing crescent, east of the Sun in the sky and "growing" each night as its elongation (distance from the Sun) increased. The later image (above) was taken before dawn, when the Moon was a waning crescent, west of the Sun in the sky and "shrinking" each night as its elongation decreased. (Note: Since the Moon was rising in one case and setting in the other case, the top image had to be rotated 90 degrees to make them appear so similar. If the waning crescent Moon had really been lit on the upper left, the Sun would have been up and to the left of it, which means it would have been daylight, and the Pleiades would not have been visible.)
An image of the waxing crescent Moon and Pleiades setting on the western horizon

New Moon
An image of the new moon during a total solar eclipse, showing that the side facing us is the same as at every other phase
The new moon of July 22, 2009 produced a total eclipse of the Sun. Aside from giving an excellent view of the Sun's outer atmosphere, this composite image faintly shows the near side of the moon, illuminated by earthlight (the light reflected from a full Earth as seen from the Moon). Note that the features visible on the lunar surface are the same as always, since the Moon always keeps the same face to the Earth. (Image reproduced by permission of © holder: Miloslav Druckmüller, Peter Aniol, Vojtech Rušin, Lubomir Klocok, Karel Martišek, Martin Dietzel)

Full Moon
An image of the full moon
A gorgeous view of the full moon of December 22, 1999. (Robert Gendler, apod000113)