Expert: Courtney Seligman - 4/18/2009

Question
At times, you can look into the sky and see the Sun and the moon at the same time.  It's almost as if you could imagine all three components (you, the Sun and the moon), each on different legs of a tripod.  It would stand to reason that if you can see the sun and the moon in the same celestial dome with nothing in between the two, that the Sun's rays should reflect off the moon and you should see a full profile image of the moon.  Sometimes though, you only see a sliver of the moon... why?

Answer
The portion of the Moon that you see lit depends upon the difference between the angle you look at it, and the angle the Sun looks at it. If the Moon is on the opposite side of the Earth from the Sun, both Earth and Sun look at it from the same direction, and see it fully lit. This occurs at full moon.

At other phases the Moon is off to the side, so to speak, of the direction opposite the Sun, and since we look at it from a different direction than the Sun, we see part of the day side and part of the night side. This corresponds to all phases other than full moon.

It doesn't make any difference, at these other than full phases, whether you see the Moon at night or during the day. As long as the angle between the Earth, Moon and Sun is the same, the shape the lit part of the Moon appears to have is the same. For instance, if the Moon was a quarter way round the sky from the Sun, as it is at first and last quarter, we would see half the Moon lit (the side toward the Sun), and half dark (the side away from the Sun). At such times the Moon would be up half of the day, and half of the night (rising at noon and going down at midnight at first quarter, and rising at midnight and going down at noon at last quarter), but whether you saw it in the daytime or at night, it would still be half lit, as seen from anywhere on the Earth.