Moon Time

Cheryl is sitting at a desk in an office and the clock shows 12:20 and the Moon is seen through the window as a thin crescent with the open side pointing downward to the right. What do you make of this scene? Where could the Sun be?

Answer

The person is at the desk at 12:20 during the noon hour, not at night. The Sun must be at the upper left because the Moon is illuminated from this direction. This daytime Moon is seldom noticed because the sky is normally bright, but the daytime Moon is up as often and as long as the Moon at night. Perhaps the easiest way to appreciate the appearances of the Moon at night and during daylight is to use a lamp for the Sun and two spheres, one representing Earth and the other the Moon. Fix the lamp position and the Earth position, but move the Moon around Earth to observe its illumination phases. Stop the motion at several points in the orbit of the Moon to observe its view from daytime locations on Earth. You also might reconstruct the scene in the diagram.