| MadSci Network: Astronomy |
Fortunately, this question is one of the few astronomy questions that can be answered with a simple experiment by anybody.
Go into a room lit by only one lamp. We're going to pretend the lamp is the Sun. Place a basketball, volleyball, or any kind of ball on the floor. We'll pretend that this ball is the Earth. The reader gets to play the role of the Moon.
How is a day defined? Essentially, a day is the interval from when the Sun is overhead (i.e., noontime) to the next time the Sun is overhead. (One could choose other ways, for instance, when the Sun is rising, the Sun is setting, etc. Choosing the Sun to be overhead is traditional and often easiest to observe.)
Stand between the Earth and Sun, facing the Sun. This is now lunar noontime, because one is facing the Sun. To the inhabitants of the Earth, they see the New Moon, because no light is being reflected from the Moon to the Earth.
There is one other important fact we'll need. The Moon always keeps the same side toward the Earth.
The Moon orbits the Earth taking one month to do so. Now, the Moon should start walking around the Earth, being certain to always keep the same side toward the Earth.
How long is it before the Moon (the reader) is once again facing directly at the Sun?
[You'll find an apparently curious relationship between the month and the lunar day. This is not a coincidence. For more reading, the reader will want to consult a previous MadSci answer about the Earth and Moon.]
Try the links in the MadSci Library for more information on Astronomy.