|MadSci Network: Astronomy|
Good question. It's always important to ask how scientists came up with the numbers that we throw around so often. The short answer to your question is: they calculated it. This means that the -400 degrees farenheight number is an estimate. Actually, the number I've seen is -382F (-230C). Absolute zero, the coldest possible temperature, is -460F (-273C).
The reason it must be extremely cold there is that the bottom of the deep, deep crater (>10km down, I think) never ever gets any sunlight. From the south pole of the moon, the sun is always just on the horizon, so a deep hole will always be in shadow.
Your subject heading mentioned the "dark side" of the moon. This refers to what is more accurately called the Far Side of the moon, because it is the side facing away from us. Because of tidal forces from Earth's gravity, the moon has been "locked in," such that only one side ever faces the earth. This does not mean that the far side is always dark. During a full moon, when the near side is lit up, the far side is of course dark; however, during the new moon phase, the so-called "dark side" is experiencing full daylight. One day on the moon is 28 earth-days long, exactly the same as the time for the moon to orbit the earth, because the two motions are locked.
The temperature on the surface of the moon generally ranges from 265F (130C) in sunlight to -170F (-110C) in darkness, because there is no air to hold in the heat like here on earth. Note that the high temperature is even above the boiling temperature of water, which is why it is impossible for water to exist anywhere on the moon that is touched by sunlight.
I assume the astronomers studying the ice on the moon used these values, which have been measured with instruments on the Apollo missions, along with guesses at how well the rock of the moon's surface would transmit heat, based on measurements of moon rock samples brought back to earth, to calculate the temperature they'd expect in an area of the moon that gets no heat directly from the sun.
For a great series of pictures and diagrams that should help visualize why this crater never gets sunlight, check out this web site: http://www.soest.hawaii.edu/PSRdiscoveries/Dec96/IceonMoon.html It also contains plenty of info on how they discovered the (possible) ice.
Jay H. Hartley
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