Susan Phelps, age 11, of Richmond, Virginia, for her question:

Is 12 o'clock midnight a.m. or p.m.?

The moment of midday needs no "a.m." or "p.m."; neither does the moment of midnight. We refer to them by their full titles as 12 o'clock midday and 12 o'clock midnight. Actually, they are fine lines that divide the 24 hour day into two equal halves. Both are based on the meridian, the imaginary line in the sky that the noon sun crosses as it reaches its highest point. The "a.m." stands for "ante meridian", or "before meridian". "The "p.m." stands for "post meridian", meaning "after meridian".

Greenwich Time is based on the Prime Meridian, which is Longitude 0 degrees, running down through England from pole to pole. According to this system, 12 o'clock occurs when the sun crosses the matching Prime Meridian in the sky. This divides the day down the middle. At this moment, a new calendar day is born along the International Date Line, along Longitude 180 degrees, on the opposite side of the globe.

Wherever you are, all the pre noon hours are a.m. and all the after noon hours are p.m.