12 Noon: a.m or p.m?

By Karthik Uppalari,

What do you consider 12 noon; a.m. or p.m.? Have you ever wondered or
confused? This is a very important question for international travelers. Most
of the international flights leave around midnight and it is a confusing issue
for less experienced travelers. It is an important issue which confuses both
local and international people.

In order to find out the proper way to write 12 noon, we should first learn
about the abbreviations. The abbreviation a.m. stands for ante meridian in
Latin (before the sun has crossed the line or before midday), and p.m. or
post meridian in Latin ( after the sun has crossed the line or after midday).
At 12 noon the sun is at its highest point in the sky and directly over the
meridian. That's why it is neither ante or post meridian, so the proper way
to write 12 midday is 12 Noon, and 12 at night is 12 midnight.

The idea of time is based on the rotation of the earth relative to sun and our
understanding of the day is the length of the time for the earth to rotate
once. The early pendulum clocks were corrected by means of astronomical
observations and they had an accuracy rate of about a second a year.
Later the clocks were replaced by quartz crystal controlled clocks which
used an atomic transition of frequency. The second was redefined and the
clocks were more accurate to one second in thousand of years.

The time measured based on the rotation of Earth on its axis with respect
to stars is called Universal time. Coordinated Universal time(UTC) was
created to keep 0.9 seconds of the time determined by the mean position of
the Sun (UT1). Leap seconds are added to keep the difference between
UTC and UT1 to a minimum. The rotation of the earth is measured by
artificial satellites in earth’s orbit to a precise determination of the location.

Time is measured in many different ways. The measure of time based on
orbital motion of Earth and other planets in the solar system is called
Dynamical Time. The measurement of time based on the rotation of the
Earth on its axis with respect to the stars is called Universal Time. Finally,
the measure o time through the oscillations of atoms is called International
Atomic Time.

The earth rotates to the Sun once a day, but in the course of the year it
makes one extra rotation due to its orbit. This will amount to a difference in
the position of the stars in the sky by about one degree, or four minutes of
time. The clocks run at slightly different rate to show this difference.
Universal Time is based on the mean sidereal time as measured in
Greenwich, England.

Eclipse predictions are presented in terms of Universal time and hours are
added to local time according to Time zone to convert local time to
Universal Time. For Example;

Atlantic Standard Time (AST)    UT - 4 hours
Eastern Standard Time (EST)     UT - 5 hours
Central Standard Time(CST)      UT - 6 hours
Mountain Standard Time (CST) UT - 7 hours
Pacific Standard Time (PST)      UT - 8 hours

The official source of the time used in the United States is the Time Service
Department of the U.S Naval Observatory.

For more information on Time please visit:
http://eclipse.gsfc.nasa.gov/SEhelp/TimeZone.html

Sources:1. NASA - Time Zones and Universal Time
2. Astronomy & time, National Maritime Museum