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Horology: What is A.M. and P.M.?


I have many things that fascinate me (as I hope you do too), and one of them is the study and measurement of time, known as horology.  Previously we’ve discussed the origin of the days of the week and a bit about the history of the calendar, but today I’d like to focus on something we all reference every day – a.m. and p.m.

Understanding what a.m. and p.m. are all about starts with understanding a useful imaginary line that astronomers (and horologists) draw across the sky called the meridian.  The meridian connects due south to due north through the top of the sky; so the meridian basically splits the sky into an eastern half and a western half.  For all astronomical objects in the sky that are in the eastern half, because of the rotation of the Earth those objects are rising up to the meridian, and for those objects in the western half they are moving down from the meridian.  So this means that when any of those objects, the Sun, the Moon, the stars, the planets, cross the meridian, that will be the highest they are in the sky all day.  A.m., p.m., and how we set our clocks are based upon specifically the location of the Sun relative to the meridian.  When the Sun crosses the meridian, that is deemed to be “noon” – thus the term “high noon”.  In the morning hours, the Sun is at various locations that are a certain amount of time “before the meridian”.  In Latin, this translates to “ante meridian” we shorten to “a.m.”.  And when the Sun crosses the meridian, it moves to locations where it is a certain amount of time “post meridian”, or “p.m.”


Follow your curiosity to the Fred G. Dale Planetarium at Wayne State College.

Dr. Todd Young hails from Minnesota and received his undergraduate degree in Physics & English from the University of Minnesota – Morris, his Master’s degree in Physics from Purdue University, and his Ph.D. from the University of Nebraska – Lincoln in Astrophysics. He has worked at Wayne State College since receiving his doctorate in 1998 and is currently a full professor of physics and astronomy. He teaches a variety of courses at Wayne State College, including university physics, astronomy, general education science, and astrophysics.
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