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AM/PM Mini-explanation
Observing >
Some of the most common words and phrases we use are based on phenomena in the skies above. All the days of the week, some months and seasons, even the names of some geographic features of our globe, can find their origins in the heavens.
And our common terms of telling time, specifically the ubiquitous AM and PM, are also based on the movements of the sun above our heads.
For most of us, AM and PM are quick and simple ways of indicating morning and afternoon or evening. But it goes a little deeper than that. Ready?
AM stands for ante meridiem, Latin for "before the middle of the day." PM is for post meridiem, "after the middle of the day."
So far, so good. Before noon, after noon - no big deal. But what exactly is the middle of the day? And is my middle of the day the same as yours? If not, how does that change things?
It takes no great strain of the imagination to see the middle of the day as the time when the sun is highest in the sky, essentially dead between sunrise and sunset. But let's be more precise.
Imagine splitting the sky with a line from true north all the way over your head down to true south. We could call it the meridian, the midday line, and that's what astronomers call it. It is the border between AM and PM. Before the sun gets there, before the meridian, we have AM. When the sun gets there, we have midday or "local noon." Afterwards, it is PM. But, as we'll see, this noon is not the noon we are accustomed to.
This whole midday line and how the sun moves through it is one of the foundations for the old sundial way of timekeeping. But the old observational ways of telling time pose a great problem in the modern world, because - cue dramatic music - not everyone's meridian is the same!
It is easy to see that when the sun reaches midway in an Arizona sky, it still has a ways to go before it reaches midway in a California sky. More dramatically, the sun at the meridian in San Diego still has 5 minutes of sky-crossing before it reaches meridian in Los Angeles. Bottom line: Go east or west and your time changes.
Imagine living back 150 years when timekeeping was indeed important, but pretty much only at the local level. You live in a village or city, and most of your life was spent doing business there. You set your watch or clock by the local noon, a time determined by the sun in your own local sky. All is well.
But then the train is introduced to modern civilization and suddenly great distances are covered in short order. Now you go a hundred miles to the east or to the west in a day and the sun isn't exactly moving with your watch. Because of this, you must constantly reset your timepiece to match the local time.
Some clever people at the end of the 19th century saw a big mess coming as more people traveled, and came up with the idea of time zones, huge areas locked into a certain time regardless of when the sun was crossing the local meridian.
Of course this move saved a lot of time adjusting timepieces, and meant people in different cities could synchronize when to meet or call or plot. But it also meant that noon was no longer true noon, AM and PM had lost their original meanings, and sundials went the way of the dodo. Scheduling a duel at "high noon" was a thing of the past.
Again we see that the great lights in the heavens, especially that bright one that rules the day, has far-reaching effects in nearly every facet of our lives together down here, even if the effect is slightly modified by man. Timekeeping, temperature, climate, weather, ocean currents, seasons, a host of astronomical influences, and of course life itself can find a common starry thread in the sun.
Until next time, clear skies!
Posted by Mark Ritter at 2007.07. 8 10:34 AM | Comments (0)
