October 11, 2004
Just about everyone has seen parking signs, time schedules, newsletters, etc. handle the notion of 12 o'clock noon or midnight as 12AM or 12PM. Shame shame. Here are the details on why this is incorrect.
First let's lay down the law:
What do a.m. and p.m. mean?
What do we mean when we say the time is 2:46 a.m or 3:51 p.m?
We know that the rotation of the earth about its axis causes day and night. The part of the earth, which faces the sun, has day, and remaining part has night.
In the past the time of day was calculated from the position of the sun in the sky. When the sun is at its highest position, we can draw an imaginary line across the sky, from the north to south. This is called the meridian. It is derived from the Latin word Meridies meaning mid day.
While the sun is to the east of this meridian, it is morning. So a.m. is an abbreviation for antemeridian or before mid-day. After the sun has crossed the meridian, it is after mid-day or afternoon. So p.m. is the abbreviation for post meridian or after midday or afternoon. Hence to express the time from 12 noon to 12 o'clock at night, we use the term p.m. and from 12 o'clock at night to 12 o'clock noon we use a.m.
However 12 o'clock midday is neither ante meridian or post meridian. It is simply 12 Noon.
It is unfortunate that modern electronic clocks choose to display the PM indicator just as it changes from 11:59AM to 12:00Noon. However, the fact they do does not mean they are right.
If you say 12 p.m. you are really saying the 12 o'clock time after midday which would translate to midnight.
If you say 12 a.m. you are really saying the 12 o'clock time before midday which would translate to the midnight of last night.
See the confusion?
A strict interpretation suggests that 12:00 a.m. and 12:00 p.m. both mean midnight as this is the only twelve o'clock time that occurs both before or after noon.
Well, John why don't we just pass a law to say 12:00 p.m. is noon?
We can just as easily pass a law that says cats are really parrots. However, even if that happens, cats will never be parrots. Such attempts at silly laws do occur from time to time such as this Indiana classic from "Fun Facts about the Hoosier State" pamphlet:
Here are some more:
So what to do? Simple.
Use Noon or 12:00 Noon when you mean midday and you will be in good company. The term Noon was first used in the year 1140AD.
Use Midnight or 12:00 Midnight when you mean the middle of the night.
You might notice that most travel schedules avoid the issue completely be never ever saying 12:00. They either add or subtract one minute to display 11:59 or 12:01. More and more street signs are begin changed to avoid 12:00 too.
So you can be the first person in your organization to correctly call mid day Noon and midnight Midnight.
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