Dorrin K Mace, Horologost

Dorrin K Mace, Horologost
The Clock Man in a pensive moment

Monday, May 27, 2013

Why does the United States use the 12 hour time standard

Actually, the counting of time from the meridian passage of the Sun (accurate time keeping) is a rather recent thing.

For a long time, daytime was divided into 12 parts (2 times 6) because 12 is a number easily divisible by 2, 3, 4 and 6. Makes it easy to assign portions of day for workers, marching armies, and so on.

The first hour began at sunrise and hours were given flexible length so the the 6th hours would finish at noon (meridian passage of the Sun) and the 12th at sunset.

The night hours were not numbered for a long time. The Romans are often thought to be the ones who invented the hours of the night in order to assign watches for the night guards. They used a 12 hours system to mirror the daylight system.

Thus, 12 hours in the day and 12 hours at night. Hours were given as, for example, the fifth hour of the day or the tenth hour of the night. In this system, there was never a number bigger than 12.

The Roman numbering systems (for distances, currency as well as time) spread out to the rest of the western world and their impact was felt the longest in England (therefore, on English speaking countries) where, for example, currency was divided in 12 and 20, feet had 12 inches and so on.

So the habit of counting the hours of the day in two bunches of 12 hours dates back to the Romans. When the start of the day was moved to midnight (before that, the day "started" at sunset of what we now consider the eve), the numbering scheme was moved along with it so that the first hour of the first group began at midnight and the first hour of the second group began at mid-day.

Like the numbering of years, hours were given ordinal numbers (there was no such thing as 0 h). The very first second of daylight was the start of hour number 1. You have to wait for the end of hour 1 for that hour to have elapsed.

When cardinal systems were being introduced, the hours of the day were numbered from 0:0:0 to 23:59:59, starting at the "lower meridian passage" of the Sun (12 hours away from noon, a.k.a. mid-night).

The definition of Meridian Passage is the simplest calculation of astronomical navigation and is when an observer determines his latitude by measuring the altitude of a heavenly object at the time of its meridian passage. Meridian passage is when the object passes the observer's meridian, i.e. passes through south or north. This is usually done with the sun for a noon latitude but can be done with any heavenly object. Noon is when the sun passes through the meridian.

So why does the United States use the 12 hour standard in telling time?  The answer is simple, that is what we are used to doing and change does not come easy for US.  (If you remember the Carter administration and the attempted adoption of the metric system, you will understand)

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