Over time, people have used a number of ways to measure the hours in each day. Astronomers measure time by the earth's rotation in relation to the stars. The sundial was one of the earliest ways of measuring time. It shows the time of day by shadows cast on a circular surface. Early peoples used ropes with knots tied at regular intervals or candles marked with regularly spaced lines. An hourglass or sandglass tells time by means of sand trickling through a narrow opening. A water clock, or clepsydra, measures time as water drips slowly from one marked container into another.
People in the Middle Ages believed flowers opened at specific times of the day, so they used flowers to mark hours. Rosebuds opened in the first hour. Hyacinths opened in the fourth hour. Pansies opened in the 12th hour. Some parks and gardens still have flower clocks.
Sailors on ships mark time in "watches" and "bells." A watch is made up of four hours. There are two "short watches," from 4-6 p.m. and 6-8 p.m. Every half-hour in a watch is marked by a bell. For example, 12:30 p.m. would be one bell and 1:00 p.m. would be two bells. The bells ring eight times during a four-hour watch.
Military groups divide the day into 24 hours. They begin counting at 1 a.m. — which is written as 0100 and pronounced "oh-one hundred." 2 a.m. is 0200. 1:00 p.m. is written 1300 and pronounced "thirteen hundred hours." Midnight is 2400 — "twenty-four hundred hours." Minutes between the hours are marked, too. For example, 4:30 p.m. is 1630, pronounced "sixteen thirty." Many scientific groups and some computers also use the 24-hour military time. The abbreviation for ante meridian, "before noon," is a.m. The abbreviation for post meridian, "after noon," is p.m.
Since the sun is not seen at the same place in the sky throughout the world at any one time, an international conference in 1884 established 24 world time zones, one for each hour of the day. The starting point is the meridian of longitude that passes through the Greenwich Observatory in England and is known as the prime meridian. Time in Greenwich is called Greenwich Mean Time.
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Twelve time zones run west and 12 run east. The 12th zone is divided in half, and the imaginary line through the center of the 12th zone is called the International Date Line. A traveler going east gains a day when he crosses that line, while a traveler going west loses a day. All the other zones are an hour apart, with zones to the west, earlier, and zones to the east, later. Zones were given irregular boundaries so that neighboring communities could be on the same time. A few places, such as polar regions with weeks of constant light or darkness, do not have standard time zones.
At one time in the United States, each locality set its own time by the sun. Railroads established railroad time along their lines to simplify their schedules, but they differed from one railroad to another. In 1883, railroads divided the United States into four standard time zones, each 15 degrees longitude apart. Each has a central meridian Eastern, 75 degrees west; Central, 90 degrees west; Mountain, 105 degrees west; and Pacific, 120 degrees west.
Today there are six zones in the United States and Canada, with the addition of the Atlantic Time Zone, 60 degrees west, and the Alaska Time Zone, 135 degrees west. Hawaii is in the Hawaii-Aleutian Time Zone, 150 degrees west. The Newfoundland Time Zone is separate. It's only one-half hour away, and it does not have a central meridian.
Resource: from "Millennium," a Deseret News educational section