Marking Time

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.

For more fun reading and other activities, try these Web sites:    • A Walk Through Time    • Clocks...Teaching Time