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| Billie Silvey |
| Marking Time |
| Sun Clocks. One of the earliest forms of clock was the sundial, a device first used in Egypt around 3500 B.C. in which the angle of the sun casts a shadow on a “face” divided into hours. Cleopatra’s Needles were tall obelisks whose shadows marked time on 12 marks on the ground. Another shadow clock was more portable. It used five variably spaced marks and an elevated crossbar. At noon, it was turned around to measure the afternoon hours. Sundials use the sun to tell time, and thus can only be used on sunny days. |
| January 2006 |
| Water clocks. Next came water clocks, or clepsydra, around 1400 B.C. Water clocks used two containers of water, one higher than the other. Water traveled from the higher to the lower through a tube. Marks on the containers told the time. Clepsydra were invented in Egypt but improved repeatedly over the years by the Greeks. Later water clocks used gears and faces. They told time at night as well as day and were more accurate than sundials. |
| Mechanical clocks. In the early 14th century, large mechanical clocks began to appear in clock towers in Italian cities. These public clocks were weight driven and were hard to regulate. Peter Henlein of Germany invented the spring-powered clock around 1510. This clock slowed down as the spring unwound. But the spring made it possible for the timepieces to be smaller and more portable. Jost Burgi added a minute hand in 1577. Glass began to be used to protect the face of the clock in the 17th century. Clocks weren’t very accurate until 1656, when the Dutch scientist, Christian Huygens, made the first pendulum clock. It was correct to within a minute a day. As the pendulum swings, it turns a wheel with teeth, which turns the hour and minute hands on the clock face. The primary problem with pendulum clocks is that they have to be restarted on a regular basis. The first pendulum clock with external batteries was made around 1840. By 1906, the batteries had been moved inside the clock. |
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| Hourglasses. Hourglasses use the flow of sand through a restricted opening between two containers. The earliest reference to hourglasses or sand glasses was in a shipping manifest in 1345. Hourglasses were common in Italy by the 14th century. |
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| Electronic clocks. Quartz clocks, invented in 1920, use a type of crystal that looks like glass. Electricity causes the quartz crystal to vibrate at a high rate of speed. This vibration moves the clock’s hands very precisely. Sources for this article included: ARCYTECH.org, About.com, and the website of the National Institute of Standards and Technology, physics.nist.gov. |
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