Clocks, considered to be one of man's earliest inventions, originated 5,000-6,000 years ago. The ancient Egyptians used obelisks as primitive sundials; the shadow cast by the sun allowed them to split the day into two parts. Markings were later added to the bases of these structures, indicating further time divisions. But mankind still needed a means of keeping time that did not rely on sunny days. Water clocks, or clepsydrae, solved that conundrum. The earliest water clocks were simply vessels with sloped sides and a small hole in the bottom. They were filled with water, which dripped out at a near constant rate; hours were measured with markings on the inner surface. However, they required consistent temperatures for accuracy, and sand glasses, which followed the same principles but were much more reliable, replaced water clocks around A.D. 330. 

The first mechanical clocks signaled the time by striking bells. The word clock actually comes from the French "cloche", meaning "bell". The mechanisms were built in tall towers and operated by slowly descending heavy weights, which turned the gears as they moved. In the early 1500s, Peter Henlein, a German locksmith, developed a smaller, spring-powered clock that quickly became popular with the wealthier classes, who could display their timepieces on the mantel. Henlein's "Nuremberg Eggs" were the precursors of today's pocket watch. In 1581, Galileo discovered that the properties of a pendulum's swing could be incorporated into clock making. The design was not perfected until well after his death, and Christian Huygens is credited with inventing the pendulum clock in 1656. The mechanics were housed in long boxes that allowed for the movement of the pendulum, and became known as grandfather clocks.

Clockworks became more and more accurate as new discoveries were made. In 1840, Alexander Bain created the first electrical clock, and in 1912, the Warren Clock Company began producing battery operated timepieces. Timekeeping was revolutionized in 1927 with the invention of a highly accurate quartz movement which operated on the vibrations of a crystal in an electrical circuit. 1949 brought about the world's first atomic clock, using an ammonia molecule for its vibrations. Cesium soon replaced ammonia, and in 1967, a second was officially defined as 9,192,631,770 vibrations of the cesium atom, becoming the new, international unit of time, and the first time the day was not solely defined by the movements of celestial bodies. 

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