Approximately 5000 to 6000 years ago great civilizations in the Middle East and North Africa began to make clocks in addition to calendars to regulate time more efficiently. No doubt before the dawn of civilization, primitive humans had some need for breaking up the day and night into more manageable increments.

Sun Clocks
As early as 3500 BCE, the Egyptians used obelisks (4-sided monuments) to divide the day into smaller parts. Obelisks were like sundials, allowing the day to be divided into morning and afternoon by watching shadows. They also showed the longest and shortest days of the year when the shadow at noon was the shortest or longest of the year.

Egyptian sundials (shadow clocks) came into use around 1500 BCE. The sundial divided the day into 10 parts plus two "twilight hours" in the morning and evening. Shadows were cast based on the Suns movement East to West across the sky. The merkhet, the oldest known astronomical tool and introduced around 600 BCE, was used to to establish a north-south line (or meridian) by aligning them with the Pole Star (North Star). Nighttime hours were marked when certain other stars crossed the meridian. By 30 BCE, sundials became quite sophisticated.

Clock Components
All clocks have two basic components:

1) A regular, constant or repetitive process or action to mark off equal increments of time. Examples in early times included the suns movement across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses (hourglasses), and in the Orient, knotted cords and small stone or metal mazes filled with incense that would burn at a certain pace. Modern clocks use a balance wheel, pendulum, vibrating crystal, or electromagnetic waves associated with the internal workings of atoms as regulators.

2) A means of keeping track of the increments of time and displaying the result. Modern clocks track the passage of time by the position of clock hands and digital time displays.

Water Clocks
Water clocks did not depend on the observation of celestial bodies. One of the oldest was found in the tomb of the Egyptian pharaoh Amenhotep I, buried around 1500 BCE. The Greeks started using them around 325 BCE and called them clepsydras. They were stone vessels with sloping sides that allowed water to drip at a fairly constant rate from a small hole at the bottom of the vessel. Other clepsydras slowly filled with water and markings on the inside surfaces marked the passage of time.

More elaborate and mechanized water clocks were developed between 100 BCE and 500 CE by Greek and Roman horologists and astronomers. The flow of water became more constant by regulating the pressure. Some water clocks rang bells while others opened doors and windows to show little figures of people, or moved pointers, and dials.

Mechanical Clocks
It wasnt until the 14th century in Italy when mechanical clocks came into being. The clocks were weight-driven and regulated by a verge-and-foliot escapement. These kinds of clocks lasted for more than 300 years. Like water clocks before them, regulating the rate with accuracy was difficult because oscillations of the escapement depended heavily on the amount of driving force and the amount of friction in the drive.

Peter Henlein of Nuremberg introduced spring-powered clocks shortly after 1500. Spring-powered clocks were less heavy than mechanical clocks and allowed for mobility. However, the clocks ran slower as the mainspring unwound.

In 1656, Dutch scientist Christiaan Huygens made the first pendulum clock, regulated by a mechanism with a "natural" period of oscillation. Galileo was studying pendulum motion back in the late 1500s. Huygens' earliest pendulum clock had an error of less than 1 minute a day, which eventually he reduced to an error rate of less than 10 seconds a day. Around 1675 he developed the balance wheel and spring assembly, still found in some of today's wristwatches. In London in 1671, William Clement built clocks with a new "anchor" or "recoil" escapement, a substantial improvement over the verge because it interferes less with the motion of the pendulum.

In 1721, George Graham improved the pendulum clock's accuracy to 1 second per day by compensating for variations in temperature. Later, John Harrison, refined Graham's temperature compensation techniques and developed new methods for reducing friction. By 1761 Harrison built a marine chronometer with a spring and balance wheel escapement. It kept time on board a rolling ship to about 1/5 of a second a day, approaching the accuracy of pendulum clocks on land.

Around 1889, Siegmund Riefler's pendulum clock achieved an accuracy of a 1/100th of a second a day and became the standard in many astronomical observatories. R.J. Rudd and W.H. Shortt refined pendulum clocks later. In 1921, Shortts clock replaced Riefler's clock in many observatories. Shortts clock had two pendulums, one a slave and the other a master. The slave pendulum pushed the motion of the master clock and drove the clocks hands. The master pendulum was then free from mechanical disturbances.

Quartz Clocks
In the 1920s, quartz crystal oscillators and clocks vastly improved on accuracy beyond pendulum and balance-wheel escapements. Quartz clock operation is based on the piezoelectric property of quartz crystals. In an electronic circuit, the interaction between mechanical stress and electric field causes the crystal to vibrate and generate an electric signal of relatively constant frequency. Quartz crystal clocks had no gears or escapements to disturb their regular frequency. However, quartz clocks still relied on mechanical vibrations with frequencies dependent on a crystals size, shape and temperature. No two crystals are exactly alike. Quartz clocks dominate the current market largely because of price.

Atomic clocks are next.

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