As best we know, 5000 to 6000 years ago great civilizations in the Middle East and North Africa began to make clocks to augment their calendars. Slender, tapering, four-sided monuments, known as obelisks, were built as early as 3500 BCE. Their moving shadows formed a kind of sundial, enabling people to partition the day into morning and afternoon. Obelisks showed the year’s longest and shortest days when the shadow at noon was the shortest or longest of the year. During the Egyptian development of around 600 BCE such astronomical tool as merkhet was used. Sundials evolved from flat horizontal or vertical plates to more elaborate forms. By 30 BCE there were around 13 different sundial styles in use in Greece, Asia Minor, and Italy.
Another type of timekeeping was Water Clock. One of the oldest dates to around 1500 BCE. These clocks were used to determine hours at night, but may have been used in daylight as well. More mechanized water clocks were developed by Greek and Roman horologists between 100 BCE and 500 CE. One of them is known today as the Tower of the Winds. Meanwhile in the Far East mechanized astronomical/astrological clock were developed between 200 to 1300 CE. One of them is the Su Sung clock tower. Since the rate of flow of water is very difficult to control accurately, a clock based on that flow could never achieve excellent accuracy. That is why people were naturally led to other approaches.
In Europe Sundial clocks evolved between 500 CE to 1500 CE and were placed mainly above doorways. They were used to identify midday and four “tides”. By the 10th century, several types of pocket sundials were developed and used in Europe. The first half of the 14th century is known in European history of watchmaking because of large mechanical clocks, which began to appear in the towers of several large Italian cities. But large mechanical clocks had the same basic problem: the period of oscillation of the escapement depended heavily on the amount of driving force and the amount of friction in the drive. Like water flow, the rate was difficult to regulate. Revolution in the watchmaking by this time was the invention of spring-powered clocks by Peter Henlein of Nürnberg (Germany), between 1500 and 1510. Clocks and watches invented by Peter Henlein were much smaller thanks to replacing of the heavy drive weights. This made them popular among wealthy individuals due to the fact that the clocks and watches were portable now and due to truly accurate timekeeping.
Dutch scientist Christiaan Huygens made the first pendulum clock in 1656. It is important to stress that Galileo Galilei is actually credited with inventing the pendulum-clock concept. He studied the motion of the pendulum as early as 1582. George Graham from Great Britain improved the pendulum clock’s accuracy to 1 second per day in 1721 and John Harrison, a carpenter and self-taught clock-maker, developed new methods for reducing friction. As a result John Harrison had built a marine chronometer by 1761 with a spring and balance wheel escapement that won the British government’s prize.
The next century was characterized by the Siegmund Riefler’s invention (1889) of nearly free pendulum clock, which attained an accuracy of a hundredth of a second a day and became the standard in many astronomical observatories. A 100% free-pendulum clock was introduced by R.J. Rudd about 1898. This made it possible the development of several free-pendulum clocks. One of them was W.H. Shortt clock, introduced 1921, which was supreme timekeeper in many observatories and almost immediately replaced other clocks popular at that time.
Quartz crystal oscillators and clocks, invented in the 1920s and developed onward improved timekeeping performance far beyond that was achieved using pendulum and balance-wheel escapements. Quartz clock operation is based on the piezoelectric property of quartz crystals. Quartz crystal clocks were a way much better because they had no gears or escapements to disturb their regular frequency. Such quartz watches continue dominate the market in numbers because their performance is excellent for their price but their performance has been substantially surpassed by atomic clocks.