The earliest know clocks were produced in Egypt around 1500 BC. They were similar to the sun dials of today and were called 'gnomons'. In those days most activities were confined to the hours of daylight and these examples are the earliest know record of a human attempt to divide the day into equal time periods. Due to the difference in the number of hours of daylight in winter when compared to summer, these divisions of daylight represented different periods of time over a full year. But it was the first attempt to monitor and record the passage of time.

In later Roman times, the sundial was a status symbol and many wealthy Romans even carried pocket versions. Today we realize that, due to the effect of latitude on the angle of the sun, sundials must be designed for specific locations for them to be accurate. In Roman times, many of the sundials in use in their major cities were built in Sicily, the sundial building province of the Roman world. These instruments were of course only accurate when used locally but they could be found in many locations throughout the empire. It appears that the ancient Romans were never aware that their sundials were largely inaccurate.

The Clepsydra

The water clock, later named the clepsydra by the Greeks, meaning 'water thief', date back to the same time as the sundial in Egypt. A clepsydra was found in the tomb of the Pharaoh, Amenhotep I, buried about 1,500 BC. They worked by the steady rising or falling of a water level in a chamber or container with markings to show the passage of time. Unlike sundials, these clepsydras could measure the passage of time at night or on cloudy days and worked consistently regardless of the season, but they still couldn't determine absolute time.

The Astrolabe

We have the Greek astronomer, Hipparchus, to thank for the development of the astrolabe, because without the accurate star maps of Hipparchus, dated about 150 BC, the astrolabe would not have been possible. Dating from around 200 AD, the astrolabe is a devise for measuring the angle of bright stars relative to ones position on earth and thus used to determine location, distances and time.

Although not strictly a clock in its own right, the astrolabe was used to calibrate sundials and therefore worthy of mention in the story of the development of clocks.

The Candle Clock

No one knows the origin and date of such a simple concept as the candle clock but Alfred the Great was recorded as having calibrated candles with marks to determine the passage of time. He used these candle clocks to organize his day into periods devoted to religion, royal duties, study and rest. Candle clocks were also unable to tell absolute time but, like the clepsydra, were effective at all times and not just when the sun was shining.

The Chinese were also users of similar instruments about the same time only they burned incenses to indicate the passing of time. They also ingeniously hung threaded weights along the incense sticks at regular intervals, which would fall into a metal plate with a clang to alert one as to the time.

Heavy metal objects were also driven into the sides of candles, making a loud noise as they eventually fell out to indicate the passing of a predetermined time period.

The Hourglass

The hourglass, or the sand glass as it was originally know, probably originated in Europe before the 14th century. It worked on the same principle as the candle clock or the clepsydras, in that it could measure a predetermined period of time. Hourglasses were used to measure the length of a sermon in church or a summation of a case by a lawyer in court.

A specific 28 second hourglass was used in conjunction with a knotted rope to determine the speed of a vessel over water. If the rope was knotted every 47 ¼ feet and then the end, suitably weighted, was thrown off the stern of the moving boat, the number of knots that disappeared over the side during the period of the hourglass, was the speed of the vessel - in knots. One nautical mile an hour or one knot equals a distance traveled of 47 ¼ feet in 28 seconds. Knot many people know that.

The Weight-driven Clock

No one knows the inventor or the exact date of invention of the weight-driven clock but Europe suddenly exploded with a profusion of them in the early fourteenth century.

They work on quite a simple principle. A weight is hung on a cord which in turn, is wound around the axis of a gear, suspended in a frame. As the weight falls, the cord unwinds and the gear turns. The gear is connected to a further arrangement of gears which turn the hands on the clock to indicate the passage of time. Because the process now can be made to run continuously, the first true clock has been born which can give absolute time. To prevent the weight from just falling to the ground in one quick movement, the clock utilizes an escapement mechanism. In these early clocks, the escapement mechanism took the form of a verge-and-foliot. Ìt is this escapement mechanism that produces the characteristic tick-tock of a weight driven clock as the pallets on the verge obstruct the movement of the toothed escape wheel and the foliot oscillates to regulate the verge.

These weight-driven clocks survived for 300 years but they had a basic problem. The period of oscillation of the foliot in the escapement mechanism depended on the amount of friction in the bearings and was dependant on temperature and thus difficult to regulate. They would never be very accurate.

Another advance in the clock industry was the invention of the spring-driven clock by Peter Henlein of Nuremberg in 1510. This meant that clocks could now be more compact and bought for the home by wealthy individuals who could hang them on the wall or place them on a shelf. But they were still inaccurate; when the main spring was fully wound the clock ran quickly but as the main spring wound down, the clock slowed down.

The Pendulum Clock

In 1656, Christiaan Huygens, a Dutch astronomer, made the first pendulum clock. He didn't exactly 'invent' the pendulum clock as Galileo was credited with that invention in 1582, although he never actually got around to building his design before he died. These early pendulum clocks of Huygens were accurate to one minute a day and he later improved on this to less than ten seconds a day. This was the beginning of accurate time keeping.

Christiaan Huygens later went on to invent the hair spring and balance wheel and jewels were introduced to help reduce the friction in the bearings. This hair spring had the same periodic motion of a pendulum but in a more compact form. Now watches could be made that would be accurate to about ten minutes a day.

In 1671, William Clement started building improved pendulum clocks in London, using the new 'anchor' escapement mechanism. This new mechanism interfered less with the normal periodic motion of the pendulum and enabled still greater accuracy.

It was George Graham, in 1721, who was able to significantly improve on the current pendulum clock by devising a mechanism that would take into consideration the changes of the pendulum length with temperature. John Harrison, a self-taught clock maker, refined George Graham's temperature compensation system even further and improved the bearings to reduce friction further. By 1761, he had built a self-contained spring and balance wheel clock that was accurate to one fifth of a second per day. It was with this clock that he won the British navel prize for inventing a means for determining longitude to within half a degree over a 3000 mile distance.

Siegmund Riefler's clock, in 1889, further advanced the pendulum clock with the invention of the 'nearly' free pendulum, accurate to a hundredth of a second a day. The famous Shortt clock was invented in 1920 with two pendulums, one slave and the other the master pendulum. The slave pendulum drove the hands of the clock leaving the master pendulum free to maintain a perfect motion. This clock became the instrument of choice in most observatories of the day.

The Quartz Clock

The quartz clock has been around for a lot longer than most people realize. Invented in the Bell Laboratories in the 1920s by J W Horton and W A Marrison, the devise took up a small room but it was accurate to about one second every ten years. Quartz clocks work on the principle of the piezoelectric properties of a quartz crystal so that when a current is passed through such a crystal, it vibrates at an exact frequency.

Of course our modern quartz watches are no where near that accurate but they are much more accurate than most mechanical watches and considerably less expensive to mass-produce. The reason they are so accurate is because the quartz crystal vibrates thousands of times a second when exposed to an alternating electric field. The battery in the quartz watch is converted too A/C and makes the quartz crystal vibrate. A tiny microprocessor monitors this high frequency vibration and reduces it to a slow constant rhythmic electric pulse which it transfers to a coil. This coil operates an electromagnet which switches in time to the pulsing current and operates the watch's geared mechanism, thus moving the watch hands very precisely.

The precision of the Bell Laboratories' quart clock was able to help earth scientists to establish that the earth, from where we had defined the unit of a second to represent 1/86,400 of a solar day, did not actually rotate at a constant speed. The search was on for a more precise measurement to define 'the second'.