Middle Era

Part 3 - Clocks

 Our hunter-gatherers ancestors built stone circles in many parts of the world, to predict equinoxes or solstices, and also used the stars, Moon and Sun to determine animal, fish and bird migrations and the availability of seasonal fruits and vegetables. About 4000 years ago, the Sumerians of Mesopotamia (Iraq) adopted the sexagesimal method of counting because the number of finger knuckles on the left hand was 12 and all the fingers and thumbs on both hands added up to 10. They also divided the night sky into 15 degree sectors as a way to study star positions. 

This was not an arbitrary number as 15 x 2 x 12 = 360, nearly the number of days in a year when the stars returned to their original position in the sky. The sky was thus divided into 24 (= 2 x 12) sectors of one hour increments. 

(The actual number of days it takes for the Earth to make one revolution around the Sun (a year) is 365.25 days). 

Egyptian priests, about 5500 years ago, divided the sky into two sectors because the day was about 12 hours and the night was also about 12 hours. They erected obelisks as giant shadow clocks, precursors to sun dials, to detect the noon position of the Sun god Ra and to determine the hours for daily rituals.

They invented water clocks 2500 years ago to determine the time at night. One of these floated a bowl, with tiny holes, in water. The bowl sank as it slowly filled with water, the time being recorded by marks on the side of the container.

In Persia (Iran) about 2300 years ago, accurate water clocks were necessary to measure the length of time each farmers could divert water from the communal qanat (a water supply from deep wells) for irrigation. Since this was the only source of water for agriculture, it needed to be shared equitably.

Time was also recorded by candles marked to record the passing of time and the sandglass or hourglass.

In many parts of Asia, calibrated slow burning incense sticks were used to measure time. These not only provide the time but also the smoke masked the odour of sewage and rotting meat. Some of these supported small weights that fell off to sound a gong at regular intervals. For longer times, spiralled incense sticks were often hung from the roofs of homes and temples.

The oldest clock with an escapement mechanism, powered by water, was made 2300 years ago in Greece. Mercury-powered escapement mechanisms (which did not freeze) were in use in China by the 10th century.

The clock towers built in China by Zhang Sixun in the 10th century, incorporated a striking clock mechanism to sound the hours. The mechanical engineer Su Song created a water-driven clock for a clock tower in Kaifeng City, in the 11th century. It used an endless, power-transmitting chain to drive an astronomical (armillary) sphere.

The Arab engineer Ibn Khalaf al-Muradi invented the first geared clock in 11th century Islamic Iberia (Spain). It used complex segmental and epicyclic gearing, that were not produced again until the mechanical clocks of the mid-14th century. A clock that struck a bell on the hour was built by Muhammad al-Sa'ati in the 12th century at the Umayyad Mosque in Damascus, Syria.

  

Initially, a sandglass was the only reliable method of measuring time at sea. In later times, the day was divided into increments of half an hour with a sandglass. A bell was run at the end of each half hour as the sand ran out and the sand glass was turned. The watch (or shift of men on duty) changed when the bell rang eight times (every four hours).

In the 15th century, sailors began to knot the line of a ship's log at intervals proportional to the nautical mile. The log was a simple device with a board (drogue) at one end of a long line. Typically 28-second and 14-second sand glasses enabled navigators to compute the ships speed through the water as the drogue was trailed behind the ship and a sailor counted the knots spaced along the line as it was paid out. When the sand glass ran out of sand the speed was recorded as the number of knots or nautical miles per hour. This information was used with the compass heading to predict the ship's future position a process known as dead reckoning. 

The first mechanical clocks with a verge escapement mechanism and a balance wheel were invented in Europe in the 14th century. They were powered by weights moving slowly down and became the standard timekeeping device until the pendulum clock was invented in 1656. The invention of the mainspring in the early 15th century allowed portable clocks to be built, evolving into the first pocket-watches by the 17th century, but these were not very accurate until the balance spring was added to the balance wheel in the mid 17th century.

Between the 11th and 13th centuries, Muslim astronomers made a number of highly accurate astronomical clocks for mosques and observatories. In 1206, Al-Jazari build a water-powered clock about 11 feet (3.4 m) high that included a display of the zodiac and the solar and lunar paths. It opened doors every hour to display clock powered, mechanical marionettes, including falcons and musicians, that automatically played music.

The city of Dublin in Ireland erected one of the first clocks for public use and by the end of the 15th century, many towns had public clocks on civic building. Early clock dials showed only hours. The minute hand was added to clock faces around 1680 in Britain. Clocks indicating minutes and seconds were uncommon until invention of the pendulum clock and the spiral balance spring in the 15th century but by the 16th-century, the astronomer Tycho Brahe was using accurate clocks, with minutes and seconds, to note star positions.

Galileo Galilei, the Italian scientist, discovered that a pendulum could be used to regulate a clock in1582 but the first pendulum clock was built by the Dutch scientist Christiaan Huygens in 1656. Early versions were accurate to within one minute per day, but this was later improved to within 10 seconds per day.

Christiaan Huygens, published drawing of his spiral hairspring attached to a balance wheel on 25 February 1675. The addition of the spiral spring made the balance wheel a harmonic oscillator like the pendulum, which oscillated at a fixed resonant frequency and resisted oscillating at other rates. This greatly reduced the error to about 10 minutes per day.