Wonders of the Universe - Brian Cox [86]
MEASURING TIME
Humans have long been measuring time, and we’ve developed our skills from the bluntest of temporal measurements to the extreme accuracy with which we can measure time today. The first attempts in chronometry may have begun thirty thousand years ago, when Stone Age humans used the lunar cycle to mark time. To early humans, the Moon would have marked out the clearest rhythm in the night sky, and by following it through its phases they were able to create the first calendars. Giving structure to the year beyond the day– night cycle allowed them to name periods of time, and so our classification and division of the cycles of the cosmos began.
Beyond the naming of the morning, afternoon and evening, the fine division of the day required the invention of one of our most enduring pieces of technology, the influence of which has been incalculable.
The first clocks were simple pieces of technology employed throughout the ancient world. Using nothing more complicated than a stick known as a ‘gnomon’ to cast a shadow, many civilisations were able to use sundials to track the passing of time during the day by measuring the movement of the shadow across a calibrated surface. Sundials are surprisingly accurate, but they have limited use as timekeepers, not least because they are difficult to use on a cloudy day and impossible to use at night!
Ancient Egypt was the first civilisation we know of that took measuring time beyond the sundial. The technique of using the flow of water to measure time may date as far back as 6000 BC, but the oldest physical evidence of a water clock can be found in the reign of Pharaoh Amenhotep III in 1400 BC. These elegant devices were simply stone vessels that allowed water to escape at a near-constant rate from a hole in the base. Inside the clock were twelve markings by which time could be measured as the water level dropped. These primitive clocks gave accurate measurements both night and day so that priests could perform their rituals at the appointed hour.
Water clocks continued to be refined and used by cultures across the globe for many centuries, and hourglasses employing the flow of sand to measure time were also used extensively. The Portuguese explorer Ferdinand Magellan used 18 hourglasses as a navigation tool on his ship when he circumnavigated the globe in 1522.
Time keeping was elevated to a completely new level of accuracy with the invention of pendulum clocks. Galileo was the first scientist to investigate the physics of a swinging pendulum. The key property of the pendulum, which makes it useful as a timekeeping device, is that the period of the swing – the familiar tick-tock of the clock – depends only on the length of the pendulum and Earth’s gravitational pull. Perhaps counterintuitively, the period doesn’t depend on how high you lift the pendulum to start the swing, as long as it’s not too high. Physics students have the formula for the time period of a pendulum permanently etched in their minds. It is:
where T is the period, L is the length and g is the acceleration due to gravity – in other words, a measure of the strength of Earth’s gravitational field, which is almost the same wherever you are on Earth; approximately 9.81 metres (300 feet) per second squared. This means that all you need to do to make a clock that ticks accurately is get the length of the pendulum right. Most grandfather clocks have a pendulum that swings with a period of two seconds, which a little simple mathematics will tell you requires a pendulum approximately one metre long. The Dutch astronomer Christiaan Huygens invented the first pendulum clock in 1656, and it remained the most accurate way of telling the time until the 1930s.
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Christiaan Huygens invented the first pendulum clock