Ken Tapping, April 28, 2010

Some time a few thousand years ago, somebody needed to know what time it was, or maybe the date. Maybe he or she wanted to use the hours of daylight efficiently, or to make an important rendezvous. Knowing the date makes it possible to know when to plant crops and when important natural events such as annual flooding of the Nile were likely to occur. As populations grew, agricultural demands increased and cities became more vulnerable to natural events, losing track of time or date could be disastrous.

The first attempt at a clock or calendar was probably a vertical stick stuck in the ground. The shadow was shortest and ran north-south at local noon, and at other times the shadow acted like the hour hand of a clock. The length of the shadow at noon indicated the date. A more sophisticated version of the stick in the ground is in the church of Saint-Sulpice, in Paris, France. A round hole high up on the southern wall of the church allows the noon sun to cast a beam onto a brass strip running north-south across the floor. Where the beam hits the strip indicates the exact date. One of the applications of stone circles like Stonehenge in the UK could have been to set up the calendar.

The problem with devices like this is that they are all useless when it is cloudy, or at night. This led to experiments to fill in the gaps in Nature's timekeeping. People made clocks using things that run at a steady rate, such as burning candles, dripping water and dry sand trickling though a small hole. For many years these were the best devices we had. Then when it became possible to make precise gears and a working escapement mechanism, the pendulum and then a spring-loaded balance wheel were harnessed to make the first mechanical clocks. In those days the gears and other components were made completely by hand by highly skilled specialists. In many cases these people specialized in making only one type of gear, so a clock would comprise the work of several or more craftsmen. When precision machining techniques became available, mechanical clocks improved dramatically. Then electronic clocks using oscillating crystals and then energy transitions in hydrogen and caesium atoms were invented. Then it became possible to make precise comparisons between clocks and the rhythms of nature, and we found that nature is not that good a timekeeper. These days we can measure time sufficiently accurately to detect small changes in the Earth's rotation, caused by tectonic plate movements and the displacements in the rocks during earthquakes. We can measure the time intervals between the pulses of radio emission from rotating neutron stars and determine the rate they are slowing down.

One of the most impressive applications of precision timekeeping is the ability to synchronize the recording of radio waves using radio telescopes on different continents to emulate a radio telescope thousands of kilometres in diameter. Canada was the first country to succeed in Very Long Baseline Interferometry (VLBI). It was a joint VLBI experiment between Canada and the UK that brought me to this side of the Atlantic.