Channel 4 Learning

Science in Focus Special: Time and Place in the Communication Age
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How we measured time in the past. Galileo and Huygens invented the pendulum clock, which was very accurate but was not transportable and therefore not useful for navigation.

John Harrison invented a pendulum clock that used springs instead of gravity. The clock was no more accurate, but it could be carried on ships. The two pendulums are linked by springs, and any effect the ship’s motion has on one pendulum is exactly countered by the effect on the other one. Knowing the time accurately enabled sailors to measure their longitude. Harrison’s clocks are now kept in the Royal Observatory at Greenwich.

How accurate are different kinds of clocks? Our units of time are based on planetary and bodily rhythms. For example, a second is approximately the time between heartbeats. Our fastest reaction time is about a tenth of a second.

07: 20—09:50
If we used various clocks to measure time throughout a typical lifetime, how accurate would they be at the end of 70 years? Our heart would be about three years out. Harrison’s clock would be about an hour out; and a very accurate pendulum clock would be about ten seconds out.

Today most time-keeping devices are electronic. Quartz clocks (including most wristwatches) work by counting the vibrations of a piece of quartz which vibrates at 30,000 beats per second (hertz).

The atomic clock, invented in 1950, uses microwaves to flip caesium (Cs) atoms between two different states. The frequency at which the atoms vibrate between the two states is almost constant, and scientists now define the second as 9,192,631,770 such vibrations. Atomic clocks are accurate to about 0.0001 seconds in 70 years. ‘Super atomic clocks’, like those being developed at the National Physical Laboratory in Teddington, will be accurate to less than 0.00001 seconds in 70 years.

Harrison’s invention introduced new levels of accuracy to navigation, and the same is true of the atomic clock. The Earth is surrounded by a system of satellites — the Global Positioning System (GPS) — each carrying an atomic clock. From anywhere on the Earth’s surface, readings can be taken from three such satellites to give a very accurate measurement of position.

Using GPS, position can now be measured accurately using a hand-held receiver. GPS is invaluable in difficult or featureless terrain such as desert or tundra.