Most people, if asked to name as many of the 43 US Presidents as they could, would probably come up with at least half a dozen names - George Washington, Ronald Reagan, both George Bushes, Bill Clinton, Richard Nixon ... a few people might even remember the odd name or two from the 19th century, such as Abraham Lincoln, Ulysses S. Grant or Andrew Jackson. But it's unlikely that any but the most knowledgeable would mention Chester Arthur, the 21st President.

A distinguished looking fellow with mutton-chop whiskers, Arthur was more a safe pair of hands than a man who set the presidency ablaze with memorable actions.

But one of his accomplishments did have a profound effect on one area of modern life. It was Arthur who, in October 1884, convened the International Meridian Conference in Washington D.C.

The most memorable outcome of this conference was to establish a single prime meridian to replace the various meridians that were in use and so ease confusion and standardise the drawing up of navigational charts. Forty-one delegates from 25 countries attended the conference, and it was concluded that the prime meridian should be the one passing through the Royal Observatory at Greenwich. In future, all lines of longitude would be calculated using the Greenwich meridian as a starting point.

All but three of the nations attending the conference accepted the designation of Greenwich as the location of the prime meridian. The delegate from San Domingo voted against, and those from France and Brazil abstained.

Greenwich Mean Time was accepted as the world's standard timekeeping referent: all local timezones were defined by the number of hours by which they differed from GMT. One major reason why Greenwich was so readily accepted as the point of reference was that the majority of the world's shipping used charts that already used Greenwich as the prime meridian. Another was that the US had already drawn up its own system of time zones by reference to Greenwich.

Since 1986 Greenwich Mean Time has been replaced as the standard referent of world time by Coordinated Universal Time (which has the confusing acronym UTC). UTC keeps track with GMT, which is still used as the timezone in which the prime meridien is located - in other words, as the UK's time zone. UTC is calculated by atomic clocks, and is a measure of absolute time. However, because the rotation of the earth is not completely uniform, it has proved necessary to introduce a relative measure of time, known as UT1. This enables navigators and astronomers to make accurate observation that take the earth's movements into account.

According to the rules governing UTC and UT1, there can never be a variation between the two time standards of more than 0.9 seconds. Since the rotation of the earth is not a phenomenon that is easy to adjust, any corrections required to keep UTC and UT1 closely synchronised have to be applied to UTC. When the gap gets too big, a leap second is added to UTC to restore the time standards to equilibrium.