Weather: A brief history of timing

If you want to know the time, ask a sundial Photograph: Brian Harris

We take clocks, watches and the Greenwich time signal so much for granted that we are liable to forget that accurate timing is both a comparatively recent and to some extent an unnatural phenomenon.

If you want to know the time - the real, natural time, free of man- made adaptations of convenience - you should ask not a policeman but a sundial. We are all so used to our neatly organised 24-hour day, and accurate clocks and watches, and the BBC putting us right when batteries fail, that we tend to forget that our system of time is all a convenient fudge.

We are not the first to take clocks for granted. In Julius Caesar, Shakespeare has Brutus say: "Peace! count the clock", to which Trebonius replies: "The clock hath stricken three." The chiming clock, however, was not invented until some 14 centuries after Caesar. And while we're on the subject, Falstaff, in The Merry Wives of Windsor, says "Let the sky rain potatoes" a long time before the potato arrived from the New World; and First Carrier in Henry V, Part One, says: "The turkeys in my pannier are quite starved", long before turkeys were known in this country.

But more modern examples of bad timing, strangely enough, all began with the railways. Until the late 19th century, the pace of life did not require widely separated places to agree on the time. Individual towns had their own local time - often officially determined by the sundial on a church wall - and travellers would reset their timepieces at each new place of call. Especially if the church wall had suffered some subsidence, causing the sundial to register the wrong time.

Oddly enough, it was the railways that led the way to a nationally agreed time - most dramatically in the United States when the National Railway Time Convention was adopted on 18 November 1883 (they called it "the day of two noons") and town halls around the country ceremonially altered their clocks to conform to a national standard time. (There is a good account of the attendant panic in Bill Bryson's book, Made in America.)

Since then we have all been happy with the time and come to view days, hours, minutes and seconds as natural phenomena. In fact, they are man- made. A day, as a sundial sees it, is the time between one sunrise and the next. It is determined both by the spin of the earth on its axis (which is not quite constant) and the orbit of the earth round the sun (which is not quite circular, and not travelled at constant speed). Our "day" is the average time between one sunrise and the next, and an hour is one 24th of that average.

This process of averaging creates a a discrepancy between Mean Solar Time (which is what our clocks and watches register) and Apparent Solar Time (as seen on accurately aligned sundials). All of which provides the answer to a question that has been asked by several puzzled readers: Why do the shortest day, the latest sunrise and the earliest sunset not all occur on the

same date?

The answer is that in Apparent Solar Time, they do. The only reason they appear not to is because the railways insisted we get our clocks sorted out so that they could issue timetables. A decision, in the light of experience, that they have good cause to regret.