18 JANUARY 1975, Page 14



Greenwich is good for you

Oliver Stewart

It is at once the tercentenary of the Royal Greenwich Observatory and the year in which we, in this country, are expected to go metric in all our measuring. But the trouble is that metric, or SI time differs from Greenwich Mean Time and that the two are incompatible.

In fact time Measurement is yet another field in which the Systeme International d'Unites is making a great nuisance of itself. It is all because SI time is meted out in _seconds by a laboratory monster they call the caesium clock, whereas the GMT second is a fraction of the tropical year. And the caesium second is not of the same duration as the GMT second.

Yet in 1967 the atomic or caesium 133 second was established as the SI base unit of time by the Conference Generale des Poids et Mesures. From then on the GMT

second and what Sir Harold Spencer Jones called "our fundamental unit of time, the day" lost its validity.

SI or metrication time now stands aloof from GMT. As its standard it exalts references to a particular atom above references to the sun and the stars. So after 300 years as a working tool for men and women in all parts of the world, that famous meridian begins to lose its relevancy.

Metrological specialists favour the SI second because it is a marvellously precise measure of duration. But there are reasons why laymen and such people as amateur air pilots, yachtsmen and others for whom time is an essential factor in their daily work are reluctant to give up GMT. In fact to many everyday affairs atomic time seems to be out of tune. The passing of the seasons, for instance, is more relevant to the human life span than pure duration. In stating people's ages it would be ludicrously precious and totally unhelpful to give the figures in SI megaseconds. Years and days are the appropriate units.

But there are plenty of other troubles. There are these tortured attempts to bring SI time and GMT together. They have brought into use those horrible little things called positive and negative leap seconds.

Intended to cope with the irregularities of the earth's rotation they are indicated on the radio by special signals either of 6 short pips and one long one, or by 4 short pips and one long one. They are supposed to make the best of both worlds and to squeeze out the differences between the two time scales. Inevitably they impose inaccuracy.

So we now have three kinds of 'time'; caesium clock time, Greenwich Mean Time and radio signal time. It is a lunatic situation. While GMT reigned it had absolute authority. The Royal Greenwich Observatory told the time; the one and only right time. But now, says officialdom, the right time is that shown by the caesium clock. GMT has become the wrong time. And radio signal time, being neither SI nor GMT, cannot be right.

It is an unacceptable situation. It could be set right if the SI second were to be re-named as what it really is, a unit of duration and not of time. Alternatively two different time scales could be allowed to co-exist without any of these absurd attempts to crush them together.

Best of all would be for those who order these things (and not many seem to know who they are) to give us back our GMT together with Sir Harold Spencer Jones's day unit, and to lock the SI second away in the laboratories. The Royal Greenwich Observatory would then be re-established as the world's timekeeper.