Observations

Keeping time: Harry reports on the September 2008 meeting of the Sydney City Skywatchers

A digital watch

Time, the fourth dimension of physics, underpins and interweaves all astronomy – and this talk, “Keeping Time” by Dr Bruce Warrington of the National Measurement Institute, encompassed the whole of scientific time from the earliest ideas for clocks to the utilisation of millisecond pulsars as time-keepers.

The speaker retold the historic “longitude” controversy and its varied solutions, some horological. We have amazing glimpses of Harrison using naked-eye star transits to check his pendulum clocks, with one in a cold room, one in a hot room, in efforts to control temperature affects. We learn of Kendall’s marvellous replica chronometers, and of K1’s three trips to Australia – lastly for 1988 celebrations (thanks Nick Lomb).

We review transit ‘scopes, and “guns” and time-balls; and the emergence of telegraphy that can repeat accurate time at remote sites. Passing through historic time we feel the loom of the twentieth century – with Einstein, and concepts of Relativistic Time. And Space, of course, where accurate time keeping is vital to detect space curvature, possible gravity waves, and the ‘flips’ of electron shells and the frequency of radiation beams.

Quartz oscillators and early Caesium clocks are dissected for us; the pace of change quickens again. An ionised mercury atom vibrates in an ‘ion trap’ no bigger than the head of a match – and amazed, we watch trapped ions emit tiny flashes of light as they change state. We learn finally of the latest prototype time-pieces accurate to one second in 400 million years! It’s an exhilarating presentation – always returning to link observed universal time with terrestrial machines designed to keep that time.

Of course Earth’s wobbly rotation rate, drifting continents and other variables must be compensated for – and the Bureau International des Poids et Mesures, Paris, has the final say in deciding, now and again, to insert a leap-second into the world’s arrays of time-keepers! And this is sometimes a controversial action.

We learn that here, in Sydney Cove, where some of the history of Time has been made, caesium atomic clocks keep time in neat white boxes with red led read-outs, watched over by two hydrogen masers and a ytterbium ion frequency unit.

We are relieved to hear that the 1194 Time Service is accurate to better than a second, indeed a tenth of a second if your reflexes permit.

There was of course much more, and Dr Warrington augmented his talk with anecdotes, wit and rich detail – which, in the setting of the old observatory and surrounded by historic instruments, seemed to come vibrantly alive, making for a most wonderful evening’s journey through Time.

Dr Bruce Warrington – National Measurement Institute, studied at University of Otago NZ, before completing his PhD at Oxford. Further research there and at University of Washington in Seattle preceded joining CSIRO National Measurement Laboratory as research scientist developing ultra-accurate timekeepers. He then led the Time and Frequency Group of the National Measurement Institute and now heads the Length, Time and Optical Standards Section of NMI.

Harry Roberts

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