The National Maritime Museum in Sydney is presently hosting a travelling exhibition from its namesake in Greenwich that should not be missed by any visitor to Sydney in the next few months.

There are many highlights, although the centrepieces are the Harrison sea clocks H1-H4, represented here by recently made official replicas (the Greenwich museum has always discouraged amateurs from making copies, perhaps just adding to the adventure of people like our own Norm Banham, who has completed unofficial replicas of all four

The version of H3 on display has perpex plates, not exactly authentic, but marvellous for seeing what goes on between them. Just like the originals back in London, H1-H3 are ticking away ponderously. For H4, the revolutionary 'watch' that won John Harrison the Longitude Prize, we see the original case and dial, as well as a facsimile of the movement. It is not running (H1-3 have no lubricants and are supposed never to wear out, but H4 will), but there is a nice animated video showing the basic functions. The exhibition concentrates on the two real contenders for the prize - the accurate portable timekeeper, and the so-called Lunar Distances method, where the relative postions of the Moon and certain stars were used to give the absolute time. This was the method championed by Astronomer Royal Nevil Maskelyne,  cast as the villain in Dava Sobel's book 'Longitude', and successfully used by James Cook on his exploration of the east coast of Australia. Maskelyne, who first produced the Nautical Almanac from 1767, is partially rehabilitated in this exhibition.

The exhibition includes much of interest to James Cook fans like myself, including the famous portrait by William Hodges who actually voyaged with Cook and presumably knew him well. Speaking of portraits, Edmond Halley and Galileo Galilei appear too, the latter the earliest known image of the great man before he became really famous. Observation of the moons of Jupiter that he discovered was potentially a way to determine absolute time but it could never be done satisfactorily on a moving ship. Halley appears here because of his work on mapping magnetic deviation around the world, potentially another method to find longitude. There is a beautiful Portuguese compass designed to make similar measurements. There are cross-staffs and octants and sextants as well as timekeepers from  famous early makers, like Kendall, Mudge, Arnold and Earnshaw. Budget a couple of hours at least.