A CELEBRATION OF THE HISTORY, CULTURE, SCIENCE AND TECHNOLOGY OF RECHERCHE BAY
CSIRO Marine and Atmospheric Research Auditorium Hobart, Tasmania
26–28 February 2007

Heaven's clocks: The first chronometers
Professor Michael Dopita

Michael Dopita, FAA was born in the Czech Republic and educated in England. He became an Australian some 30 years ago. He is a Professor at the Research School of Astronomy and Astrophysics at the ANU. Elected a Fellow of the Australian Academy of Science in 1996, he now sits on the Council of that body. His research on galaxy formation and evolution, the physics of the interstellar medium, plasma diagnostics and spectral synthesis, star formation and the feeding and the galactic environment of black hole has resulted in over 450 publications, including over 240 publications in refereed journals. He is the co-author of major text book in the field, Diffuse Matter in the Universe. He became became an inaugural Australian Federation Fellow and was awarded the Australian Centenary Medal in 2003. He is a member of a number of editorial boards, and is an Editor-in-Chief of Astrophysics and Space Science. He has chaired a number of HST and Spitzer Space Observatory peer review panels, and currently provides policy advice to NASA though the Science Oversight Committee for the Hubble Space Telescope Wide Field Camera #3. He also serves on ARENA providing science policy advice for the development of Antarctic Astronomy at Dome C.

The precise mapping of the world could only have happened when both the latitude and the longitude of any point on the earth could be reliably fixed. Measuring latitude is easy, one simply needs to measure the maximum angle that the sun reaches above the horizon an any date. To fix longitude we need a precise clock – a chronometer – to compare local solar time with respect to Universal Time defined at some reference meridian (initially Paris, but later adopted to be that of the Greenwich observatory). In the late 18th century, reliable chronometers were still in development. However, the ancient art of astronomy was able to assist by providing two precise celestial clocks. The first of these, developed by Cassini in France, relies on timing the eclipses for Galilean satellites as they weave in their regular orbits about Jupiter. Indeed the timing of these eclipses could be so precise that Olaf Rømer found deviations in their predicted time throughout the year and suggested to the Academie des Sciences that these could caused by a finite speed of light – a speculation which took some 53 years to confirm. Observations of the Jovian satellites needed a fairly respectable telescope at a land-based observatory. For observations taken at sea in a moving boat, something easier had to be found. This led to the development of the lunar tables, which predicted the exact location of the moon against the fixed stars as a function of the Universal time. Simplifying somewhat – all that is needed is to compare local time with this Universal time: each hour difference between these two indicated fifteen degrees of longitude. The Earth revolves on its axis twenty-seven times faster than the Moon takes to orbit. This multiplies any observational error in the lunar position by twenty-seven times in longitude. This fundamental limitation of the technique was eventually solved by the development of chronometers which kept precise Universal time against which the local solar time could be checked.