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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The last transit of Mercury for 26 years
From: Herbert Prinz
Date: 2006 Nov 05, 13:43 -0500
From: Herbert Prinz
Date: 2006 Nov 05, 13:43 -0500
Thanks to Peter for reminding us of the upcoming transit of Mercury. However, lest some prospective observers may miss the event, let me point out that the date and time given was severely biased by a rather "extreme point of view"! The transit will start on Wednesday, November 8, at 19:12 UT. For viewers on the east coast of the USA, his translates to 14:12 EST on the same day. What a pretty coincidence! Edmond Halley, who played an important role in the observation of transits of the inner planets, was supposedly born 250 years ago on the exact day of this year's transit. No birth record survived, but an anonymous _Memoir_, printed at MacPike, 1932, gives October 29. (See Allan Cook, _Edmond Halley_, Oxford, 1998.) If this is correct, it is to be interpreted as Old Style. The Gregorian Calender was ten days ahead, so that the date according to our reckoning was November 8. One of the major concerns of astronomers at Halley's time was solar parallax. Beyond its cosmological significance, its precise value is required for practical purposes such as positional astronomy, navigation, and in particular, lunar distances. The latter would hardly have been feasible without a solid idea of solar parallax. The parallaxes (i.e. distances) of all planets of the solar system are connected by theory. Copernicus already had a good concept of the relative distances, Kepler an even preciser one (3rd law). What was missing was at least one absolute distance measurement. Efforts to measure the parallax of Mars during opposition by Cassini and others had questionable results. James Gregory suggested to employ Venus transits. Halley developed the idea further and tried it out by timing a Mercury transit in 1677. Venus would have been the better choice geometrically, because it's farther from the sun and closer to the earth. But there were no Venus transits during Halley's lifetime. The ones in 1761 and 1769 were observed around the world using Halley's procedure and, indeed, yielded values for solar parallax that came close to the truth. Herbert Prinz Peter Fogg wrote: >" The planet Mercury will appear to pass in front of the disc of the >Sun on the morning of Thursday 9 November 2006 ...The entire transit >is visible from the east coast of Australia, Tasmania, New Zealand, >the Pacific Islands, Antarctica and the west coast of North America. >Elsewhere only part of the transit is visible or, as in Africa and >Europe, it is not visible at all." > >From: >http://www.sydneyobservatory.com.au/events/whatson.asp > > --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---