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    Re: The last transit of Mercury for 26 years
    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."
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