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    Re: transit of venus 1769
    From: J Cora
    Date: 2006 May 27, 10:41 -0500
    Greetings George,

    You are quite correct, I only extract the one line from "the SHIP".  

    First a comment about the book, I have not completed reading it but it is of high quality,
    comparable to an art book with color maps
    of the era and many photographs.  I would
    recommend it.

    On the transit of venus and related topics
    Using google I searched on the topic "transit of venus" and was pleased to find many additional details on the topic.  I did not mean to imply that the atmosphere of venus and the blackdrop were the same effect but that more than one factor affected the observations.  It is
    mentioned in wikipedia, after the 1761 transit of venus was observed a russian
    predicted the existence of an atmosphere on venus.  The work of James Short and Thomas Hadley in the analysis from the 1769 observations brought the size of the AU to within 1 percent of todays figure. 

    One very interesting aspect about all this is that the Board of Longitude awarded monetary prizes to the widow of Tobias Mayer for his lunar tables and an unsolicited gift to Leonhard Euler for his mathematical works used by Mayer for the tables in 1765, which was well before the transit observations.  Also Nevil Maskelyne began printing an almanac in 1767 along with lunar tables in 1767, so the precise value of the AU was not required to compile fairly accurate  ephemerides at that time.

    To respond to Frank Reed,  I seem to recall that the transit was not visible from  my location near Los Angeles, California but I cannot swear to it.  Dont know it the next transit will be visible from here either.

    On 5/27/06, George Huxtable <george@huxtable.u-net.com > wrote:

    Coralline Algae wrote-

    "I am starting to read  "the SHIP retracing Captain Cook's Endeavour
    Voyage"    which I found while browsing the stacks at our local

    On page 27 to summarize: by observing from different parts of the
    globe and
    measuring the time the planet venus took to cross the sun,  it would
    the measurement of the suns diameter and also the earth's distance
    from the
    sun, the Astronomical Unit.

    The observations of the transit of venus in tahiti were not as precise
    hoped since the timing of when venus first met the suns disc could not
    determined with accuracy as a halo due venus atmosphere and an optical
    effect called blackspot."


    response from George-

    Although I didn't travel on that voyage, I was involved as an adviser
    to the BBC (who organised it) about navigational matters, and also in
    correcting errors in  the resulting book, "The Ship", that Coralline
    is reading.

    Determination of the Sun's actual diameter from its angular diameter
    was an indirect result of knowing the Earth - Sun distance, now known
    as the Astronomical Unit. Measuring that distance, rather than the
    Sun's diameter, was the main aim of the international expeditions to
    observe the transit of Venus. Not that what Coralline wrote was
    actually wrong, but the emphasis should be different.

    Coralline describes the reasons for the imprecision of the
    measurements as "a halo due to venus atmosphere and an optical effect
    called blackspot". Are those, I wonder, Coralline's own words rather
    than words found in "The Ship"? What the book does is quote the
    observers' description of "an atmosphere or dusky shade around the
    body of the planet". Many later commentators have ascribed this, in
    part at least, to the atmosphere of Venus, but in fact that does not
    contribute to what is seen. The "black drop" effect is entirely
    generated here on Earth, due to the muddying of thermal effects in a
    heated daytime atmosphere combined with optical defects in the
    instruments, seeking the ultimate resolution of which they are
    capable, when heated by the Sun. Those, combined with a deficiency
    within the human eye known as "irradiation", which affects sextant
    observations also, and makes it difficult to define exactly where lies
    the exact boundary between a bright area and a dark area, are
    sufficient to explain the black drop: not the atmosphere of Venus!


    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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