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    Transit of Venus; the "black drop"
    From: George Huxtable
    Date: 2004 Jun 9, 00:53 +0100

    When Venus crosses the Sun's disc, there are four instants which could
    conceivably be timed-
    First contact: the first appearance of Venus' disc as it starts to take its
    first tiny bite out of the edge of the Sun's disc. This in almost
    impossible to time with any precision by eye, as there is no advance
    warning of when that first contact will occur. However, modern
    moving-imaging should overcome that problem, in allowing a time-stamped
    picture-sequence to be replayed in reverse.
    Then more and more of Venus crosses on to the Sun's disc, until-
    Second contact: the instant when the whole of the planet has crossed onto
    the bright Sun's disc. It was assumed that this would be suddenly indicated
    by a sliver of light appearing behind the planet, which could be accurately
    Then Venus slowly crosses the Sun, until-
    Third contact: the instant when the planet's disc starts to leave the
    bright disc of the Sun, breaking the last sliver of light between the Sun's
    edge and the planet. Again, it was expected that this would be an
    instantaneous event, and so possible to time with precision.
    Then Venus moves gradually off the Sun, leaving a decreasing dark "bite"
    out of the edge of the Sun's disc, until-
    Fourth contact: The final moment when the last touch of Venus leaves the
    Sun's disc.
    It was acknowledged that the first and fourth contacts would be difficult
    to time with precision, but the interval between second and third contacts
    was expected to be very clear-cut, and it was this interval that
    expeditions were mostly asked to measure, with as much precision as they
    could muster. On return to their home-base observatory, the way in which
    this time-interval varied with latitude would be used to derive the
    distance of the Sun, and the size of the solar system, which was the object
    of the exercise.
    At some of the observing stations, several observers would be timing
    together, each with his own instrument. At the 1761 transit, it was found
    that second and third contacts were less clear-cut than expected, and
    different observers at the same station would disagree, often by many
    seconds, about the instants of second and third contacts.
    What the observers reported was that at second contact, the expected thin
    bridge of light did not jump across the dark gap behind the planet,
    immediately the planet's disc came entirely on to the Sun, as expected.
    Instead, the planet's disc appeared to draw in behind it, from outside the
    Sun's disc, a dark patch or thread, and this dark patch wasn't crossed by
    the expected bridge of light until several seconds later. This was
    described as the "black drop". So there was no unambiguous moment, the
    timing of which all observars could agree on. A corresponding uncertainty
    took place at third-contact. This bedevilled the astronomers' hopes of a
    precise result for the distance of the Sun.
    The "black-drop" effect wasn't really understood at the time of the 1769
    transit of Venus, but it seems to have been hoped that its baleful effects
    would have gone away. Far from it, unfortunately!
    For the Nineteenth-century transits, photography had started to make its
    appearance, but still the "black drop phenomenon degraded the results.
    Some erroneous reasons have been put forward for the effect, including a
    popular and long-held misconception that it was related to the Venus
    atmosphere. A Nav-l posting from David Edwards correctly identified it as
    being produced in the Earth's atmosphere.
    Brad Schaefer has cleared matters up with an authoritative paper entitled
    "The Transit of Venus and the notorious black drop effect", in JHA (Journal
    of the History of Astronomy), xxxii, (2001).
    He explains the black drop in terms of the degraded "seeing" in a
    telescope, coupled with the effect of "irradiation", to which the eye is
    For the first time, this latest transit will have been photographed to
    death, and presumably some will have been taken from outside the Earth's
    atmosphere. It will be interesting to see what this has done to the black
    drop effect.
    At this transit, American observers will have seen only third and fourth
    contact. I wonder if any saw evidence of the black drop at third contact?
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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