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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
timed.

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
subject.

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?

George.

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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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