Well...
>
> Not the first I'd seen, but just as good as the others. Beats me why so many
> people bothered to photograph it (myself included), really just a black dot
> on a
> light disc, though mine will be slightly different as I was upside down
> (relative to
> Carlos) at the time. Is there any scientific value gained from this exercise
> ?
>
If you measure the times where Venus initially "touches" the solar disc
(I Contact) and where Venus following limb "touches" the inner edge of
the solar disc (II Contact) i.e. Venus is fully in transit, or the
opposite on the other limb of the Sun, III and IV Contact, you can
calculate an angle. If you have a suitable baseline and another observer
measuring the same timings then you can calculate the distance to the
Sun and Venus, hence quantifying the Astronomical unit, the Earth-Sun
distance, (1AU=149.607 million Km). Knowing the distance to the Sun and
Venus you can then use Keplers' Third Law to calculate the distances to
the other planets. Basically the transit of Venus provides a key
measurement to give a absolute scale to the entire solar system. In 1882
this was BIG science. The only problem was that as Venus has an
atmosphere when it is near the solar limbs it looks like an elongated
drop of water due to refraction of sunlight by its atmosphere so it is
difficult to obtain precise timings of the contact points. Consequently
in past attempts this "black drop" effect caused measurements to vary by
several tens of millions of kilometers.
The same parallax technique used to calculate the distance of the Sun
and Venus was used to calculate (IIRC by Hertzsprung in 1913) the
distance to a Cepheid variable, discovered by Henrietta Leavitt (one of
my personal scientific heroes!) in about 1912 who found the Cephied
period luminosity relationship, knowing their period you can determine
their luminosity and then their distance. This then provided a scale for
the Universe. So its clear that the Transit of Venus was of great
importance to our scientific understanding of the Universe. Now its just
spine-tinglingly brilliant to watch!
All the best,
Gareth.
--
Gareth.J.Martin
Research Postgraduate
School of Geographical Sciences
University of Bristol
University Road
Bristol
BS8 1SS
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