Published in Communiqué, the newsletter of La Société Guernesiaise, April 2004
122 years ago the
knowledge we had of the solar system was very different from that of today. The
five ‘wanderers’ in the sky - the
five naked eye planets -
were augmented by the discovery of Uranus in 1781 and Neptune in 1846.
Venus, named after
the goddess of love, is the brilliant planet now shining in the west after
sunset. We now know that this planet
is probably the nearest thing you could get to hell. It is a scorching 400
degrees centigrade, ninety times the atmospheric pressure at sea level on Earth,
and blanketed under thick clouds of sulphuric acid
- a truly nasty place. Probes
that have managed to land on the surface and avoided being crushed for long
enough to send back pictures, have revealed a desolate landscape, but before the
space age there was no knowledge of what may lay beneath those thick clouds.
As Venus takes 224
days to travel around the Sun and the Earth takes 365 days, one would expect
that they would line up with the Sun pretty often
- every 1.6 years. The
reason they would line up is that the orbits are not in the same plane, so for
the transit to take place the three bodies must line up at the point where the
planes of the orbits cross. If they
do not, Venus will be either above or below the Sun and there will be no
transit.
Transits occur in
pairs eight years apart. There are
two December transits eight years apart, a wait of 121 ½ years, then two June
transits followed by 105 ½ years wait; then it repeats itself. So
we have the transit this June and then another one in June 2012. If
it is cloudy on both these days then there is along wait until 2117 and then
2125, but unfortunately none of us will able to wait around for them!
The transit of Venus
will be visually interesting to watch, but over 250 years ago Edmund Halley, of
the comet fame, realised that if two observers watched a transit from widely
separated latitudes they would be able to compare their observations and use
them to calculate the distance from the Earth to the Sun. Each
observer would time the transit from start to finish, and the shift in the
position of the planet would be used to calculate the distance. For
the 1761 transit expeditions were planned to different locations to record the
observations and hopefully calculate the Earth-Sun distance. They
had a mixed success rate, as an unexpected phenomenon affected the accuracy of
the timings. As the disc of Venus
entered and exited the solar disc a smearing effect was noticed, and, as it was
crucial to record the exact timings, this made it difficult to be precise. This
effect was first thought to be due to the atmosphere of Venus.
As the disc of Venus crosses on to the solar disc it appears to be joined
to the limb of the Sun by a dark area. The
effect, called the “black drop”, is caused by refraction through the dense
clouds of Venus. However, long after
the 18th century transits, the recordings were subject to much scrutiny and
calculation. In 1824, Johann Encke
(also of comet fame as he discovered the comet with the shortest orbital period)
reviewed all the measurements, and came up with the Earth-Sun distance as being
about 95,280,000 miles. This was
more than the currently accepted distance of 93,000,000 miles, but was more
accurate than any previous measurements.
122 years ago, in
1882,
It is most important
to remember that it is never safe to
look directly at the Sun at any time, without specially designed filters. Remember
all the safety warnings for the solar eclipse in 1999.
Debby Quertier
The transit of Venus on 08 June 2004 starts at 6.21 am and ends at 12.23 pm.
Graphic by David Le Conte