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
Graphic by David Le Conte
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