From 1882 to 2004  -  a transit of Venus

by Debby Qertier

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.

The planet Venus is the connection between 1882 and 2004.  This summer, on the 8th June, there will be a transit of Venus, the first since 1882 and a phenomenon that nobody alive today has ever seen - an event, therefore, that will arouse considerable interest in the astronomical community.  The last transit before 1882 was in 1874.  Earlier ones occurred in 1761 and 1769, and 1631 and 1639  -  eight year gaps between each transit, and over one hundred years between each pair.  A transit occurs when the planet Venus (or Mercury, as happened in May 2003) is seen to travel in front of the solar disc, taking several hours to do so.  The transit of Venus occurs when the Sun, Venus and the Earth are in a line.  Venus, in the middle, is seen crossing the Sun, as long as appropriate safety measures are taken.

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, Guernsey was a very different place to what it is today.  From a scientific point of view 1882 was a significant year due to the transit of Venus, but it was also the year that La Société Guernesiaise was founded as a scientific society with interests and aims in conservation and history.  There was no astronomy section  -  in fact there was not one until the 1970s  -  but in 2004 the members of the Astronomy Section will meet to watch this special event, as will astronomers in many locations.  Let us hope that the weather does not let us down, and we are able to watch this memorable event whilst enjoying the sunshine.

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


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