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January 9, 2012

The Transits of Venus

Dr Robert Massey
Report by: Chris Gadsden

It was a pleasure to welcome back an old friend, Dr Robert Massey, now Press Officer at the Royal Astronomical Society. He was previously Public Astronomer at the Royal Observatory and has appeared many times on radio and TV to explain various aspects of modern astronomy and space exploration.  Robert was instrumental in helping to found the Flamsteed Society in 1999.

Robert began by referring to the last transit of Venus in 2004.  On that occasion there were clear skies and the entire transit was visible from the UK.  He explained what was meant by a “transit” –  a view of Venus as it passes between the Earth and the Sun and therefore appears as a small black spot passing across the face of the Sun – and pointed out that there have been only 11 transits of Venus between 1396 and 2012.  They are rare events because of the inclination of the orbit of Venus relative to the orbit of Earth and a transit of Venus can only occur when Venus and Earth are close to the line of nodes of their orbits; also, transits occur only in June and December pairs with a frequency pattern of 8 years/121.5 years. There wasn’t one in the 20th century and the last one was in 2004, the next will be in 2012.

Johannes Kepler discovered that planets move in elliptical orbits and realized that from the silhouette at the time of a transit, the angular diameters of Mercury and Venus (which orbit between the Earth and the Sun) could be determined.  Robert showed a slide of a drawing made by Jeremiah Horrocks of the 1639 transit and followed this by giving us a fascinating review of some of the attempts to make accurate observations in the 18th and 19th centuries.

From observations of the transits it is possible to calculate the Earth-Venus and Venus-Sun distances, and hence (using Kepler’s laws) the scale of the entire solar system.  Edmund Halley, legendary second Astronomer Royal, developed detailed proposals for calculating the Astronomical Unit (AU — Sun-Earth distance) by using observations made during a transit and he presented these to the Royal Society in 1716.  Joseph-Nicholas Delisle proposed in 1743 that it was sufficient to read the ingress and egress of the silhouette made by the planet on the Sun.  However, due to the “black-drop effect” (the phenomenon whereby the black disc, which is Venus seen against the Sun’s face, does not appear to break away cleanly at the points of ingress and egress because of the Venusian atmosphere) accurate timing of these points did not prove possible as was hoped.  Nevertheless, observations of the 18th and 19th Century transits enabled better estimates of the scale of the solar system, although at some cost to the observing expeditions…

Many observers attempted to make accurate measurements of the 1761 transit, including Maskelyne (soon to become Astronomer Royal), Mason & Dixon (later to be immortalized in the Mason-Dixon Line), Pingre (French), Le Gentil (also French), and the Russian, Lomonosov.  This last-named observed a luminous ring around Venus just as it entered the solar disc and thus concluded that Venus had an atmosphere.

The attempts of Le Gentil were perhaps the most unfortunate.  He set sail in 1760 for India in order to observe the 1761 transit but England and France were at war and his destination, Pondichery, was overrun while he was at sea.  He was only able to make some useless observations on board ship somewhere in the Indian Ocean.  He stayed in the East to observe the 1769 transit but when the critical moment came there was cloud cover.  By the time he arrived back in Paris he discovered that he had been presumed dead and his heirs were dividing up his estate.

In 1768 Lt. James Cook was sent by The Royal Society to Tahiti, together with the astronomer Charles Green, Joseph Banks and others, on the Endeavour to make observations of the 1769 transit.  Tahiti was chosen in order to have a large difference in latitude between observing stations.  Unfortunately Cook’s observations were blighted by the black-drop effect, the timing errors were too great and the results were not as accurate as hoped.  After Tahiti Cook’s expedition sailed in search of the undiscovered great South Continent, which he failed to find but instead charted New Zealand and became the first European to sight and claim New South Wales.  Despite Cook’s attention to the health of his crew many, including Green, contracted diseases in Batavia and died during the journey home.

The 1874 transit was recognised as a major scientific event and many international expeditions were mounted to make observations.  Eight expeditions were sponsored by the US alone. Hundreds of photographs were taken in an attempt to reduce the errors of the timings caused by the black-drop effect but the results were uncertain to the extent of 1.5 million km.

Transits of Venus are no longer important to determine the scale of the solar system which has been measured in other ways.  ‘Parallax’ observations of Mars, and later the near-Earth asteroid 433 Eros, produced better results, and in modern times it is possible to use radar to measure the distance to Venus and Mars with high precision.  Transits of Venus however continue to offer terrific spectacle.

The most recent transit of Venus was on 8th June, 2004 and was viewed in its entirety from the UK.  Many astronomers, including a team from the Flamsteed, set-up solar telescopes at the ROG and almost 1000 visitors were able to enjoy the thrilling sight from start to finish.  The next transit will be on 6th June, 2012, but will only be partially visible from the UK, the best locations for viewing being Russia, Greenland, the Pacific and east Australia.

Robert explained how exo-planets can now be detected by the “transit method” (or blink method) by observing the effect of planets moving in front of distant stars.  When this occurs the brightness of the star reduces. The change in the light curve can be measured and the size, mass and density of any exo-planets can be deduced.  To date around 700 exo-planets have been detected.

This fascinating account of the Transit of Venus has surely inspired many of us to take especial interest in this year’s Transit, the more so considering that the next one will not occur until December, 2117, but it is worth noting that there will be a transit of Mercury in 2016.

Our sincere thanks are due to Dr Robert Massey, not just for the lecture, but also for his responses to many questions from the audience afterwards.

Posted under: Flamsteed, Flamsteed Lecture, Meeting Report