Flamsteed Astronomy Society

‘The Transits of Venus’ by Dr Robert Massey - January 9, 2012

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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.


Dr Robert Massey

[Pic Mike Dryland]

[Pic Mike Dryland]

Diagram of transits of Venus and the angle between the orbital planes of Venus and Earth

Observation of 1639 transit of Venus in Venus in sole visa by Jeremiah Horrocks, published in 1662 by Johannes Hevelius