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The 2012 International Transit of Venus



Undergraduate Thesis 2009 by Matthew Cooper

Supervised by Dr Craig Roberts


Venus in transit *




History of transit observations


Transit of Venus first predicted and observed


Transits of Venus were first predicted by Johannes Kepler in the early 17th century, as a result of his revolutionary work on determining the elliptical orbits of the planets about the Sun. The first transit of Venus to be scientifically observed was the transit of 1639, which Jeremiah Horrocks predicted after refining Kepler’s work in planetary orbits (Sheehan & Westfall 2004). Horrocks observed this transit using a telescope to project the image of the Sun onto a piece of paper, outlining the image of the Sun and marking the position of Venus at intervals. He and accomplice William Crabtree were the only people in the world to observe this transit (Sheehan & Westfall 2004). He made an estimation of the Astronomical Unit from this, but his estimation was based on assumptions on the size of Venus (Sheehan & Westfall 2004).


Halley and the Earth-Sun distance


Early in the 18th century, Edmund Halley proposed a method of calculating the distance from Earth to Sun using transits of Mercury or Venus. His method was to observe a parallax shift of the Sun between two locations on Earth. Halley’s method required the duration of a transit be recorded from the moment Venus enters onto the Sun’s disk to the moment it leaves it. Such observations were to take place at two locations where the entire transit would be visible, separated by as large a distance as possible to obtain maximum parallax.

Later, Joseph-Nicolas Delisle proposed a variation on this method that didn’t require the entire duration of the transit to be timed. Delisle’s version required only ingress (entry) or egress (exit) of Venus to be observed at a pair of stations, and recording the absolute time (in Greenwich or Paris time) of the event at each station (Sheehan & Westfall 2004, p.138). Delisle’s method is discussed in Section 2.3.


Cook and the 1769 transit observations


The transits of 1761 and 1769 were observed by many people, in different locations around the world. The 1769 transit is the one observed notably by Captain James Cook in Tahiti. For this transit, exploration teams were sent to the South Sea (Pacific Ocean) while observations also took place in Europe and North America. All observation teams were supplied with the same types of instrument for observing the transit (Howse 1990, p.180). Cook and his team of explorers, plus astronomer Charles Green, sailed to Tahiti to observe the Transit of Venus, which was successfully performed before they proceeded on to discover Australia and New Zealand.


An observing station was set up on Point Venus (named after the event) in Matavai Bay, Tahiti by Cook and his astronomer, Charles Green. A two foot Gregorian reflecting telescope was used, with a micrometer to measure small angles within the field of view of the telescope (such as the diameters of the Sun and Venus) (Howse 1990, p.181).


Since Cook and Green were using Delisle’s method to observe the transit to calculate the AU (Sheehan & Westfall 2004), accurate time had to be determined of ingress or egress, plus latitude and longitude of the observing location. A Shelton astronomical regulator clock was used to determine time of the event, and this clock had to be checked daily against local noon (Howse 1990, p.181). The method to do this was to observe (using a large astronomical quadrant) and time equal altitudes of the Sun at either side of noon, with the mean between the two establishing the moment of local noon. The time of the clock at local noon indicated its daily pendulum gain or loss (Howse 1990, p.181).


Latitude was determined by observing the Sun and stars with the quadrant, while longitude was determined by observing lunar distances with a Hadley sextant.


On the day of the transit (3 June 1769), Cook notes that the skies were clear and the weather was hot. Three men observed the transit – Cook, Green and botanist Dr Solander. Cook and Green were using identical telescopes – Gregorian-type reflecting telescopes of 2ft focus made by Mr. James Short (one of which equipped with an object glass micrometer, as used to determine the exact latitude and longitude of the observation site). Solander used his own telescope, being a different model and having a higher magnification. Exact time was kept using a Shelton astronomical regulator clock, which was housed in a purpose built tent on a wooden frame (isolating it from possible disturbance and weather).


Cook and Green both noticed a “halo” around the edge of Venus as it made its way onto the Sun. This they called the penumbra, and the result of it was that they both missed the exact timing of the first contact of Venus with the Sun. For the second contact, however, Cook and Green agreed exactly on its timing. Only Dr Solander differed (by 13 seconds). Cook and Green differed by 6 seconds for the time of the 3rd contact, while the fourth contact was difficult to time (because of the penumbra), with no time recorded in Cook’s notes.


Cook and Green were reportedly disappointed with their results, having undertaken the project with high expectations of accuracy. However, the results have since proven to be much better than they believed, according to Lomb (2004).


Observations of the 1874 Transit of Venus in Australia


In Australia, a team of men led by Henry Chamberlain Russell, the Government Astronomer at the time, scientifically observed the 1874 transit of Venus in various locations around New South Wales. A value for the Astronomical Unit was to be calculated from this transit, even though other methods for deriving this distance were emerging. Hence Russell gathered a large team of experienced observers and created four teams to observe from four different towns. The towns were chosen based on probability of sunshine from meteorological data, and observing at four separate locations would reduce the chance of cloud cover preventing any observations.


One of the observing stations was at Woodford in New South Wales. Here a photo-heliograph was set up, with a device called a ‘Janssen’s apparatus’, or ‘photographic revolver’ to capture the images. With this instrument short-exposure images could be taken in quick succession on circular glass photographic plates (Lomb 2004). This setup is typical of the early form of graphical recording system for post-transit viewing of events, which is being investigated in this thesis.


Of all the members of Chamberlain’s teams, only two observed the black drop effect (Lomb 2004). Others noted effects such as halos around Venus, distortion of the rim of the Sun near Venus’ edge at third contact, a spot of light on Venus’ North Pole near fourth contact, a bulge in Venus’ rim, a ‘parachute effect’ where a dark line formed between Venus’ limb and the inside edge of the Sun, and a jumping effect between Venus’ limb and the inside edge of the Sun (a dark patch quickly forming between the edge of Venus and the Sun).



Quick Facts about the 2012 Transit of Venus


Transit will commence at approximately 22:00 (Universal Time)


Transit duration will be about 6 hours 40 minutes


(Times above are approximate and will vary according to observer’s location)


For Sydney:

First contact: 8:16 AM

Last contact: 2:44 PM

The transit and the distance from Earth to Sun

History of transit observations

Observation methods

Observing conditions

Collaborating data







Matthew Cooper 2009

Last modified 30 October 2009