Transit of Venus

The Transits of Venus


A central question to scientist is the size of the solar system. The distance between the Sun and Earth is for the purpose of this story of particular importance, and this distance is referred to as the Astronomical Unit (A.U.).

Scientist like Kepler, Newton and Halley laid the foundations for measuring the A.U. by scientific method. Preceding Kepler and the scientific revolution classical “astronomers” made size and distance measurements, which were accepted as “truth” for many centuries. Aristotle (384 – 322 BC) reported mathematicians had estimated the Earth’s circumference to be 63 000 km (39 000 miles). Eratosthenes (276 – 195 BC), by measuring the difference in angle of shadows from two sticks a known distance apart, calculated the circumference of the Earth to be 39 690 km (24 608 miles). This is very close to the modern accepted value of 40 097 km (24 857 miles) around the equator. Aristarchus (310 – 230 BC) measured the distance from Earth to the Moon by estimating the dimensions of the Earth’s shadow cone during a solar eclipse. He concluded that the Moon was ¼ the size of the Earth, and that the distance to the Moon was about 60 times the radius of the Earth. Both of these values are close to the modern values. He also tried to calculate the A.U. but with much less success. [Ferguson pp.1 – 28.] These are just some of the more spectacular work done by classical scholars.

Johannes Kepler (1571 – 1630 A.D.) laid the basis for explaining the path of the planets through the sky. Kepler’s third law of planetary orbits defined the scale of the solar system, which gives the relative distance of all the planets from the Sun. Know one planets orbital radius and you know them all. [Sheehan, p. 34] Incidentally, Kepler was also the first person to predict the transits of Mercury (7 November 1631) and Venus (6 December 1631) in advance. [Dick, p.75.] In 1687 A.D. Newton, with the theory of gravity explained the dynamics of how the celestial bodies moved through the sky. But still, how far away are all the planets from each other?

A contemporary of Newton, Sir Edmund Halley used Newton’s laws to predict the return of a comet today known as Halley’s comet. Halley was also the person who realised that using the Transit of Venus the distance problem can be solved. (Halley is cited as the originator of the idea but independently Joseph-Nicolas Delisle of France [Sheehan, p. 34.]  and James Gregory (1663) [Koorts – British, p. 36.] also worked along the same lines)

Thus the transits were set to become events of prime importance to be studied by astronomers.


A very infrequent transit

The Transits of Venus are very rare phenomenon, occurring in sets of two transits with a time span of 8 years between them. Then there is a time span of 105.5 years before the next transit occur. The whole cycle repeats every 121.2 years. [Dick, p. 74; Koorts – Huguenot, p. 200.] Only two sets of “Transits of Venus ago” (1769) did Captain Cook sail to Tahiti to observe the event, and in the process visited the continent of Australia, and due to his impressions the British Government decided to colonise Australia. However I am now getting a bit ahead in the story.

The reason that the transits are so infrequent is that the Earth and Venus do not move in the same plane around the Sun. There is a three and a quarter (3.39) degree difference in the angle to the plane. This means that due to the revolutions of Earth and Venus around the Sun we should see Venus transiting the Sun every 8 years, but due to the difference in the angle when we move around the Sun Venus is usually above or below the Sun [Gingerich, O. Sky and Telescope, June 2004, p. 78.; Dick, p.74.]. For only two consecutive transits we are lined up to see the transit, and then we are out of alignment for 105 years. When we are in alignment we refer to the line as the node.


The black-drop effect: that is the problem

There is however a practical problem which rendered the transits impractical as a method to determine the Astronomical Unit. For the method to work it relies on exact time measurements. When Venus reached the contact points during Ingress and Egress it looks to observers on Earth as if it changes shape. From a round planet its silhouette becomes amoeba-shaped, it distended to look like a drop of water about to drip from a leaky faucet [Sheehan, p. 34.; Westfall, pp. 75 – 78.]. This phenomenon is referred to as the black-drop effect and is due to a variety of optical effects, most notably due to the fact that Venus has an atmosphere (as well as the earths atmospheric influence [Sheenan, p. 36.]) . For the 1874 and 1882 transits observers were specially trained to compensate for the effect. Artificial transit machines were built to simulate the transit and black-drop effect. These machines consisted of a white triangular background and then a round metal disk was pulled across it. Observers watched the machine through telescopes from a distance away and learned how to take accurate time measurements.  However with the actual transits observers at the same sights still reported up to 50 seconds differences in their timings. For the method to work observers needed to be accurate to within one second (Halley’s expectation was one part in 500 [Sheehan, p. 34; Koorts – British, pp. 36.]). Thus ultimately the method proved to be, well, rather useless.

When photography was invented the observers saw this as a possible solution to eradicate human error. French astronomer Pierre Jules Janssen designed (in time for the 1874 transit) an early version of a cinematomatic (movie) camera in order to take many photographs in quick succession. The ultimate result was still disappointing [Sheehan, pp. 35 – 36.].


The different transits.

The first observed transits were more a curiosity to men of science. In 1716 Halley proposed the transit method to determine the A. U. Astronomers had decades to prepare and the 1761 transit was a flurry of excitement and expeditions were organised all over the world. The results were disappointing. In the meantime astronomers using other methods obtained better results. Thus the 1882 transit was still observed by hopeful observers but the enthusiasm was much tempered by this stage. The 2004 transit had hardly any scientific value but was observed by the astronomical public at large for its rarity value.

Reference is made in the text to the Southern African Connection, abbreviated as S.A.C.

1631 December 6:

This transit was the second one to be predicted in advance, by Kepler. The event did not attract great attention amongst men of science, mainly because it was not visible in Europe. Kepler himself died the year before but did spread the word around the world. No one is known to have observed it. [Dick, p.75.]

S.A.C. As the 1631 and 1639 transits happened before the European settlement of South Africa in 1652 there were no South African involvement.

(The first transit to be predicted in advance was the transit of Mercury, which took place on 7 November 1631. Three men is known to have observed it, most notably the French natural philosopher Pierre Gassendi) [Dick, p.75.; Ferguson, p102]

1639 December 4:

Jeremiah Horrocks realised that the next transit would occur on 4 December 1639. (Horrocks, an English astronomer listed the date as 24 November because England did not adopt the Gregorian calendar until 1752.) Horrocks and William Crabtree are the only two persons known to have observed the event, and Horrocks estimated the apparent diameter to be one arc minute. [Dick, pp.75 – 76.]

S.A.C. There was no South African involvement.


In 1716 Halley realised the importance of the Venus transits as a means to solve the distance problem. Due to the sudden scientific value that the transits now obtained there was a flurry of excitement in preparing for this event. Expeditions were organised to places all over the world but the results were not as good as expected. The black-drop effect, a phenomenon that astronomers hardly knew, now rendered their observations inaccurate. The measured values of the solar parallax ranged from 8.3 to 10.6 arc seconds, a very wide range. [Dick, p.76.]

Some of the expeditions were very dramatic indeed. Frenchman Guillaume Le Gentil went to Pondicherry, near Madras in India, only to find the town occupied by British forces. France and England were enemies at the time. (Seven years war) Not being able to observe the 1761 transit he decided to stay on for the 1769 transit, only to be clouded out. Not having informed anyone of his decision to stay on meant that when he returned to France he was presumed dead, his estate divided and his post filled. [Hoskin, p183.; Ferguson, pp. 126 – 127.] Jean d’Auteroche led a French expedition of four astronomers across Mexico to California. Three of the four died of disease. [Ferguson, p.127.] Reverend Nevil Maskelyne, sent by the Royal Society to St Helena had a much better time. His expenditures were 292 pounds, out of which 141 pounds were for his personal liquor expenses. [Ferguson, p.127.]

S.A.C. Two English Astronomers, Jeremiah Dixon and Charles Mason were on their way to Sumatra to observe the 1761 transit of Venus. Their ship was delayed, so they did their work at Cape Town instead. The transit was successfully observed from Concordia Gardens, a social club that used to be behind St Mary’s Cathedral in Cape Town. [Koorts – British, p. 36.]


More expeditions were arranged and taking the black-drop effect into account a much narrower range of values were obtained – 8.43 to 8.8 arc seconds – translating into distances of 149 million to 156 million km for the A.U. (93 million to 97 million miles)

The famous voyage of Captain Cook on the Endeavour to Tahiti was to observe this transit. David Rittenhouse, a pre-eminent American scientist fainted from excitement after peering through his telescope. [Dick, p.76.]

1874 December 9:

The hopes of astronomers were pinned on this transit to obtain the value for the A.U. Many expeditions were organised by scientifically active countries, for example American (8);British (12); French (6); German (6); Russian (26). Exorbitant amounts of money were spent on equipment, personnel and other resources. The American Congress appropriated $177 000, equal to $2 million today. There was even a private expedition by Lord Lindsay to Mauritius (He took David Gill along). [Dick, p.77; Sheehan, p35]

It is important to note that there was great excitement for the transit of Venus method as a means to establish the A.U. This was despite the fact that, as discussed later in this article, astronomers were achieve better results before 1874 by using other methods. Journals such as Scientific American avidly followed the progress of the expeditions. Perhaps the reason for the excitement was confidence that the black drop effect could be eliminated.

The different observers initiated specific training in order to teach astronomers to counter the black-drop effect. Artificial transit machines were designed as discussed under the heading “The  Black-drop Effect: that is the problem“. Different expeditions made extensive use of photography as this was recently invented.

Bad weather afflicted most of the expeditions and the black-drop effect played havoc with time measurements, even after all the training. The results were inconclusive and disappointing. The French published results, but with wide error bars. America only published a value of 8.883 arc seconds in 1881, on the eve of the next transit. [Dick, pp.77 – 78; Sheehan, pp. 33 – 36.]

S.A.C. South Africa was not well placed to observe this transit so the expeditions went elsewhere.

1882 December 6:

Despite the disappointment of the 1874 transit, different countries mounted more expeditions. The British Government, driven by the momentum of past expeditions formed a Transit committee. A wide range of sites was selected, to South Africa, Jamaica, Barbados, Bermuda, Madagascar, Canada, New Zealand and Australia [Koorts – British, pp. 41 – 42].

America organised eight expeditions, but with less fanfare than for the 1874 transit. The media questioned the amounts of public money to be spent on a method that was now being questioned. [Sheehan, pp. 36 – 37.] American expeditions were sent to South Africa, New Zealand and South America. They published a value of 8.809 arc seconds, yielding a Sun-Earth distance of 149 342 000 km (92 797 000 miles). [Dick, p.78.]

S.A.C. Southern Africa was well placed for this transit and the British and Americans organised expeditions to South Africa.

As South Africa was then a British colony the locally organised efforts can be seen as part of a bigger British expedition. A previous director of the Cape Observatory, E.J. Stone, now occupying the post of Radcliffe Observer at Oxford, was appointed directing astronomer for the British Transit committee.

The director in 1882 at the Royal Observatory, Cape of Good Hope was David Gill (1879 – 1907). The Observatory observed the transit with six instruments.  The transit was deemed with such importance that a  second permanent observatory was established in South Africa, the Natal Observatory situated in Durban with Edmund Nevill (also known as Neison) as the director.

Apart from the local observations at the Royal Observatory, Cape of Good Hope and Natal Observatories the British transit committee send out to South Africa another four instruments and they were manned at two sights, namely Montagu Road (renamed Touwsrivier) and Aberdeen Road. All the South African sights experienced clear conditions although a strong wind was a factor at Montagu Road. All stations managed to make successful observations. For more information on the British efforts in South Africa read the article by Koorts – MNASSA April 2004, Vol. 63 nos. 3 & 4, pp. 34 – 57.

The Americans under Simon Newcomb send an expedition to Wellington. They set up their instruments at the Huguenot Seminary School for Girls, a school with very strong ties to America. They made successful observations in perfect weather conditions. For more information on the American effort read the article by Koorts – MNASSA October 2003, Vol. 62 nos. 7 & 8, pp. 198 – 211

2004 June 8:

By 2004 the A.U. has been well established. Astronomers used radar in the 1960’s to refine measurements, and by sending space craft to the Moon and other planets, and having them arrive at the right place and the right time is proof that we got it right. Thus the transit observations in 2004 hardly had any scientific value.

S.A.C. Professional and amateur astronomers manned telescopes mainly to satisfy their own curiosity and to help educate the public. “Expeditions” were launched to some of the historical sights namely Wellington and Touwsrivier (Montagu Road). Other sights were at Bloemfontein, Graaf-Reinet, Harare, Pietermaritzburg, and a professional conference at Pilansberg National Park. (For more information: MNASSA, August 2004, Vol. 63 nos. 7 & 8.)

2012 June 6:

The event came and went with hardly a whimper. Very little fuss was made of the event. Few articles appeared in Journals compared to the 2004 event.


Modern value of the Astronomical Unit (A.U.)

The transit of Venus turned out to be less than perfect as a method to determine the A.U. Another method that was much more practical was the mean solar parallax. Looking at Mars and carefully noting the positions of background stars when we are at various positions orbiting the Sun use the difference in parallax to determine our orbit, thus our distance from the Sun and other planets.

During his memorable visit to South Africa in 1751 – 1753, La Caille make observations using the solar parallax method and determined an A.U. of 131 500 000 km (81 710 317miles) [Koorts – British, p. 36.]

Using observations from the 18th Century transit of Venus, Johann Franz Encke in 1824 from Seeburg Observatory in Germany announced a value of 8.58 arc seconds, translating to 153 340 000 +/- 660 000 km (92 180 000 +/- 410 000 miles) This was 2.5% larger than the modern value. [Sheehan, p. 34; Dick, p.76.]

Southern African astronomers helped to contribute to the A.U. using the solar parallax method. Thomas Henderson did exemplary work during the 1832 opposition of Mars. Thomas Maclear observed the oppositions of Mars during 1849 / 50 and 1851 / 52 but unfortunately there were no complementary Northern Hemisphere observations. Edward James Stone also made observations during the 1862 opposition of Mars. [Koorts – British, p. 36.] Most notable was David Gill, who in 1887 derived a value of ,within 0.2% of the modern value. This attempt was much better than the combined effort of all the results from the 1874 transit.  [Sheehan, p. 36.]

Since the 1960’s scientists bounced radar signals of the surfaces of the Moon and planets and refined the value to a modern accepted value of 149 597 870 km (92 955 807 miles) accurate to a few meters. [Sheehan, p. 36; Koorts – British, pp. 35 – 36.] Today we accept that the mean Sun-Earth distance is constantly changing due to oscillations around the Sun-Moon centre of mass, and the pull of other planets and asteroids.



Humans are very inquisitive. We want answers to questions but do not always know how to answer them. For most of the general public an answer that borders on myths, based on no factual basis, usually suffice. For most of mankind’s history anyone could have given you any old answer and we could not have known if the answer was correct. To find the correct answer took millennia and a great many brilliant minds spend unknown hours pondering over the problem, instead of indulging in life’s other distractions. 300 Years ago the transits of Venus were seen as the solution to the problem. It sadly turned out no to be so. The Sun Earth distance answer had to be solved by other methods.

Using the transits of Venus as a method turned out to be costly with lots of wasted energy and talent. It was an impractical method. However, the moral of the story is that even though with hindsight we tend to criticize and know better, at one time in our past it was a brilliant idea. Actually it was probably the best idea we ever had to solve this particular problem. More than that, it still is a good idea. Today we have the technology to solve the practical aspects, which flummoxed the 18th century astronomers. But because the distance problem was successfully solved by other means we don’t need the transit method anymore. Sad, isn’t it?



  • Dick, S.D.: The Transit of Venus; Scientific American, May 2004, pp. 72 – 79.
  • Ferguson, K.: Measuring the Universe: The Historical Quest to Quantify Space; Headline Book Publishing, 1999.
  • Hoskin, M.(ed.): The Cambridge Illustrated History of Astronomy; Cambridge University Press, 1997.
  • Koorts, W.: The 1882 transit of Venus and the Huguenot Seminary for Girls; MNASSA October 2003, Vol. 62 nos. 7 & 8, pp. 198 – 211.
  • Koorts, W.: The 1882 transit of Venus: The British expeditions to South Africa; MNASSA April 2004, Vol. 63 nos. 3 & 4, pp. 34 – 57.
  • Sheehan, J.: The Transit of Venus, Tales from the 19th Century; Sky and Telescope, May 2004, pp. 32 – 37.
  • Sinnott, R.W.: Timing the Transit; Sky and Telescope, June 2004, pp. 79 – 80.
  • Westfall, J.: The June 8th Transit of Venus; Sky and Telescope, June 2004, pp. 73 – 79.



External Links:
The Tansit of Venus: Historical Observations