The Transit of Venus in the 18th Century
The article draws a sketch on the historical collections
of the museum,s response to the 18th-century transits of
Venus. It also includes a reconstruction of a large-scale mechanical
demonstration of the 1769 transit.
The Transit occurs when a planet crosses the face of the sun. The transit of Venus on 8 June 2004 was only its sixth ever to be observed.
In the 18th century, this rare event held the key to calculating the size of the solar system – “the most noble problem in nature”.
The transits of 1761 and 1769 were observed all over the world, requiring an unprecedented level of international organisation in science. This exhibition focuses on the British enterprise.
Expeditions and observatories were equipped to make precise measurements. Astronomers then gathered, compared and computed the results, all to establish the distance from the earth to the sun.
But the transits were not just for the scientific elite. Through books, prints and apparatus, enterprising lecturers and authors competed to engage a wide audience. The exhibition shows both the public and scientific impact of the transit in 18th-century Britain.
A transit of Venus is similar to a solar eclipse, occurring when the planet comes between the Earth and the Sun. Because the planet is so much further away than the Moon, it appears as only a tiny circle on the bright disc of the Sun.
Just as with an eclipse, where you are on the Earth determines what you can see and when. For two widely spaced observers, there are measurable differences in the exact timing and length of Venus’ path across the sun. With the separation of the observers taken as a base line, the distance to the sun can be calculated.rints were a publishing phenomenon of 18th-century Britain. Print shops sprang up in London catering for a new middle-class market hungry for engravings of Old Master paintings, landscapes, celebrities and political satires.
Science was part of this fashionably image-conscious world. Large-scale prints were produced to illustrate and explain notable events such as solar eclipses. Transits were given the same treatment.
One of the first to see the possibilities of astronomical prints was William Whiston. His transit print, showing future transits of both Venus and Mercury, was crammed with text and carried rather spartan images. Though appealing more to the understanding than to cultivated taste, it was sufficiently successful to be reissued. Displayed here is the original version of 1723 and the 1736 revision.
Edmond Halley was the first to develop a method for making the complex calculations. He published his most important paper on the transit of Venus in 1716, knowing he would not live long enough to witness the next transit in 1761.
The most spectacular demonstration of the transit of Venus in the 18th century was produced by Benjamin Martin. We have recreated his mechanical version of the transit of 3 June 1769, using the original engraving illustrated here.
Martin’s theatrical demonstration was staged in London. It was set up in the lecture room of his instrument shop in Fleet Street.
The scene itself is to the north of the city, looking westwards from Islington towards Hampstead. When set in motion, a clockwork mechanism pulls the Sun down the slot towards the horizon. At the same time, a hidden gear train moves the black dot of Venus across the face of the Sun, mimicking the appearance of the real transit.
The transit of Venus is an astronomical phenomenon. But geography was needed to make use of it.
Astronomers had to determine not just when a transit would take place, but also from where it could best be viewed. The more widely-spaced the observing stations, the better the resulting value for the Earth-Sun distance.
When observers on expeditions finally reached remote sites, much of their work was to establish exactly where they were. The necessary astronomical instruments were at least as important as those used to view the transit itself.
Both latitude and longitude had to be determined with extreme care. The longitude was particularly demanding, requiring the independent observation of eclipses, lunar distances and the moons of Jupiter.
Timing was crucial to observing the transits. Edmond Halley’s method required the recording of Venus’ two moments of “internal contact” with the Sun.
The official British expeditions of 1769 were supplied with clocks made by John Shelton, like the two shown here. Each has dials for minutes and seconds. Unlike domestic timekeepers the display of the hours was relatively unimportant.
The larger clock, known as a regulator, was the principal timekeeper. It was kept undisturbed in its own tent or enclosure and its rate checked against the motions of the stars.
The smaller journeyman clock was a secondary timepiece, installed alongside the observatory instruments. The example on the left has lost its original hood, allowing the movement to be seen.
During a transit, Venus appears as a small dark circle on the face of the Sun. Although it can be seen with the (suitably protected) naked eye, a telescope is essential to make useful observations of its motion and position.
The favoured instruments of the major British expeditions of the 18th century were reflecting telescopes, which use a mirror rather than a lens as the primary optical component. The preferred supplier was James Short, who specialised exclusively in the manufacture of reflectors.
The telescopes were used together with an expeditions clocks to see and time the key moments of a transit. By adding a micrometer, a telescope could also be converted into a measuring instrument, so that small angles such as the diameter of Venus could be accurately determined
(Courtesy Museum of the History of Science,
University of Oxford)