On those very rare occasions when Venus is passing in front of the Sun, the jet-black circular outline of Venus has a diameter that is about thirty times smaller than the visible solar disc. A spot this size is just detectable to the unaided eye. Interestingly, however, the ancient astronomical histories record no such observations. Transits of Venus were unsuspected. Things changed in the seventeenth century. The German astronomer Johannes Kepler (1571 – 1630) used Tycho Brahe’s accurate planetary observations not only to work out the exact shape of planetary orbits but also to produce a reliable ephemeris of the ever-changing planetary positions in future years named the Rudolphine Tables in honour of the German Emperor Rudolf II (published in 1627). Kepler predicted that Venus would cross the Sun on 6^th December 1631. Unfortunately it was night-time in Europe and no observations are known from other places on Earth.

Jeremiah Horrocks (1618 – 1641) a young Liverpudlian astronomer had a similar fascination with astronomical tables, and was annoyed about their inconsistencies. He made his own evaluations of the position of Venus, and especially the times of its inferior conjunction (when the planet passes between the Earth and the Sun). Horrocks realised that Venus would actually transit the Sun on Sunday 24^th November 1639. (This date is ‘old style’. When England adopted the Gregorian Calendar in 1752 it became 4^th December). Not only did Horrocks confirm Kepler’s prediction that transits were spaced by about 120 years, he also discovered that they usually occurred in pairs, early December 1631 and 1639; early June 1761 and 1769; early December 1874 and 1882; early June 2004 and 2012 and so on. Horrocks was living in Much Hoole, Lancashire (some ten miles west of Preston), probably as the tutor to the children of the Stones family of Carr House. This obscure twenty-year-old predicted the 1639 transit less than a month before the event. He set up his simple refracting telescope on the third floor of Carr House so that it projected a six-inch diameter image of the solar disc onto a graduated circle. This circle was marked out in degrees so that Horrocks could measure the size of the disc of Venus, the direction of its path across the Sun and the speed at which it moved. For five hours on that cloudy, wintry Sunday Horrocks stayed with his telescope. At 3.15 p.m., half an hour before sunset, the western clouds broke. There was the large black disc of Venus on the edge of the Sun, and in the remaining thirty minutes he made three accurate measurements of its movement. Not only did this transit observation provide a key reference point on the orbit of Venus, it was also a major breakthrough in planetary astronomy, leading to an accurate estimation of the diameter of Venus, the Earth’s twin. Horrocks wrote up his observations and they were published as Venus in Sole Visa (Venus visible on the Sun) by the great Danzig astronomer Johannes Hevelius in 1662, twenty-one years after Horrocks’s untimely death when aged about 22.

Thirty-eight years after the Horrocks observation there was another planetary transit. This time it was the planet Mercury, and another precocious twenty-year-old English astronomer; Edmond Halley (1656 – 1742). Halley had dropped out of Oxford University to travel to the South Atlantic island of St Helena to observe the southern stars and the predicted transit of Mercury. On 28 October 1677 he recorded both the ingress and egress of Mercury onto the solar disc. (Ingress and egress are astronomical terms describing the planets ‘moving onto’ and ‘moving off’ the solar disc.) Jean-Charles Gallet also saw the egress from Avignon in France. All observers in England were defeated by clouds. Halley realised that an accurate timing of the transit by two widely spaced observers would “give a demonstration of the Sun’s Parallax, which hitherto was never proved, but by probable arguments” (see p. 40, Correspondence and papers of Edmond Halley, ed E. F. MacPike, Taylor & Francis, 1937). Unfortunately the 1677 observations were incomplete, the timings were inaccurate, and the final result was useless. But this observation sowed the seeds of a greater endeavour.

By 1716 Professor Halley (as Edmund had now become), was describing to the Royal Society (see Philosophical Transactions, Volume 29, pp 454 – 464) how the distance between the Earth and the Sun (a quantity known as the Astronomical Unit, or AU) could be calculated, if the time it took Venus to transit the Sun was measured accurately from two known sites separated by a large north-south distance. Subtracting one transit time from the other Halley noted that “if we have this difference true to two seconds it will be certain what the sun’s parallax is … to within its 500th part at least.”

The importance of the AU is underlined by the fact that our knowledge of the size of planets, and the distances, masses, sizes and energy output of stars depends on it. The accurate measurement of the AU became a major goal of renaissance astronomy. To quote Giovanni Antonio Rocca, writing in 1651, “the problem of solar distance and parallax was one of the most important in astronomy, well worth a lifetimes work by any astronomer.” Halley’s “500th part at least” galvanised eighteenth century astronomy and was a clarion call to adventurous observers who scurried to the four corners of the Earth to observe the Venus transits of June 1761 and 1769.

There were two approaches to the problem of measuring the AU. Halley’s method required observers to see both the ingress and the egress of Venus on the solar disc and accurately time the interval between them. Typically it takes Venus between five and six hours to cross the solar disc. The difference in transit times between northern and southern observers is about twenty minutes or so, if the lines of transit stay well away from the centre of the solar disc. It is this difference that needs to be timed to ± 2 seconds if Halley’s “500^th part at least” is to be attained. Joseph-Nicholas Delisle (1688 – 1768), professor of astronomy at the College de France, Paris proposed the second approach. Here observers needed to time either the ingress or the egress (and not both). The difference between ingress (or egress) times for eastern and western observers is about seven minutes or so. An accuracy of ± 1 second is required to reach Halley’s limit.

Both methods required collaborations. Each needed two successful observers, at widely separated, cloud-free sites. Single observations were useless. Because of the vagaries of the weather, success could only be guaranteed if many astronomers travelled to observe the event from different places. Both methods required the largest possible base-line. Halley’s had to be north-south, so astronomers chose cool, high latitudes in both the northern and southern hemisphere. Delisle’s method works best with a long east-west base-line (although it is possible to use a variant using a North-South baseline), so observers mainly travelled towards the sweltering, humid equator to spots as far apart in longitude as possible. Accurate maps were drawn indicating the exact segments of the Earth from which (for Halley’s) both ingress and egress could be seen (typically about 25% of the Earth’s surface), or (for Delisle’s) either ingress or egress (typically about 50% of the Earth’s surface in each case) could be seen. Delisle published such a chart in August 1760, less than a year before the 1761 transit. Weather records were scrutinised; the distribution of suitable continents, islands, cities and political affiliations were noted; ease of access was considered. Places where the Sun was too low in the sky (altitudes less than about 10°) were usually ruled out. The friendliness of the natives and the freedom from disease were borne in mind.

The observers needed to do two things. First they had to establish where they were. Today, with satellite navigation, this can be done to an accuracy of a few metres, in a minute or so. In those days it required many days of detailed solar and stellar observations taken using delicate transit instruments, coupled with accurate timings and the use of well-regulated state-of-the-art clocks. Even so accuracies of ± 100s of metres was the best that could be hoped for.Secondly the transit had to be timed. Consider ingress. There is an instant (t₁) when the disc of Venus first touches the disc of the Sun. This is difficult to time because you have to predict the exact place on the edge of the solar disc where this happens and point a well aligned, high magnification telescope at that spot so as to capture the moment of contact. Venus then appears to slowly move onto the solar disc, and there is a second instance (t₂) when it is just ‘on-board’, and the two discs are again just touching. The interval (t₂ – t₁) is about 1200 seconds.

The Halley and Delisle methods both required instants like t₂ to be timed to an accuracy of about one second. This proved to be completely impossible. The Sun is a huge sphere of hot gas, and its circular edge seems to be ‘boiling’ when observed through a telescope, especially after the sunlight has passed through the Earth’s turbulent atmosphere. The transiting disc of Venus is jet black. There is a huge difference between the intensity of the solar luminosity and the darkness of the disc of Venus and this contrast produces an irradiance which the eye finds great difficulty in dealing with. Venus does not break away cleanly from the solar rim. Venus appears pear-shaped, the neck lingeringly attached to the edge of the Sun. When the neck finally breaks Venus has seemingly jumped well onto the disc. Good, trained non-communicating observers, at the same site, found that their timings of the t₂ moment differed by as much as 30 to 40 seconds. This so-called black-drop effect was first noticed in 1761. James Ferguson (1710 – 1776) the Scottish astronomer, horologist and artist concluded that some of the timing error was due to using different sizes of telescopes with different magnifications (the larger the magnification the earlier t₂ was recorded.) It was also very clear that all eyes were not “equally quick and good.” Unfortunately the error was not diminished when photography was introduced in the nineteenth century. Instead of Halley’s hoped for “500^th part at least”, his method had plummeted to “a 15^th”, and Delisle’s was nearly a factor of two worse.

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Content last updated: 01/06/2004

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