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History of Venus Transits


Transit of Venus in 1882“There will be no other transit of Venus till the twenty-first century of our era has dawned upon the Earth, and the June flowers are blooming in 2004. When the last transit occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity which has led to our present knowledge was just beginning. What will be the state of science when the next transit season arrives, God only knows.”

Those famous words, written in 1882 by William Harkness about the rarity of the phenomenon, are a sobering reminder that no single person alive today has witnessed a transit of the planet Venus, across the Sun.


The great German astronomer, Johannes Kepler, correctly predicted a transit in 1631, but this was only visible from America, where people did not then have the telescopes to witness it. However, Horrocks, a 20-year-old amateur astronomer from England, spotted something about the movement of the planets that Kepler had missed that led directly to the first observation of a transit. Kepler had not calculated that there would be a date in 1639 when it would happen again and this is put down entirely to Horrocks' own originality. From a mixture of his own meticulous observations of Venus, the Sun and the planets, combined also with his analysis of the errors of the best available Astronomical tables, he realised that the inferior conjunction of 1639 - when Venus passes between the Earth and Sun - would neither be above it or below it but right across its middle. The transit took place on the 4 th December 1639 starting at 4pm but as the Sun had set by 5pm, only the early stages were visible.

Horrocks realised that there was about to be a transit just weeks before it happened and wrote to a friend, a cloth maker called William Crabtree in Salford, to keep an eye open. "I beseech you therefore with all thy strength to attend diligently with a telescope," Horrocks wrote. Both men subsequently observed the transit. They did this by projecting the image of the sun produced by their telescopes onto a piece of white cardboard supported in a frame behind the telescope eyepiece.

On this card is drawn a 6-inch circle in diameter, graduated from 0 to 360 degrees and oriented with the 180-degree mark toward the zenith, the point directly overhead. The distance of the card from the eyepiece is adjusted so that the Sun’s image exactly fills the 6-inch circle.

When Venus touches the Sun’s disc the position of the planet’s silhouette is marked on the card and the time recorded. The time is recorded again as the transit ends. If these measurements are made from a number of places, a known distance apart, the Sun’s parallax (that is the size of the diameter of the Earth) in arc seconds as seen from the Sun, can be estimated. From this, the distance between the two bodies can be calculated.

However there is a snag with this observation. The timing of the precise moment of the beginning and end of the transit is crucial to the accuracy of the subsequent calculations. This cannot be done at the instant Venus’s disc touches the Sun’s edge, or limb, because it cannot be seen until it has encroached at least a little bit onto the Sun and then the precise moment is past. The time when Venus just breaks contact with the inside of the Sun’s limb can also be recorded, but this instant is also not clear-cut. As Venus leaves the limb, a dark “neck”, known as the black drop effect, is drawn out between the planet and the Sun’s limb, which makes the time measurement uncertain. We now know that this is due to Venus’s dense atmosphere. 

This correct prediction and observation of the first transit of Venus led to the birth of English astronomy. Horrocks got it spot on, in other words, he predicted and observed the transit. He then went on with Crabtree to draw a number of fundamental facts about the nature of the solar system which, frankly, if they were done today would be classed as Nobel prize-winning discoveries. He discovered first of all that when you saw Venus in transit across the Sun, it was very small, much smaller than Kepler, Galileo and Tycho Brahe had suggested it should be. This led Horrocks to ask: why should an object appear to shine so brilliantly in the night sky but appear tiny in transit? What it shows very importantly is that Venus is a world. We might miss the significance of that but Horrocks was a Copernican and he was a staunch believer that the Earth moved around the Sun, He was aware that in Copernicus's ideas, the planets were globes, rock solid like our own. In the astronomy of Ptolemy and the geocentric Greeks, the Earth itself was the only solid body in nature. Horrocks was therefore using the transit to prove Copernic's theories to be correct. He is saying this object is rock solid ... and like a ball. We know today that Venus can be considered a twin planet to Earth, but with a very different atmosphere, where a runaway greenhouse effect causes temperatures to soar to 470°C.


Jeremiah Horrocks, the first person to predict and see a transit of Venus, lived in the Lancashire village of Much Hoole when, at the age of 20, he made his observation in 1639. Although Horrocks was a self-funded, self-educated astronomer with a telescope costing half a crown (12.5p), historians believe he should be considered the father of English astronomy. No-one is certain but Historians suspect that Horrocks was working as a tutor or schoolteacher, or possibly a Bible clerk for his parish church, who, in his spare time, dabbled in astronomy. Horrocks collaborated closely with William Crabtree, a cloth maker living in Salford who shared his interest in the stars and planets. Both men were totally self-educated in astronomy, because although we know that Horrocks went to Emmanuel College, he tells us quite plainly that nobody was taught mathematics in Cambridge in those days. Most of great continental astronomers had wealthy patrons, but not so Horrocks and Crabtree. The transit of Venus in 1639 was the first time that a pair of Englishmen had taken the whole of the great Continental tradition in astronomy and not only encapsulated it in terms of research but taken it one stage further. They had discovered things about nature that none of the great, officially patronised figures in astronomy had done. It was the beginning of English astronomy.


In 1768 a ship, the Endeavour, under the command of James Cook who discovered Australia, sailed from England to Tahiti in the Pacific ocean, from where the whole transit of 1769 would be visible. Measurements of the 1769 transit gave the Sun’s parallax as between 8.5 and 8.8 arc seconds. This compares with the modern value of 8.571 arc seconds.


Let’s not forget though, that the Transit of 6 th of December 1882 was widely observed from Ireland, with mixed fortunes. At Armagh Observatory it was observed by Dr JLE Dreyer (of New General Catalogue fame!), with the Grubb 15-inch reflector stopped down 7-inches. His assistant the Rev. Charles Faris, used the 3.8-inch achromatic finder, but snow showers hampered the early stages of observation. At Dunsink Observatory Dublin, Sir Robert Ball also observed with the 12-inch “South Telescope” which was stopped down to five inches. The weather here was not good either with heavy snow showers.

In Cork the weather was somewhat better and the transit was observed from the Queen’s College using the 8-inch Grubb refractor, stopped down to five inches, in a clear blue cloudless sky. Dr William Doberck also had good clear conditions at Markree Castle Observatory, Sligo, where he used the great 13-inch refractor stopped down to five inches.

The same was true of Dr William E Wilson at Daramona Observatory, West Meath, where he observed with the 24-inch reflector stopped down to 12-inches. He also used the 4-inch finder with a power of x75. He recorded that “the entire disc of Venus could be seen against the corona. The planet seemed much darker than the corona, and was surrounded by a thin “ring” of silvery light”. During the forenoon of the 6th, Wilson also observed in the chance of detecting the transit of a satellite of Venus, but with a negative result. Wilson also took a number of photographs on glass plates but none of these is known to have survived.



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