William Crabtree, Venus genius
The date is 1639 and star-gazing as we know it was about to be discovered. Most records attribute the calculation of the transit of Venus to Jeremiah Horrocks, but he was in constant correspondence on the subject with William Crabtree of Salford, and so the honour should be shared between both men.
The transit only happens once in a blue moon and it last occurred 8 June 2004 exactly as Crabtree predicted it would. The most recent previous transit was in 1882.
William Crabtree was one of only two people to spot the first recorded transit of Venus in 1639, but he remains the unsung hero of the event; an event which gave birth to astronomy as we know it in modern times.
The transit of Venus - which takes place when Venus passes directly between the earth and the sun - was actually calculated and predicted by Crabtree's friend, Jeremiah Horrocks, an astronomer living at Much Hoole in Lancashire. But while Horrocks is seen as the grandfather of British astronomy, Crabtree's contribution was crucial. Born in Broughton, now part of Salford, in 1610 and educated at Manchester's Grammar School, Crabtree was a cloth merchant who had become so wealthy he had plenty of spare time to practice his favourite hobby - astronomy.
Crabtree and Horrocks never met .According to the historian Crofton, Horrocks died around circa 1639, a few weeks before an arranged meeting could take place.
Using that and his own observations, Horrocks was able to calculate the real courses of Venus, the sun and the earth, and realised that a transit of Venus would take place on 24 November 1639. This was a major breakthrough. Another astronomer Johannes Kepler, had spotted a transit in 1631, but failed to spot that the events happened in twos, separated by eight years.
Horrocks, using his and Crabtree's work, realised this and predicted the 1639 date just weeks before it occurred - as well as predicting future transits, including 8 June 2004. He then wrote enthusiastically to Crabtree, adding: "I beseech you, therefore, with all thy strength to attend diligently with a telescope." In their respective homes the two then sat freezing on a cold winter's day, watching for the small black spot that signalled Venus crossing the sun.
They finally saw it at 3.15pm, just as the sun was going down, having spent about five hours looking out through their telescopes. This was not a popular activity, as 24 November 1639 was a Sunday and respectable gentlemen were not expected to waste their time on frivolities such as astronomy.
Crabtree's position was crucial. It was by comparing their record of exactly when Venus appeared on the edge of the sun that the two were able to make further calculations about the distance between the different astronomical bodies. They were then able to use that information to discover more about the courses of different planets as they orbit the sun, and predict future key events that would aid their work even further.
Their discovery formed the basis for what we know about planetary motion today, although we know little more about Crabtree's work, which if it continued, was lost to history after Horrocks - a far more recognised astronomer - died just over a year later in 1640.
Recognised Jerimiah Horrocks may be, but without William Crabtree's help and background calculations it is unlikely this event would ever have taken place. It is also likely that, had Crabtree not been attending diligently with his telescope, astronomy would not be where it is now, and so the honour of discovering the Venus transit should be shared.
The next Venus transit will take place on 6 June 2012, after which there will not be another until 2117.
Astronomer William Crabtree of Broughton recognisedIn June 2004 a commemorative street nameplate (see picture below) in memory of William Crabtree was unveiled at the junction of Lower Broughton Road and Priory Grove. This site marks the location that is thought most likely to have been the home of Crabtree and his family at the time when he was working with Horrocks. The Mayor of Salford, Cllr James Hulmes, and members of the Salford Astronomical Society were in attendance.
Local residents Carl Barry and Lilian Fletcher have compiled a detailed account, available to download (PDF document, 26Kb), of the events leading up to the unveiling of the new street nameplate.
This page was last updated on 22 October 2007
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