page_title: 1639 transit of Venus
section_title: Observation of the transit
hierarchical_section_title: 1639 transit of Venus / Observation of the transit
caption_attribution_description: A representation of Jeremiah Horrocks' observation of a Venus transit across the Sun in 1639, publshed by Johannes Hevelius with Horrocks' Venus in sole visa in 1662. Hevelius seems to have been confused about the orientation of the image projected by Horrocks' Galilean telescope and has shown an inverted mirror image of what he would have seen. Horrocks wrote: "I found that the shadow of Venus at the aforesaid hour, namely fifteen minutes past three, had entered the Sun's disk about 620 30', certainly between 600 and 650, from the top towards the right. This was the appearance in the dark apartment; therefore out of doors beneath the open sky, according to the laws of optics, the contrary would be the case, and Venus would be below the centre of the Sun, distant 620 30' from the lower limb, or the nadir, as the Arabians term it." The image also shows Venus as three evenly spaced disks whereas the observations were made at irregular intervals. see: http://transitofvenus.nl/wp/2012/01/25/what-did-horrocks-really-see/
context_page_description: The first known observations and recording of a transit of Venus were made in 1639 by the English astronomers Jeremiah Horrocks and his friend and correspondent William Crabtree. The pair made their observations independently on 4 December that year; Horrocks from Carr House, then in the village of Much Hoole, Lancashire, and Crabtree from his home in Broughton, near Manchester.
  The friends, followers of the new astronomy of Johannes Kepler, were self-taught mathematical astronomers who had worked methodically to correct and improve Kepler's Rudolphine tables by observation and measurement. In 1639, Horrocks was the only astronomer to realise that a transit of Venus was imminent; others became aware of it only after the event when Horrocks's report of it was circulated. Although the friends both died within five years of making their observations, their ground-breaking work was influential in establishing the size of the Solar System; for this and their other achievements Horrocks and Crabtree, along with their correspondent William Gascoigne, are considered to be the founding fathers of British research astronomy.
context_section_description: Horrocks was concerned that the weather would be unfavourable for the transit as he believed the rare planetary conjunction would produce severe weather:
  The chance of a clouded atmosphere caused me much anxiety; for Jupiter and Mercury were in conjunction with the Sun almost at the same time as Venus. This remarkable assemblage of the planets (as if they were desirous of beholding, in common with ourselves, the wonders of the heavens, and of adding to the splendour of the scene), seemed to forebode great severity of weather. Mercury, whose conjunction with the Sun is invariably attended with storm and tempest, was especially to be feared. In this apprehension I coincide with the opinion of the astrologers, because it is confirmed by experience; but in other respects I cannot help despising their more puerile vanities.
  — Jeremiah Horrocks, Venus in sole visa
  At around midday on 23 November Horrocks darkened his room and focused the rays of sunlight coming through the window onto the paper where the image could be observed safely. At his location in Much Hoole (the latitude of which he determined to be 53° 35'), he calculated that the transit should begin at about 3:00 pm on Sunday the 24th, but he began his observations the previous day fearing that he might miss the event if his calculations proved to be inaccurate. On the Sunday he began observing at sunrise, the weather was cloudy, but he first saw the tiny black shadow of Venus crossing the Sun at about 3:15 pm, and observed for half an hour until sunset at 3:53 pm.
  When the time of the observation approached, I retired to my apartment, and having closed the windows against the light, I directed my telescope, previously adjusted to a focus, through the aperture towards the Sun and received his rays at right angles upon the paper ... I watched carefully on the 24th from sunrise to nine o'clock, and from a little before ten until noon, and at one in the afternoon, being called away in the intervals by business of the highest importance which, for these ornamental pursuits, I could not with propriety neglect ... About fifteen minutes past three in the afternoon, when I was again at liberty to continue my labours, the clouds, as if by divine interposition, were entirely dispersed ... I then beheld a most agreeable spectacle, the object of my sanguine wishes, a spot of unusual magnitude and of a perfectly circular shape, which had already fully entered upon the Sun's disk on the left ... Not doubting that this was really the shadow of the planet, I immediately applied myself sedulously to observe it ... although Venus continued on the disk for several hours, she was not visible to me longer than half-an-hour, on account of [the Sun] so quickly setting ... The inclination was the only point upon which I failed to attain the utmost precision; for, owing to the rapid motion of the Sun, it was difficult to observe with certainty to a single degree ... But all the rest is sufficiently accurate, and as exact as I could desire.
  — Jeremiah Horrocks, Venus in sole visa'
  Crabtree made his observations using a similar set-up but had insufficient time to make any measurements, as it was cloudy in Broughton, and thus he only saw the transit briefly. According to Horrocks: "Rapt in contemplation he stood for some time, scarcely trusting his own senses, through excess of joy ... In a little while, the clouds again obscured the face of the Sun, so that he could observe nothing more than that Venus was certainly on the disc at the time."  Afterwards, he made "so rapid a sketch" of Venus as it had passed across the Sun's disc, allowing Crabtree to estimate the angular size of Venus to be 1' 3", accurate to within 1 second of arc of its actual size; Horrocks's estimate of 1' 12" was less accurate.
caption: An image of the solar disk Hevelius added to his report, based on Horrocks's description of his observation. The image does not, however, give a true representation of what Horrocks would have seen.[note 2]
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