On 6 June 2012 Venus crossed the sun for the second time in a decade. Transits of Venus occur in pairs, approximately 8 years apart, at intervals of over a century – prior to 8 June 2004 the last occurred in 1874, and the next will be in 2117 – and have yielded important astronomical data, not least as the basis for measuring the distance between the earth and the sun. To mark this important occasion, the last in our lifetimes, we take a look at the earliest record of a transit in the Whipple Collection.
Observing the transit of Venus only became possible following the invention of the telescope in the early seventeenth century, and the first to leave a written record was Jeremiah Horrocks, of Toxteth Park in Lancashire. Horrocks had studied at Emmanuel College Cambridge from 1632 to 1635 where he developed a serious interest in astronomy, and, as his later correspondence with William Crabtree, of Broughton, near Manchester, shows, he was much enamoured of the work of Johannes Kepler. Kepler had correctly anticipated the transits of both Mercury and Venus in 1631, but, as the latter occurred at night in Europe, it went unobserved, and Kepler believed the next would occur only in 1761. However, by observing the skies and revising existing tables Horrocks realised in the early autumn of 1639 that Venus would in fact cross the Sun again on or around 24 November that year.
Horrocks later recorded in his treatise Venus in sole visa how he communicated the news by letter to his younger brother Jonas and his friend Crabtree so that they could also prepare to observe the transit. Unfortunately the sky was too overcast in Liverpool for Jonas to catch a glimpse, and Crabtree was resigned to a similar fate in Salford until the sky cleared just before sunset (at approx. 3.35pm) to afford him a view. Horrocks, in the village of Much Hoole near Preston, was more fortunate:
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, and I was once more invited to the grateful task of repeating my observations. 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 disc on the left, so that the limbs of the Sun and Venus precisely coincided, forming an angle of contact. Not doubting that this was really the shadow of the planet, I immediately applied myself sedulously to observe it.
As Allan Chapman has demonstrated, besides recording the celestial events of 24 November and the calculations he derives from them, Horrocks’s Venus treatise was the product of several years of astronomical thought and study, and firmly aligned Horrocks with the New Astronomy movement originating in continental Europe. It was his enthusiastic advocacy of an observational and experimental approach to astronomy, in contrast to the more traditional computational approach via tables that made Horrocks popular with the pioneers of the Royal Society.
Horrocks died young in January 1641, before any of his work had been published, but by the efforts of a number of his Cambridge contemporaries, including John Wallis and John Worthington, some of his papers were preserved. Manuscript copies of Venus in sole visa were in circulation in the late 1650s, and by 1660 one had reached Huygens in Holland. According to Whatton’s memoir, this was the route by which Horrocks’s treatise reached Johannes Hevelius in Danzig, who published the Venus text alongside his own treatise on the transit of Mercury in 1662.
Hevelius occupies an important position in the history of scientific communication in the seventeenth century because of his close attention to detail in depicting his astronomical observations – even to the extent of describing the construction of the instruments – and the time he spent supervising his books through the press. Horrocks’s Venus in sole visacan only have benefitted by association with Hevelius in this way, reinforcing the Lancastrian’s credentials as an experimental astronomer.
R.S. Whipple purchased his copy of Hevelius’s works for £45 from his regular book dealer Thomas Court of Harrow on 13 June 1938. The volume contains 4 works – Epistolæ II (1654), Dissertatio, de Nativa Saturni Facie (1656), Mercurius in sole visus Gedani (1662), and the rare Machinæ Coelestis (1673) – and Whipple had it re-backed for 13s 6d the following year.
The stamp on the title page of the Epistolæ II shows that the volume previously belonged to the Radcliffe Observatory in Oxford. Construction of the Radcliffe Observatory began in 1772 at the suggestion of Dr Thomas Hornsby, Savilian Professor of Geometry, after he had observed the 1769 transit of Venus from a room in the neighbouring Radcliffe Infirmary, but the Observatory was closed and the building sold in 1934. It is now part of Green Templeton College. The Radcliffe Observatory Library was sold at auction by Sotheby’s in May 1935. Whipple was later to acquire a further four volumes from the collection, strengthening the representation of seventeenth-century astronomy found in the Whipple Collection today.
 Royal Society website: http://royalsociety.org/exhibitions/transit-venus/
 http://www.oxforddnb.com/view/article/13806, accessed 4 June 2012. Allan Chapman, ‘Jeremiah Horrocks, the transit of Venus, and the ‘New Astronomy’ in early seventeenth-century England’, Journal of the Royal Astronomical Society 31 (1990), 333-357.
 Johannis Hevelii Mercurius in Sole visus Gedani, anno christiano MDCLXI, d. III Maji, st. n. cum aliis quibusdam rerum coelestium observationibus, rarisq[ue] phaenomenis. Cui annexa est Venus in Sole pariter visa, anno 1639, d. 24 Nov. st. v. Liverpoliae, a Jeremia Horroxio, nunc primum edita … quibus accedit succinta Historiola, novae illius, ac mirae stellae in collo Ceti .. Nec non genuina delineatio paraselenarum & pareliorum quorundam rarissimorum. (Gedani, Simon Reiniger, 1662). Whipple Library, STORE: 13:12 OS
 A. B. Whatton, The Transit of Venus across the Sun: A Translation of the Celebrated Discourse Thereupon by the Rev. Jeremiah Horrox, to which is prefixed a Memoir of his Life and Labours (London, William Macintosh, ), p. 124.
 Chapman, ‘Jeremiah Horrocks’, pp 337-341.
 Whatton, Transit, p. 62.
 Johannis Hevelii Mercurius in Sole visus Gedani, anno christiano MDCLXI, d. III Maji, st. n. cum aliis quibusdam rerum coelestium observationibus, rarisq[ue] phaenomenis. Cui annexa est Venus in Sole pariter visa, anno 1639, d. 24 Nov. st. v. Liverpoliae, a Jeremia Horroxio, nunc primum edita … quibus accedit succinta Historiola, novae illius, ac mirae stellae in collo Ceti .. Nec non genuina delineatio paraselenarum & pareliorum quorundam rarissimorum. (Gedani, Simon Reiniger, 1662). Whipple Library STORE 13:12 OS
 Mary G. Winkler and Albert Van Helden, ‘Johannes Hevelius and the visual language of astronomy’ in J. V. Field and Frank A. J. L. James (eds), Renaissance and Revolution: Humanists, scholars, craftsmen and natural philosophers in early modern Europe (Cambridge, Cambridge University Press, 1993), 97-116.
 Many copies of Machinæ Coelestis were destroyed in a fire at Hevelius’s observatory in 1681. See Silvia De Renzi, Instruments in Print: Books from the Whipple Collection (Cambridge, Whipple Museum of the History of Science, 2000), p. 16.