Most of us don’t know, but we have been deceived for hundreds of years with regard to Jeremiah Horrocks’ observation of the 1639 transit of Venus. Every picture showing the Cytherean silhoutte on the solar disk as recorded by Horrocks turns out to be plainly wrong. The source of all error: seventeenth century astronomer Johannes Hevelius.
Horrocks didn’t leave us any drawings of what he saw on that fateful day of December 4, 1639. All we have to go by is the wordy account of his observation, of which it is a miracle that it has come down to us at all. After his early death, several unfinished autograph manuscripts of his account Venus in sole visa were found in 1659 by John Worthington, Master of Jesus College, Cambridge, among the papers of the young astronomer’s friend and co-witness of the transit William Crabtree. Worthington sent two of the latest drafts to Samuel Hartlib, who in turn sent them to the mathematician Nicolaus Mercator, so that these could be combined into a single document – a publication that never came to be. The Greenwich Observatory and the Cambridge University Library currently hold three handwritten copies, which could be identified as identical to one of the copies sent to Mercator. A poor transcription and conflation based on these two manuscripts was obtained by Christiaan Huygens from Paul Neile, who sent it to Johannes Hevelius. The latter would finally publish Horrocks’ Venus in sole visa as an annex to his book Mercurius in sole visus Gedani, published in 1662.
Hevelius added an image of the solar disk to the report, based on Horrocks’ description of his observation. This is the first image that’s in error.
Hevelius’ image (to the left) shows the solar disk, with the zenith at the top and the three positions of Venus as observed by Horrocks’ between ingress and sunset. However, if the path of Venus relative to the zenith is computed for the 1639 transit as observed from Much Hoole, the difference with Hevelius’ rendering becomes readily apparent: in the image of Hevelius, Venus is moving towards the horizon, whereas in reality Horrocks must have seen the planet moving towards the zenith. Also, the three positions of Venus are spaced equally in Hevelius’ drawing, whereas the three observations by Horrocks were made at intervals of 20 and 10 minutes respectively.
This representation of the solar disk is not how Horrocks would have seen the image of sun on his screen. The solar image projected by the Galileian telescope used by him, was inverted top to bottom, but not left to right. So, while Venus was in the bottom-left corner of the sun in reality, Horrocks would have seen Venus in the top-left corner of the projected image. Hevelius however, in his printed publication of Horrocks’ account, changes the astronomer’s own words to place Venus in the top-right corner of the projected solar image:
Primò pro Inclinatione Lineâ diametrali perpendiculariter ad Horizontem insistenti circuli tamen plano ob Solis altitudinem aliquantum reclinato, inveni Veneris umbram hora dicta 3 15’ Solis discum intrasse grad. 62 30′ circiter (certe inter gr. 60 & 65) à vertice ad dextram.
(In the first place, with respect to the inclination, the line of the diameter of the circle being perpendicular to the horizon, although its plane was somewhat inclined on account of the Sun’s altitude, I found that the shadow of Venus at the aforesaid hour, namely fifteen minutes past three, had entered the Sun’s disc about 62 30′, certainly between 60 and 65, from the top towards the right.)
The three handwritten manuscripts of Venus in sole visa all have “ad sinistram” (to the left) instead of “ad dextram”, as does a mid-1659 letter of Nicolaus Mercator to Samuel Hartlib, quoting directly from Horrocks’ own writing. Still, Hevelius thought Horrocks was in error and changed it to “ad dextram”. Also, John Wallis added the note “lege dextram” (read right) after “sinistram” in his Opera Posthumus of Horrocks. Both Hevelius and Wallis must have been confused about the orientation of the projected image by a Galileian telescope. Their error is also reflected in three historic paintings of the 1639 observation of the transit of Venus.
The 1903 painting of Jeremiah Horrocks by J.W. Lavender (to the left) and the mural by Ford Madox Brown from the 1880s showing William Crabtree (in the middle) both have the silhouette of Venus at the top-right corner of the solar disk, while is should have been in the top-left corner. The third painting, a 1891 oil painting by Eyre Crowe (to the right), doesn’t show Venus at all, but the eyes of Horrocks are directed to the upper right corner of the solar image. The 1859 stained glass window in the church of Much Hoole also shows Horrocks viewing the projected image of the sun; here, Venus is incorrectly at the bottom-left corner.