‘This observation was made in an obscure village where I have long been in the habit of observing, about fifteen miles to the north of Liverpool, the latitude of which I believe to be 53° 20', although by common maps it is stated at 54° 12', therefore the latitude of the village will be 53° 35', and longitude of both 22° 30' from the Fortunate Islands, now called the Canaries. This is 14° 15' to the west of Uraniburg in Denmark, the longitude of which is stated by Brahé, a native of the place, to be 36° 45' from these islands.
‘This is all I could observe respecting this celebrated conjunction during the short time the Sun remained in the horizon: for although Venus continued on his disc for several hours, she was not visible to me longer than half an hour on account of his so quickly setting. Nevertheless, all the observations which could possibly be made in so short a time I was enabled by Divine Providence to complete so effectually that I could scarcely have wished for a more extended period. 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, and I frankly confess that I neither did nor could ascertain it. But all the rest is sufficiently accurate, and as exact as I could desire.’
Besides having ascertained that the diameter of Venus subtends an angle not much greater than one minute of arc, Horrox reduced the horizontal solar parallax from fifty-seven seconds as stated by Kepler to fourteen seconds, a calculation within one and a half second of the value assigned to it by Halley sixty years after. He also reduced the Sun’s semi-diameter.
Crabtree, to whom Horrox refers as ‘his most esteemed friend and a person who has few superiors in mathematical learning,’ made preparations to observe the transit similar to those already described. But the day was unfavourable, dark clouds obscured the sky and rendered the Sun invisible. Crabtree was in despair, and relinquished all hope of being able to witness the conjunction. However, just before sunset there was a break in the clouds, and the Sun shone brilliantly for a short interval. Crabtree at once seized his opportunity, and to his intense delight observed the planet fully entered upon the Sun’s disc. Instead of proceeding to take observations, he was so overcome with emotion at the sight of the phenomenon, that he continued to gaze upon it with rapt attention, nor did he recover his self-possession until the clouds again hid from his view the setting Sun.[4]
Crabtree’s observation of the transit was, however, not a fruitless one. He drew from memory a diagram showing the exact position of Venus on the Sun’s disc, which corresponded in every respect with Horrox’s observation; he also estimated the diameter of the planet to be 7/200 that of the Sun, which when calculated gives one minute three seconds; Horrox having found it to be one minute twelve seconds. This transit of Venus is remarkable as having been the first ever observed of which there is any record, and for this we are indebted to the genius of Horrox, who by a series of calculations, displaying a wonderfully accurate knowledge of mathematics, was enabled to predict the occurrence of the phenomenon on the very day, and almost at the hour it appeared, and of which he and his friend Crabtree were the only observers.
Having thought it desirable to write an account of the transit, Horrox prepared an elegant Latin treatise, entitled ‘Venus in Sole Visa’—‘Venus seen in the Sun;’ but not knowing what steps to take with regard to its publication, he requested Crabtree to communicate with his bookseller and obtain his advice on the matter.