When you’re working at the Radcliffe Camera and the Bodleian Old Library, you sometimes end up fielding questions about the history of these establishments from curious readers, and so a colleague advised me early on to do a little bit of reading on the subject. We even keep a helpful printout of the Wikipedia page for the Bodleian Library at the Proscholium (the main entrance), and as I was looking through this a sentence caught my attention:
“The astronomer Thomas Hornsby observed the transit of Venus from the Tower of the Five Orders in 1769”. 
Interesting, I thought – having studied astrophysics at university, I’m a little bit of a space nerd. So, I started diving deeper into the topic.
What is the transit of Venus, and why was it important to observe?
The transit of Venus simply refers to Venus crossing directly between the Earth and the Sun, like the moon does during a lunar eclipse. Since Venus is significantly further away from us than the moon, it appears much smaller, and so during its transit we would see a small black dot moving across the face of the Sun. The last two transits of Venus occurred in 2012 and 2004, and the next one won’t be until 2117.  Nowadays, an event like the transit of Venus is interesting to watch, and is a good way to get people interested in astronomy, but back in the 18th century, it was also of real scientific significance.
Edmond Halley (1656-1742), of Halley’s Comet fame, was the one to suggest that the 1761 and 1769 transits of Venus would be the perfect opportunities to take some measurements which could be used to calculate the distance of the Earth from the Sun, a question that became known as “the most noble problem in Nature”. 
But how would this be done? The answer lies in a phenomenon called parallax.  The simplest demonstration of parallax is to hold a finger a little distance in front of your nose, and close one eye, then the other. You should notice that your finger seems to move, because you’re now looking at it from a different angle. If you experiment with holding your finger at different distances from your face, the size of this effect will change. Similarly, if you watch the transit of Venus from multiple places on Earth, it will cross the edge of the Sun at very slightly different times, and if these times are measured accurately enough, you can work out the distances involved.
The 1769 transit
To get the best results, observations need to be made as far apart as possible. James Cook and his crew were to journey to Tahiti to observe the phenomenon there , and many scientists and keen amateurs planned to make their own observations all around the world . As Bridgerton fans may recall, even King George III observed the transit.
The phenomenon really captured the public imagination. Lectures were held in the lead up to the event, and a wide range of prints and instruments were sold .
Observations in Oxford
Thomas Hornsby, the Savilian Professor of Astronomy at the time, chose to make
his observations from the top of the Tower of the Five Orders at the Bodleian Library . He described his reasoning as follows, in an article published by the Royal Society:
“I proposed to observe the transit of Venus and the Sun’s eclipse in the upper room of the tower of the Schools, which, though the floor of it be very unsteady, yet from its elevated situation afforded me the clearest view of the north-west part of the horizon, and is indeed the best place for making occasional observations in different parts of the heavens, and at different altitude, which this place at present affords.” 
Others made their own observations in locations including New College Tower and “an unfurnished room of the Hospital”. 
Hornsby described that although initially “the wind sometimes blew so hard as to incommode the observer”, the weather conditions soon became favourable to observe the transit.  However, he encountered the same problem as all the other observers: a phenomenon known at the time as the black drop effect, whereby Venus appears to stretch out and become pear-shaped as it meets the edge of the Sun’s disk. This, combined with the fact that the edge of the Sun’s disk appears darker than the centre, makes it very difficult to accurately judge the time at which Venus crosses the edge of the Sun. 
Thomas Hornsby was one of several scientists who combined some of the data from different locations to attempt to calculate the distance from the Earth to the Sun. I found it fascinating that in his paper he does discuss ideas about errors and accuracy, albeit not in the quantitative way that a modern scientist would:
“From the near agreement of the several results before found… and affected only by the necessary error in observing, the accuracy of the observation… is abundantly confirmed”. 
I also enjoyed the following sentence, which I can’t imagine ever seeing in a modern scientific paper, in which he explains an alteration he has made to the data gathered by the French astronomer Pingré:
“And Mr. Pingré… will probably be of the opinion, that an error of one minute was committed in writing down the time of his observation, as was conjectured by many persons, as well as myself; a mistake to which the most experienced observer is sometimes liable”. 
By the end of his calculations, Hornsby arrived at a figure of 93 726 900 miles  as the distance from the Earth to the Sun. Remarkably, there is only a 0.8% error in this compared to the modern value of 92 955 800 miles. 
However, different astronomers produced a wide range of different values , meaning that unfortunately, what we now know was a highly accurate result for Hornsby was a lucky fluke. Astronomers realised there were large errors in their data: instead of timings being precise to within a second, as they had hoped, there were uncertainties of about a minute, due to the black drop effect and the dark appearance of the edge of the Sun’s disk.  The final verdict was that the problem remained disappointingly unsolved. 
Despite the lack of a conclusive answer, I think this remains a fascinating part of the history of astronomy. The worldwide nature of the observations to my mind echoes modern enterprises such as the Event Horizon Experiment, which combines radio telescopes all around the world into effectively one huge telescope and so was able to take the first photo of a black hole in 2019.  Furthermore, the story of transit observations continues today as a key way in which astronomers are able to discover planets orbiting stars outside of our Solar System.  And all of this, to me, makes the connection to our own Bodleian Library site all the more exciting.
 Bodleian Library – Wikipedia Accessed 6 Nov. 2023.
 Transits of Venus | The Royal Astronomical Society (ras.ac.uk) Accessed 6 Nov. 2023.
 “The Most Noble Problem in Nature” (ox.ac.uk) Accessed 6 Nov. 2023.
 Cook, James, and Charles Green. “Observations Made, by Appointment of the Royal Society, at King George’s Island in the South Sea; By Mr. Charles Green, Formerly Assistant at the Royal Observatory at Greenwich, and Lieut. James Cook, of His Majesty’s Ship the Endeavour.” Philosophical Transactions (1683-1775), vol. 61, 1771, pp. 397–421. JSTOR, http://www.jstor.org/stable/106113 Accessed 6 Nov. 2023. https://solo.bodleian.ox.ac.uk/permalink/44OXF_INST/ao2p7t/cdi_jstor_primary_106113
 Hornsby, Thomas. “An Account of the Observations of the Transit of Venus and of the Eclipse of the Sun, Made at Shirburn Castle and at Oxford. By the Reverend Thomas Hornsby, M. A. F. R. S. and Savilian Professor of Astronomy in the University of Oxford.” Philosophical Transactions (1683-1775), vol. 59, 1769, pp. 172–82. JSTOR, http://www.jstor.org/stable/105821 Accessed 6 Nov. 2023. https://solo.bodleian.ox.ac.uk/permalink/44OXF_INST/ao2p7t/cdi_jstor_primary_105821
 Hornsby, Thomas. “The Quantity of the Sun’s Parallax, as Deduced from the Observations of the Transit of Venus, on June 3, 1769: By Thomas Hornsby, M. A. Savilian Professor of Astronomy in the University of Oxford, and F. R. S.” Philosophical Transactions (1683-1775), vol. 61, 1771, pp. 574–79. JSTOR, http://www.jstor.org/stable/106123 Accessed 6 Nov. 2023. https://solo.bodleian.ox.ac.uk/permalink/44OXF_INST/ao2p7t/cdi_jstor_primary_106123
 “The Most Noble Problem in Nature” (ox.ac.uk) Accessed 6 Nov. 2023.