Astronomy at the Old Bodleian: The 1769 Transit of Venus

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”. [1]

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?

Venus appears as a small black dot visible against the Sun, which appears large and orange.
The transit of Venus as photographed in 2004.

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. [2] 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”. [3]

But how would this be done? The answer lies in a phenomenon called parallax. [2] 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 [4], and many scientists and keen amateurs planned to make their own observations all around the world [3]. 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 [3].

Observations in Oxford

Thomas Hornsby, the Savilian Professor of Astronomy at the time, chose to make

The Tower of the Five Orders. It is built of pale stone and is ornamental pilllars and statues decorating it..
The Tower of the Five Orders today.

his observations from the top of the Tower of the Five Orders at the Bodleian Library [5]. 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.” [5]

Others made their own observations in locations including New College Tower and “an unfurnished room of the Hospital”. [5]

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. [5] 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. [2]

The results

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:

A series of drawings entitled "Appearances of Venus by Capt. Cook" showing Venus as a black circle with a grey halo around it , with the lower edge of the planet seeming to spread out as it crosses the edge of the Sun's disk.
Cook’s drawings of the black drop effect. [4]
“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”. [6]

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”. [6]

By the end of his calculations, Hornsby arrived at a figure of 93 726 900 miles [6] 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. [2]

However, different astronomers produced a wide range of different values [7], 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. [2] The final verdict was that the problem remained disappointingly unsolved. [7]

Final remarks

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. [8] 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. [2] And all of this, to me, makes the connection to our own Bodleian Library site all the more exciting.

References

[1] Bodleian Library – Wikipedia Accessed 6 Nov. 2023.

[2] Transits of Venus | The Royal Astronomical Society (ras.ac.uk) Accessed 6 Nov. 2023.

[3] “The Most Noble Problem in Nature” (ox.ac.uk) Accessed 6 Nov. 2023.

[4] 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

[5] 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

[6] 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

[7] “The Most Noble Problem in Nature” (ox.ac.uk) Accessed 6 Nov. 2023.

[8] Press Release (April 10, 2019): Astronomers Capture First Image of a Black Hole | Event Horizon Telescope Accessed 6 Nov. 2023.

 

Happy Halloween (or autumn) from the Oxford Library trainees!

Whether you celebrate Halloween or not, it is hard to deny that autumn is an atmospheric, spooky

Radcliffe camera with a sunset illuminating its top half and shadow on the bottom. Blue cloudy sky behind.
Photo of the Radcliffe Camera at sunset.

time to be in Oxford. Walking around the city in October, the trees are turning auburn, the mist is setting in, and, as the clocks change, the orangey streetlamps lamps are illuminated earlier and earlier. The night sky turns a deep royal blue. The clamour of ghost tours resonates around Radcliffe Square, reminding readers in the Old Bodleian of the city’s ghastly and rather gory history.

To honour this, I thought that there was no better time to explore some of the more ghoulish stories from the history of the Bodleian (and Oxford College) Libraries. For a Library which can trace its history back to 1320, you can imagine that these stories are plentiful. Through periods of history such as the town vs gown riots, which saw a Brasenose student killed by a local murderous mob, Radcliffe square has a hidden gruesome history. Thanks to Old Bodleian trainee Nia Everett, I have been informed about an even more prolifically gory historical function of this beautiful square: the Old Bodleian Anatomy school.

Founded in 1617, the Old Bodleian Anatomy school was active in this location for over 60 years, with frequent dissections occurring to instruct students on all aspects of physical human anatomy, including the slimy bits! For a current student or librarian, only allowed to take liquids into the library if contained in a keep-cup or sealable bottle, such activities are impossible to imagine taking place within the walls of the current Bodleian. However, to 17th century readers in anatomy, this study formed a crucial part of their daily scholarly practice. Such readers were able to demand the body of anyone executed within 21 miles of Oxford for use in their dissections and examinations.

A particularly oddly-spooky story from the Anatomy school is the tale of Anne Green’s

Early modern woodcut reading 'behold god's providence' with an image of anne green being hung and her resurrected from her coffin.
Image of Anne Green’s hanging

resurrection at the hands of William Petty, the Reader in Anatomy at the time of her execution. Anne was hung in Oxford castle yard, accused of committing infanticide. On a cold December night, awaiting to be dissected on the operating table, Anne Green was resurrected by Petty and his colleagues after they detected a faint pulse. Though she was brought back to life, her story ends with arguably another early modern horror: being married off to a man.

 

Turning away from horrors carried out within the Bodleian building, and towards its collections, there are plenty of spooky, magical, occult and alchemical manuscripts within the Bodleian’s special collections. For example, the Ripley Scroll, almost 6 metres long, as illustrated in this excellent TikTok from the Bodleian libraries: The Ripley Scroll. This manuscript instructs the reader how to create the philosophers stone- an item which Harry Potter readers will know can grant immortality. Additionally, many grimoires used by medieval priests to exorcise demons still remain within the Bodleian’s collections, such as this manuscript whose facsimiles are hosted on Digital Bodleian: MS. Rawl. D. 252.

To finish, I would ask you to consider the history all around you. Students at many Oxford Libraries can look up and see hundreds of years of architecture and artistry. College libraries can be over four hundred years old (Corpus Christi), or can be moved to cloisters (Brasenose) or be so grand, like my library at All Souls, that there are metres and metres of intricate cornices, shelving and windows to gaze up into. At the time of Halloween, All Souls requiems and frosty mornings, the vale between the present and the past becomes ever thinner, and the weight of history ever heavier.