Blast from the pastMeasurement and morals in the early Transits of Venus

Stephen Johnston Museum of the History of ScienceUniversity of Oxford

PHYSTAT 05 - Statistical problems in Particle Physics, Astrophysics and CosmologyOxford, September 2005

The ChoiceThe past is a foreign country

They do things differently there - so it’s not directly comparable or relevant to today

There’s nothing new under sunDoesn’t matter whether it’s 1769 or 2005, there are fundamental similarities in the predicament of the researcher

Benjamin Martin, A View of 17 Transits of the Planet Venus (1757)

‘A Transit of the Planet Venus over the Sun’s Disk is not only the most rare but also the most curious Phaenomenon of the Heavens. And since the most Noble Problem in Nature, viz: the Dimension of the Solar System, is solvable only by it, & thereby renders it of the Greatest Consequence to Mankind, I concluded a proper Representation of all the Transits that happen in 1000 Years would prove no unacceptable Present to the Public.’

Benjamin Martin, 1757

Benjamin Martin (1705-1782)

“The Great Retailer of the Sciences”

SchoolmasterAuthor Lexicographer Itinerant lecturer Inventor Instrument maker Shopkeeper PublisherEntrepreneur

‘A Transit of the Planet Venus over the Sun’s Disk is not only the most rare but also the most curious Phaenomenon of the Heavens. And since the most Noble Problem in Nature, viz: the Dimension of the Solar System, is solvable only by it, & thereby renders it of the Greatest Consequence to Mankind, I concluded a proper Representation of all the Transits that happen in 1000 Years would prove no unacceptable Present to the Public.’

Benjamin Martin, 1757

Benjamin Martin, Venus in the Sun (London, 1761)

Transits of Venus:18th-century challenges

• One-day event in 1761 and 1769

• Global scattering of observers

• Mix of established observatories and makeshift observing stations

• International competition with nations at war

How to avoid error and inconsistency?

Captain James Cook

(National Maritime Museum, Greenwich)

Bark Endeavour (modern replica)

Tahiti, 1769 - through European eyes

Captain Cook in Tahiti: the Endeavour anchored off Fort Venus

Internal contact between Venus and the limb of the Sun

Transit observations by Cook and Green, Tahiti 1769 Philosophical Transactions (1771)

Transit observations by Cook and Green showing the ‘black drop’, Tahiti 1769,Philosophical Transactions (1771)

Samuel Dunn, ‘A Determination of the exact Moments of Time when the Planet Venus was at external and internal Contact with the Sun’s Limb’, Philosophical Transactions (1770)

Radcliffe ObservatoryFounded 1772

Thomas Hornsby (1733-1810)

1760 - Fellow of Corpus Christi College, Oxford

1763 - Savilian Professor of Astronomy

1763 - Professor of Experimental Philosophy

1772 - Radcliffe Observer

1782 - Sedleian Professor of Natural Philosophy

1783 - Radcliffe Librarian

Thomas Hornsby and the Transits of Venus

Observing:1761 - Shirburn Castle, Oxfordshire (Earl of Macclesfield)1769 - Tower of the Five Orders, Old Schools, Oxford

Publishing (in Philosophical Transactions):1763 - comparative analysis of 1761 transit1765 - planning suitable stations for 1769, including South Seas1769 - organising and reporting observing groups in Oxford1771 - comparative analysis of 1769 transit

‘It is well known to your Lordship, that the method practised by astronomers, in order to diminish the errors arising from the imperfections of instruments, and of the organs of sense, by taking the Mean of several observations, has not been so generally received, but that some persons, of considerable note, have been of opinion, and even publickly maintained, that one single observation, taken with due care, was as much to be relied on as the Mean of a great number.’ Thomas Simpson, 1756

‘And the more observations or experiments there are made, the less will the conclusion be liable to err, provided they admit of being repeated under the same circumstances.’

Thomas Simpson, ‘A letter to the Right Honorable George Earl of Macclesfield, President of the Royal Society, on the advantage of taking the mean of a number of observations, in practical astronomy’, Philosophical Transactions, 49 (1756), 93

Solar parallax from Mars in opposition, 1751

Thomas Hornsby, ‘A Discourse on the Parallax of the Sun’, Philosophical Transactions, 53 (1763)

Solar parallax from Mars in opposition, 1751 - according to Lacaille

Thomas Hornsby, ‘A Discourse on the Parallax of the Sun’, Philosophical Transactions, 53 (1763)

Planning for error reduction: Thomas Hornsby on the effect of solar altitude, 1765

Solar parallax from the transit of Venus, 1761

Thomas Hornsby, ‘A Discourse on the Parallax of the Sun’, Philosophical Transactions, 53 (1763)

Success in 1769

Thomas Hornsby, Philosophical Transactions (1771)

The solar system from Oxford; Thomas Hornsby (1771)

Thomas Hornsby’s calculation of solar parallax from the Transit of Venus, 1769, Philosophical Transactions (1771)

Thomas Hornsby transit calculations, 1769 (MHS Radcliffe MS 7)

‘If Oxford be supposed only 5.0 W of Greenwich then….’

Hornsby juggling observationsPonoi and Norriton are dropped, then Cook is tried out as the only reliable witness from George’s Island (Tahiti)