Wednesday, 29 June 2011

Maskelyne, Mason and Dixon, 1761

2011 is the 250th Anniversary of Nevil Maskylene’s visit to St. Helena to observe the June 1761 Transit of Venus and is also the bicentenary of his death.  Charles Mason and Jeremiah Dixon visited St Helena in October the same year returning from observing the Transit in South Africa.

When I first started reading about Maskelyne the most widely available picture of him was the one below.
Nevil Maskelyne by John Downman, BHC 2854. National Maritime Museum

The Board of Longitude blog, recently posted that the National Maritime Museum now describe this picture as "Formerly called Nevil Maskelyne" as there is some doubt that it is of Maskelyne for the sensible reason that it does not look all that much like the other known portraits.  The late Derek Howse, formerly Head of Astronomy at NMM and biographer of Maskelyne firmly believed that this portrait is misidentified on the basis of its great dissimilarity to Louis van der Puyl's undoubted Maskelyne portrait of 1785 owned by the Royal Society and that this otherwise fine portrait is therefore probably another astronomer, so far unidentified.

Van der Puyl shows him in clerical dress appropriate to his formal status as a clerk in Holy Orders against the background of the Royal Observatory, Greenwich.

In 1716 Edmond Halley, who had visited St Helena in 1677 and 1700, noted that an accurate timing of the transit of Venus would aid the calculation of the distance between the Earth and the Sun.  Although Halley knew he would not live to see the next anticipated Venus transit in 1761, his detailed plans and papers ensured that expeditions were sent out to the furthest reaches of the globe to make observations.  The 1761 transit  observations constituted the largest international scientific undertaking up to that time and despite the fact that it took place during the latter half of the Seven Years' War (1756-1763), a worldwide conflict involving the major powers of the time and their colonies, it was to be observed by many astronomers at over one hundred locations involving 170 observers scattered from Peking to Newfoundland though, as we shall see, not always from the location planned.
Janisch records that The Directors of the East India Company in London wrote to The Governor of St Helena (Letters from England, Dec. 31, 1760)
His Majesty having been graciously pleased to encourage the making observations on the transit of the planet Venus over the Sun's disk on the 6th June next and proper persons being engaged by the Royal Society for the purpose two of them, Mr. Charles Mason and Mr. Jeremiah Dixon proceed to Fort Marlborough* on H. M. Ship Seahorse and the other two Revd. Mr. Nevil Maskelyne and Mr. Robert Waddington take passage on the Prince Henry to St. Helena.  As this is done to make some improvements in Astronomy which will be of general utility the two last named gentlemen are upon their arrival and during their stay to be accommodated by you in a suitable manner with diet and apartments at the Company's expense and you are to give them all the assistance as to materials, workmen, and whatsoever else the service they are employed upon may require.

*Fort Marlborough was the EIC base at Bencoolen, a pepper trading centre on the southern part of the west coast of Sumatra.  A garrison had been established there in 1685 and the Fort was built in 1714.  St Helena and Bencoolen were linked both through trade and personnel.

The Governor replied (Letter to England, May 26, 1761)

Rev. Nevil Maskelyne and Mr. Robt. Waddington shall be accommodated in a suitable manner with diet and apartments at the Company's expense. We have already erected an observatory for them in the country and shall furnish whatever else the service may require.

Transit Diagram

A graduate of Trinity College, Cambridge, Maskelyne (1732 - 1811) became assistant to the then Astronomer Royal, Bradley, in 1757.  He left England for St. Helena in January 1761 to observe the June 6th transit, with the aim of using this information to calculate the distance of the Earth from the Sun.  Having set up his observatory on the high ridge behind Alarm House low cloud prevented any useful observations being made, though several people reportedly saw it in Jamestown.  He returned to England in May 1762. 
He evidently had a taste for good living. Chambers Edinburgh Journal for 1848 records the following in regard to his trip. “In a curious estimate which he drew up of his expenses for the voyage and sojourn on the island for one year we find 13 guineas set down for washing, for board 109 guineas, for liquors 141 guineas. 5 shillings per day was reckoned as the charge for drink while on the island and £50 for the same item for the voyage out and home.  Maskelyne was a clergyman but his habits would have ill accorded with our present notions of temperance.”
Mason and Dixon are remembered today primarily for their work between 1763 and 1767 in the resolution of a border dispute between British colonies of Maryland and Pennsylvania/Delaware in Colonial America. The disputants, the Calverts of Maryland and the Penns of Pennsylvania, engaged the astronomer Mason and surveyor Dixon, to survey what became known as the Mason–Dixon Line, and paid £3,512 to have the 327 miles surveyed.  In popular usage, especially since the Missouri Compromise of 1820, the Mason Dixon Line symbolises a cultural boundary between the Northeastern United States and the Southern United States (Dixie).
Having been chosen, in 1760, by the Royal Society to go to Bencoolen to observe the 1761 transit Mason suggested that Dixon should go as his assistant.  On January 10th 1761 their ship, the HMS Seahorse, was attacked by L’Grande a 34 gun French frigate and they had to return to Portsmouth with 11 killed and 37 wounded. Wanting to call the expedition off they were threatened with dire personal consequences “they may assure themselves of being treated by the Council with the most inflexible resentment, and prosecuted with the utmost Severity of Law” and were prodded back to sea by the Royal Society. Their second voyage wouldn't have got them to Sumatra in time (and Bencoolen had been captured by the French in August 1760), so they stopped at Cape Town, South Africa, as guests of the local governor and set up their equipment. They obtained several accurate measurements of Venus's position on the solar disc, providing some of the most useful data from the 1761 event, as it was the only successful observation made from the Southern Hemisphere.  On the passage home, they stopped at St Helena in October and, after discussion with Maskelyne, who had failed to observe the transit there, Dixon returned temporarily to the Cape with Maskelyne's clock to carry out gravity experiments.  Returning via St Helena he and Mason reached England in February 1762, each £230 richer. 

Despite being watched by so many observers the results were disappointing.  Bad weather, navigational errors, and difficulty timing Venus's image against the sun, all contributed to the failure and with no method for accurately calculating the longitude of most observation sites, no useful results were obtained.  Venus was expected to transit the Sun again on 3 June 1769, but if this opportunity was also lost, it would be another 105 years before the transit next occurred and another attempt at recording it could be made.
With this in mind, in 1768 the Royal Society sent a petition to King George III requesting assistance to send a scientific expedition to the South Seas to observe the forethcoming transit.  The petition, supported by the Greenwich Observatory, sought the sum of £4,000 to defray the costs of a Pacific expedition which would, it was stressed, enhance Britain’s imperial ambitions and scientific reputation, and improve navigation and trade.  The petition was soon approved.  Maskelyne, by this time Astronomer Royal and fellow of the Royal Society, had calculated that the best possible vantage point south of the equator was between the Marquesas Islands and Tonga.  The Royal Society informed the Admiralty that Tahiti, located almost at the centre of the area calculated by Maskelyne was its desired site for the observation of the Transit and also requested that naturalist Joseph Banks and his party be permitted to join the expedition.  In August 1768 HMS Endeavour left Plymouth on Cook’s first voyage of discovery. Cook had much better weather than Maskelyne had on St. Helena, writing: The Measurements gathered by Cook and others in 1769 were pretty rough but could be used to estimate the distance of the Earth from the Sun and, by extension, the size of the solar system and the universe.
June 3rd 1769. "not a clowd seen the whole day, and air perfectly clear, so we had every advantage we could desire in observing the whole passage of the planet Venus over the suns disk".
Returning to England, Cook and Banks spent from 1st to 4th May 1771 on St Helena.

Maskelyne, later known as "The Seaman's Astronomer" used his journey to St Helena to develop a method of calculating longitude called the lunar distance method and he used this method to establish for the first time the precise longitude of St Helena.  A vested interest in an astronomical solution to the longitude problem could have been seen as a conflict of interest, but this did not stop the Board of Longitude sending Maskelyne to Barbados in 1763 to test Harrison's No. 4 timekeeper.  His advocacy of this method meant that he has gone down in history as the villain of the longitude saga, the man who despised and cheated the self-educated genius John Harrison out of the prize he earned for his brilliant timepieces, which helped fix the exact time at sea and so determine longitude and the position of sailing ships on the ocean.
He was appointed the fifth Astronomer Royal in 1765 and died at the Greenwich observatory, still in office after 46 years, in February 1811.  He is buried in the churchyard of St Mary the Virgin, the parish church of the village of Purton Wiltshire where the Maskelynes had lived from the 16th century.

After completing the boundary survey in America, Mason worked under Maskelyne at Greenwich where he contributed to the Nautical Almanac.  In September 1786 he wrote to Benjamin Franklin that he had returned to America with his wife, seven sons, and one daughter.  He provided no explanation for his return and he died on October 26, 1786, in Philadelphia.

On returning to England from America Dixon sailed to Norway in 1769 to observe another transit of Venus after which Dixon resumed his work as a surveyor in Durham.  He died unmarried in January 1779.

Edmund Halley Blog post

The National Maritime Museum and the Department of history and Philosophy of Science, University of Cambridge, are currently working on a five-year research project on the British Board of Longitude, funded by the Arts and Humanities Research Council.

Bencoolen Link.

Cook on St. Helena

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