Fig. 13 – Title page of Introductio Geographica, Peter Apian (Ingolstadt, 1533)
{National Maritime Museum, Greenwich, London}
Fig. 14 – Decorated ivory cross-staff, by Thomas Tuttell, c.1700
{National Maritime Museum, Greenwich, London}
There were some surprisingly early attempts to determine longitude by versions of this method. In July 1612, the English Arctic explorer William Baffin observed the Moon’s transit, or passing, across his local meridian to determine longitude while in Greenland, but found it ‘somewhat difficult and troublesome’.11 He took the observation from land, recognizing that it would be impossible from a moving ship. Three years later, on an expedition in search of the North-West Passage, he made observations from the ship to determine longitude from the angular distance of the Moon from the Sun. The range of observations Baffin made on these voyages suggests that he was quite unusual among English mariners. For most, these forays into the complexities of astronomical navigation would have been entirely unfamiliar.
Even in the seventeenth century, the method was not well known beyond mathematical circles. Thus, when Charles II heard of St Pierre’s claims to have solved the problem of longitude by using this method, a commission was appointed to examine their validity. The commissioners foreshadowed those appointed forty years later by the 1714 Act. They included the President of the Royal Society, the King’s Master of Mechanics, professors of astronomy and mathematics, including Christopher Wren (1632–1723) and Robert Hooke, and other Fellows of the Royal Society. Most of them had also been responsible for judging Henry Bond’s magnetic scheme the year before and it is likely that St Pierre hoped for a reward, given the commissioners’ recommendation that the King ‘grant some present support for Mr Bonde’.12
However, as happened in France, it became clear that the method was unworkable without a vastly improved catalogue of stars and theory of the Moon’s motion. In both cases, the resulting recommendation was to found an observatory and appoint astronomers. Already co-opted to the Commission was John Flamsteed, a young astronomer who had impressed several Fellows of the Royal Society and had an influential patron in Jonas Moore, Surveyor-General of the Ordnance. On 4 March 1675, Charles II signed a royal warrant that appointed Flamsteed his ‘astronomical observator’ and charged him ‘to apply himself with the most exact Care and Diligence to the rectifying the Tables of the Motions of the Heavens, and the places of the fixed Stars, so as to find out the so much desired Longitude of Places for perfecting the art of Navigation’.13 An observatory, designed by Wren and Hooke, was built at Greenwich (Fig. 15) and Flamsteed began his long series of observations there on 16 September 1676.
It was in the second of Flamsteed’s tasks that he had greatest success, although it was over a lifetime of observation and was the cause of some tribulation. His great legacy was a much larger and more accurate catalogue of ‘fixed stars’ than previously existed. On the way to producing this master-work, Flamsteed published tables of Jupiter’s satellites and other observations, calculations and commentaries. He also shared data with astronomers and mathematicians across Europe as part of a reciprocal, scholarly correspondence. His relationship with one of the most important figures of the period, Isaac Newton, broke down when he felt this scholarly etiquette was ignored. Newton, desperate to have access to as many observations as he could for the improvement of his theoretical work, demanded, in Flamsteed’s view, too much and gave too little in return.
Fig. 15 – Royal Observatory from Crooms Hill, British School, c.1696
{National Maritime Museum, Greenwich, London}
Fig. 16 – The effects of the Sun on the Moon’s motion, from Isaac Newton’s Philosophiae Naturalis Principia Mathematica (Cambridge, 1713) (detail)
{National Maritime Museum, Greenwich, London}
A serious quarrel took place over Flamsteed’s catalogue. Newton, as President of the Royal Society, urged its publication and Prince George of Denmark, Queen Anne’s consort and Lord High Admiral, agreed to pay. Newton, Halley and other Royal Society ‘referees’ for the publication gained the upper hand by persuading Queen Anne to appoint a Royal Society committee as a Board of Visitors to the Royal Observatory, with power to tell the Astronomer Royal what to observe and publish. In 1712, an edition of Flamsteed’s valuable catalogue was published prematurely, without his knowledge and to his lasting fury. The process of assembling his own edition took the rest of his life and, thanks to the dedication of his wife and two assistants, appeared posthumously in 1725.
Newton’s interest in Flamsteed’s observations had been particularly intense when he was struggling with the second edition of his Principia. The first edition, with its mathematically expressed laws, including the inverse square law of universal gravitation, provided a new and essential framework for predicting the motions of the planets. However, the complexity of the Moon’s motion, caused by the interplay of the gravitational influences of Earth, Sun and Moon, was great enough to defeat Newton. He struggled again with lunar theory and the so-called three-body problem for the book’s 1713 edition (Fig. 16), building on observational data from Flamsteed and others and recalling later that ‘his head never ached but with his studies on the moon’.14
Newton wished to devise a theory, based on both mathematics and empirical observations, that was accurate to two minutes of arc (that is, to one-thirtieth of a degree). He felt that this high level of accuracy was necessary in the theory in order to achieve accuracy to within one degree in practical navigation observations. However, in this he failed. Despite his best efforts, his evidence to the parliamentary committee of 1714 had to state that the Moon’s ‘Theory is not yet exact enough’ to determine longitude at sea within one degree. Nevertheless, he gave the impression that this improvement would be forthcoming and that it was here that the long-awaited solution would lie.
‘a Watch to keep Time exactly’
The accuracy of Flamsteed’s observations depended on a recent revolution in timekeeping. One of the coordinates that indicates the position of stars is expressed as time: the moment at which a heavenly body passes, or transits, the observer’s local meridian (line of longitude). In order to record this accurately and precisely good clocks are required. Clocks had become capable of acting as scientific instruments, known as astronomical regulators, once they incorporated pendulums, the timekeeping properties of which had been observed by Galileo. It was left to the Dutch mathematician and astronomer Christiaan Huygens (1629–95) to apply this to a clock in 1657.
The Octagon Room of the Royal Observatory at Greenwich was designed around the pendulum clocks installed there (Fig. 17). Made by Thomas