While Halley did not produce updated charts, others did and they were put to use. However, the fact that this inexact, localized, practice-based and changeable method was not mentioned at the 1714 parliamentary committee underlines Newton’s view at the time that Parliament should be looking for a more complete solution. While Whiston and Ditton’s scheme had to be mentioned – and, by Whiston’s account, Newton’s initial ignoring of it risked the complete rejection of the proposed legislation – it perhaps served as a contrast to the great aim of finding a method that could be applied confidently at any location. It was, Newton suggested, only the astronomical and timekeeper solutions that held out that promise.
‘the Eclipses of Jupiter’s Satellites’
While rocket signals and magnetic schemes were about finding a means of fixing position relative to a known location (a moored hulk or charted magnetic feature), the other methods focused on the long-understood relationship between time and longitude. These were the only universal solutions to finding longitude at sea and, as Newton explained more than once to the Admiralty, only astronomical methods could be used to find longitude if it had been lost. The downside of astronomy was that observations could usually only be done at night, and sometimes irregularly if the target object was not in the right position, which meant that dead reckoning and other techniques were still required. Observations could also be also hampered by clouds, although this was equally true for calculating latitude and local time, without which neither astronomical nor timekeeper methods were effective.
The use of lunar and solar eclipses was the earliest of several potential astronomical methods for finding longitude. One key branch of research was to find ways of using the Moon’s position on a more regular basis, while the discovery of the moons (satellites) of Jupiter, with much more frequent eclipses, opened up new opportunities. The satellites were discovered in 1610 by Galileo Galilei (1564–1642, Fig. 9) with the use of a new instrument – the telescope. It revealed that Jupiter was orbited by four satellites that would disappear and reappear with useful regularity as they passed in front of or behind the planet (Fig. 10). They provided, in essence, a celestial timekeeper, visible at the same time from different points on Earth.
Galileo quickly realized that this was a potential means of finding longitude and, having drawn up provisional tables to predict the satellites’ motions, he attempted in 1616 to interest the Spanish in a proposal to make 100 telescopes and teach navigators the method. Having failed to convince them, he began protracted negotiations with the Dutch government in 1636, which only ended with his death six years later. The scepticism of the Spanish longitude committee was undoubtedly related to the practicalities of the method. Observing objects as small as Jupiter’s satellites with a telescope from a moving ship was clearly going to be very difficult. Even a century later, when the production of telescopes and lenses had vastly improved, Newton noted that ‘by reason of the length of Telescopes requisite to observe them & the motion of a ship at sea, those Eclipses cannot yet be there observed’.6
Galileo recognized this problem and looked for a means of steadying the observer. He designed a helmet, the celatone, which supported a telescope that could be adjusted continually to counteract the ship’s movement. At least one was made and tried on board a ship in the harbour at Livorno. It impressed a member of the powerful Medici family, who apparently ‘judged this invention more important than the discovery of the telescope itself’.10 Another idea was a hemispherical vessel in which the observer could sit and which would, in theory, be kept level by floating in a bath of oil (see Chapter 3, Fig. 29). Given the small number of telescopes at this time that could show Jupiter’s satellites at all, let alone a sharp image, such adaptations for use at sea were perhaps premature. Nevertheless, chairs, platforms and other devices that would ease shipboard observations continued to be explored.
Fig. 8 – Edmond Halley’s world sea chart on two sheets, showing lines of equal magnetic variation, 1702
{National Maritime Museum, Greenwich, London}
Fig. 9 – Galileo Galilei, attributed to Francesco Apollodoro, c.1602–07
{National Maritime Museum, Greenwich, London}
Fig. 10 – Galileo’s journal of the observations of Jupiter and its satellites, 1610
{Biblioteca Nazionale Centrale di Firenze}
Ongoing attempts to make the method workable at sea were encouraged by the successful use of Jupiter’s satellites to establish longitude on land. This method began to flourish with the availability of improved telescopes and the publication of more accurate tables by Giovanni Cassini (1625–1712) in 1668. As director of the newly established observatory in Paris (Fig. 11), Cassini promoted the use of his tables on expeditions and in the mapping of France. In 1693, the Académie des Sciences published a map that compared the position of France’s coastlines on the new maps with the old (Fig. 12). Although Louis XIV, it is said, complained that the astronomers had taken more territory from him than his enemies, he and the Académie continued to finance the method and ambitious expeditions to map the nation and her empire.
Cassini’s method and tables were taken up elsewhere, including Britain. There, the new observatory at Greenwich focused on longitude, and its first director, John Flamsteed, produced his own tables of Jupiter’s satellites, published by the Royal Society in 1683. He doubted their use could be made practical at sea but encouraged sailors to learn the method for use on coastal surveys (or, rather, berated them for having not already begun to do so). By the early eighteenth century, it was clear that the use of simultaneous observations of Jupiter’s satellites to establish longitude on land could, with the best equipment and careful observers, be extremely effective. The main focus of research into astronomical methods for use at sea, meanwhile, moved elsewhere.
Fig. 11 – Paris Observatory, 1729. An astronomical quadrant with a telescopic sight and a large telescope with a mast and pulley for raising it are shown
{National Maritime Museum, Greenwich, London}
Fig. 12 – Map of France that compares the position of its coastlines on maps using new astronomical data with older maps, from Recueil d’Observations (Paris, 1693)
{National Maritime Museum, Greenwich, London}
‘the Place of the Moon’
While the leading astronomers at Paris and Greenwich had championed the use of Jupiter’s satellites, the founding of both observatories had been prompted by interest in what was known as the lunar-distance method. In France it was raised by a physician and Professor of Mathematics at the Collège Royal, Jean-Baptiste Morin, in 1634, and in London in 1674 by Le Sieur de St Pierre, another Frenchman, about whom almost nothing is known beyond his title. The method had advantages over Jupiter’s satellites in terms of what navigators would be required to observe at sea but it also had significant disadvantages with regard to the complexity of the Moon’s motions.
The