Harriot's maps of the Moon: new interpretations

Stephen Pumfrey

Abstract

July 2009 is the 400th anniversary of the first telescopic observations of the Moon, made by the English scientist Thomas Harriot. Galileo's later drawings were more influential, but some historians now question the traditional view that Harriot's were inferior. Galileo's revealed the mountainous topography of the Moon, but Harriot's arguably had the different, cartographical aim of plotting lunar features precisely. This article suggests that, influenced by the remarkable work of his contemporary William Gilbert, Harriot may have devised or used his splendid Moon map, like Gilbert, to observe the phenomenon of lunar libration decades before Galileo announced its existence.

On Sunday, 26 July 2009 the event ‘Telescope400’ will take place in Syon Park, Middlesex, UK.1 The outstanding mathematician and scientist Thomas Harriot, sometimes called ‘the English Galileo’, lived there, and on that day in 1609 he made the first recorded observations of the heavens with a telescope. Claims that the telescope was first unveiled in Elizabethan England and not in The Netherlands in October 1608, remain speculative,2 but it is indisputable that, when Harriot used his 6× Dutch telescope as an astronomical instrument to observe the Moon, he was several months ahead of Galileo Galilei in Italy. Of course, Galileo's researches achieved immediate fame and influence: he made sure of that by publishing his Sidereus Nuncius in March 1610. Harriot's manuscript records and drawings remained unknown until 1784, and the image in figure 1 was not published until 1965. Only recently has the leading historian of selenography Ewan Whitaker inferred from them that Harriot was ‘the first to observe the Moon's libration in latitude’.3 However, Harriot's patron and owner of Syon Park, the 9th Earl of Northumberland, was imprisoned in the Tower of London, more concerned about perishing himself than urging Harriot to publish or perish.

Experts have come to acknowledge Harriot's priority, but until very recently they have universally judged his sketches of the Moon to be inferior to Galileo's. Robert Huerta still considers that when ‘Harriot and others tried to describe what they saw, the schemas or mental constructs they brought to the process of vision were not up to the task of correct interpretation. That task required a man like Galileo, the type of investigator who has been described as a “paradigm observer”.’ A common criticism of Harriot is that, at least until he read Galileo, he did not properly record the existence of the Moon's mountains and craters, whose shifting patterns of light and shade were signs of the Moon's rough, Earth-like surface.4 It is true that Galileo masterfully depicted and deployed such evidence to argue that the Moon was a satellite of the Earth, as Copernican astronomers believed, and not one of the perfect, completely smooth planets that differed profoundly from our imperfect ‘sublunary world’. Indeed, as a ‘paradigm observer’, Galileo effectively created the first research programme in telescopic astronomy: the topographical task of establishing the lunar surface in its three-dimensional roughness in an effort to refute the distinction between the heavenly and terrestrial realms.

By contrast, Harriot's drawings are more like maps, approximate delineations of the locations of visible features. Certainly they improved, from the simplest sketch maps in July 1609 to the striking composite map from late in 1610 that is reproduced in figure 1.5 Like Scott L. Montgomery, Alexander suggests that we should stop interpreting Harriot's images as poor executions of Galileo's topographical programme, and should instead see them as inspired by his own background in cartography.6 In the 1580s and 1590s Harriot had been a client of Sir Walter Ralegh, supplying him with expertise in branches of practical mathematics such as ballistics, astronomical navigation, surveying and mapping. All of these skills were crucial when Harriot sailed with Ralegh's expedition to establish the colony in America named Virginia in honour of Elizabeth I. Harriot might have lacked Galileo's skill in Florentine disegno techniques, but he knew how to map unfamiliar coastlines and territories.

Figure 1

Harriot's map of the Moon. Source: Petworth House Collection, HMC 241/9, ff. 26–30. (Courtesy of Lord Egremont and Leconfield.)

Harriot saw the patches of light and dark on the Moon as lands and oceans: he referred to one patch as ‘the Caspian’. The conviction that the Moon had seas and continents, forests and plains just like Earth's was common to the small number of radical Copernicans, which included Galileo, Kepler, Harriot and his English contemporary William Gilbert. Thus, for Harriot the boundaries he drew on this map were literally littoral. This kind of cartography is concerned with the accurate depiction of relative spatial relations in two dimensions; it lacks the three-dimensional emphasis of topography.

By setting them in the context of English colonial exploration, Alexander has removed Harriot's maps from misplaced criticism. However, if Galileo's programme was driven by his desire to prove the Moon's Earth-like topography, what was Harriot's motivation? Alexander suggests that it, too, was colonial, a belief that to map was to possess. This is surely part of the answer, but I am proposing another, more scientific reason.

The very first map of the Moon was made by Harriot's Elizabethan contemporary, the natural philosopher and royal physician William Gilbert (1544–1603) (figure 2). Gilbert is still renowned today for his work on magnetism and for his claims that the Earth and the Moon are giant magnetic spheres. However, his venture into Moon mapping has been criticized. For example, the expert in lunar astronomy Professor Charles Wood considers thatGilbert's drawing is about the minimum a careful observer would notice! The locations and sizes of the maria are poor, and no craters—not even Tycho—are shown. This drawing is famous for being the only pre-telescopic drawing, but it is not a careful rendition of what is visible.7

Figure 2

Gilbert's map of the Moon. Source: William Gilbert, De Mundo Nostro Sublunari Philosophia Nova (1651; reprint edn, Elsevier, Amsterdam, 1965). Copy in the author's possession.

Gilbert shared Harriot's pride in England's nascent imperialism, but I think he had a more scientific reason for mapping the Moon: he wanted his map to be used for a programme of careful cartographical observations designed to answer astronomical questions. His Selenographia, he wrote, wasrequired of necessity in observations of the lunar body to determine whether it does not revolve at all (as was argued by Aristotle, and accepted) or whether it turns itself to either side; then to determine how it inclines in its monthly motion, with its poles (that is, its body's poles) [inclined] towards some fixed parts of the heavens.8

A lunar astronomer will be amazed to read these words. As I have recently argued,9 they show that Gilbert constructed the map to help him and others detect the librations of the Moon in longitude and latitude. Libration is the small but significant apparent wobble of the Moon in its monthly orbit, which until now was thought to require a telescope for its discovery by Galileo some time before 1632.10 However, Gilbert seems to have deduced its existence before setting out to look carefully for the minute effects.

The dominant cosmological theory of the sixteenth century remained broadly Aristotelian. Planets such as Mars or the Moon were considered to be embedded in solid spheres or orbs concentric with the Earth. They were moved by the orbs and had no motion of their own. As Aristotle himself had argued, this explained why the Moon kept exactly (so it was thought) the same face towards the Earth, because it was carried round by its sphere.

Gilbert had a radically different Copernican cosmology. For him, each heavenly body possessed its own principle of motion, by which it moved itself through empty space. The Earth's principle, like the Moon's, happened to be magnetism. The lesser Moon orbited the greater Earth, and both were moved about the Sun by its luminous power. However, stressed Gilbert, the orbits were not circular, nor was their velocity regular, because of the constantly interacting forces of the heavenly bodies.11 Gilbert seems to have predicted that these irregularities in the lunar orbit would have the visible consequence we call libration. A wobbling Moon would be visible proof of his radical cosmology.

He was right, most obviously concerning what we call libration in longitude: it is a direct and proportional consequence of the Moon's elliptical orbit and the resultant variation in angular velocity. Libration in latitude is a different, optical effect, and the libration we see is a resultant of both (and several more minor) components. Gilbert did indeed observe it, some 50 years before Galileo. Only one record survives. Gilbert noted how ‘observation of the full Moon in [the constellation of] Capricorn teaches that the distance of the Moon's southern spot from the outer edge of shining light of the orb is greater than when the Moon is in Cancer.’12

What Gilbert called his Selenographia was seen by very few, because it existed only in Gilbert's posthumous manuscript work ‘De Mundo Nostro Sublunari Philosophia Nova’, a copy of which was given to the Royal Library of James I. Francis Bacon read the book thoroughly and, some two years after Harriot's first telescopic observations, he commented that the ‘selenography or map of the Moon, which Gilbert conceived, seems now by the industry of Galileo and others to be nearly attained.’13

However, De Mundo was first mentioned by Thomas Harriot. In July 1608 he informed Johann Kepler of some chapters he had read, including those in which Gilbert argued for an interstellar vacuum, a belief that Harriot shared and Kepler opposed.14 Harriot came to agree with Kepler that the Moon and the planets moved in ellipses. It seems likely, although impossible to prove, that he would have unfolded and wondered about Gilbert's unprecedented Moon map, and then read on to discover that Gilbert wanted the map to be used to accumulate evidence for a new theory of the Moon's orbit.

With this in mind we can return to Harriot's telescopic map. As Whitaker pointed out, Harriot annotated it to record that some parts of the Moon had seemingly moved: ‘1611 Decemb. 14th. ho. 8½ mane I noted that the darker partes of 28, 26 were nerer the edge then is described.’ Whitaker adds:maximum libration towards the north occurred only two days after this observation: Harriot was thus the first to observe the Moon's libration in latitude, although whether or not he considered this a true movement or just an effect of inaccurate drawing is not known.15

When Galileo claimed in 1632 to have discovered libration, he did not know of Gilbert's selenography. Harriot, who made no such claim, very probably did, as did Bacon. If so, then, 400 years on, we can admire Harriot's map even more. His depiction was not an inferior version of Galileo's but an independent one using mapping skills that Galileo lacked. Moreover, Gilbert had shown how lunar cartography could advance the Copernican cause by establishing how the Moon orbited the Earth. Harriot's image represents the success of an English, non-Galilean yet pro-Copernican approach to depicting the Moon. As such it might also represent the very first intentional, dated and precise observation of lunar libration in history.

  • Received December 15, 2008.
  • Accepted March 9, 2009.

Notes

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