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The Moon

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The Moon

‘What, I wonder, would the science of astronomy be like, if we could not properly discriminate among the stars themselves. Without the use of unique names, all observations, both ancient and modern, would be useful to nobody, and the books describing these things would seem to us to be more like enigmas rather than descriptions and explanations.’

Johannes Hevelius, Selenographia, quoted by Whitaker (2003), p. 51.

Edward Worth’s fascination with the works of Johannes Hevelius, 1611-1689, is understandable given their visual impact.

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Johannes Hevelius, Selenographia (Danzig, 1647), portrait.

Hevelius developed the art of astronomy to a level that had never been witnessed before. Convinced of the importance of accurate visual representation he not only drew the maps himself but also engraved them in order to ensure that no errors were made during the process of printing. The result, as the following illustration of a Gibbous Moon demonstrates, were illustrations of an astonishingly high order:

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Johannes Hevelius, Selenographia (Danzig, 1647), Gibbous Moon plate.

Indeed, Winkler and Van Helden (1993) argue that Hevelius’s lasting legacy did not depend on the accuracy of his observations but rather on his enduring style of visual representation of the Moon. Exactly why it was so enduring is hard to say: Winkler and Van Helden make a compelling argument when they point to the structure of Selenographia. Though undoubtedly concentrating on the Moon the book does not launch into its principal topic immediately but gently draws the reader into a discussion of previous observations of the solar system and the observational methods used by Hevelius. In effect Hevelius was trying to convince the reader of the veracity of the process before producing his stunning results. As Winkler and Van Helden suggest, Hevelius was trying to draw his reader along with him in his process of discovery. By explaining the method, by depicting the construction of lenses, Hevelius hoped to prove the authority of his visual representations. That he felt the need to do this is perhaps testament to the state of astronomical illustration prior to his work. Galileo had been the first to attempt any naturalistic imaging of his observations but even these were relatively crude drawings which were ultimately dependent on their textual explanation: Galileo painted his pictures with words. Hevelius, on the other hand, placed the emphasis on the image and attempted to make it as naturalistic as possible as we see in his depiction of the Full Moon:

 The Moon3

Johannes Hevelius, Selenographia (Danzig, 1647), Fig. O – Full Moon.

Picturing the Moon was no easy task. First Hevelius had to have a sufficiently powerful telescope and even the Dutch or Galilean telescopes could only view part of the Moon at any one time. He then had to make his observations – many, many, observations in order to catch the various phases of the Moon and make sure that he had delineated them correctly.

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Johannes Hevelius, Selenographia (Danzig, 1647), Fig N – The Phases of the Moon.

Since Hevelius wanted to depict it at every stage this meant constantly checking that he had captured it correctly – up to ten times, as he says himself in the Selenographia. The image had then to be engraved – a lengthy process again undertaken by Hevelius – indeed, he specifically draws attention to this on various plates by placing ‘Autor sculpsit’ at the end of the page. What made his books unique was the fact that he undertook all these tasks himself – and could therefore vouch for the reliability of the process. The illustration on the Home page of this website, of Hevelius looking through his telescope, ably sums up the message of his works: these were images which he had recorded, drawn and reproduced for the viewer. He was, in short, representing exactly what he had observed.

And that was the problem. The wonderful images he drew were exactly what he saw himself but they were not always correct. As Winkler and Van Helden (1993) point out, Hevelius made much of his depiction of the phases of Mercury as one of the first representations of this phenomenon but his sightings were actually incorrect by-products of his method of observation. Even more seriously, his observations of lunar features, though wonderfully represented, left something to be desired when it came to accuracy. As Whitaker argues (2003), the areas near the terminator were usually portrayed correctly but the farther Hevelius moved from the terminator into areas more difficult to see, his observations and his consequent depictions became less accurate. Ultimately the reader as witness could only witness what Hevelius himself saw. This fault line which runs through the work eventually led to Hevelius’s text being superseded by works of later astronomers when the errors in his observations began to be clearly visible.

Hevelius was superseded in another area also: the issue of the nomenclature of the lunar landscape. Hevelius produced hundreds of new names for lunar features, predominantly using names from regions of earth that would be familiar to his readers and classical characters. In this way he underlined his adherence to Copernicanism for by giving the Moon terrestrial names he was once again emphasising its similarities with the Earth. The problem was that the names he chose were long-winded and essentially quite difficult to remember. His decision to produce a slightly different map of the Moon with the names attached also did not help his cause as the visual disparity between his normal maps and his nomenclature map jarred on the reader. It must have been galling for Hevelius when the Jesuit astronomer Giambattista Riccioli, 1598-1671, produced an alternative nomenclature system just four years after his own in his seminar work Almagestum novum (Bologna, 1651) – a work again collected by Worth.

Riccioli produced the first lunar map, by another Jesuit astronomer, Francisco Maria Grimaldi, which we see here:

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Giambattista Riccioli, Almagestum novum (Bologna, 1651), Grimaldi lunar map.

As his accompanying tag makes clear, this was not the Moon as displayed by Hevelius who in a sense had attempted to picture a snap-shot of one phase at a time, but was a composite work, including not only the observations of Hevelius but also the lunar observations of others – most notably Michiel Van Langren who had produced the first attempt at naming lunar features. Riccioli, then, was not basing his authority on his own sightings and the excellence of their visual depiction, but on the accuracy of the combination of sightings of leading lunar experts. As Whitaker has shown (2003), Riccioli’s maps, though they might not have the visual appeal of those of Hevelius, have been proven to be far more accurate representations. This was perhaps one reason why his system of nomenclature became the standard one but there were other more practical reasons why much of his nomenclature still stands today.

The first involves the intellectual climate of seventeenth-century astronomical enquiry: instead of following Hevelius’s practice of using recognised terrestrial names for lunar features Riccioli created a whole new lexicon for the Moon. As Vertesi argues (2007), undoubtedly one reason he did this was because, as a semi-Tychonian, he did not accept Copernican views of the similarity of the Moon and therefore did not want to depict the Moon as being essentially the same as the Earth. The corollary of this stance was that Riccioli’s system of nomenclature was acceptable to the Jesuit order and could be used in their network of colleges across Europe. On the other hand, he was careful not to alienate Copernicans, and named various lunar features after them. Indeed the tracts of lunar landscape he allocated to them were so prominent that is has caused some historians, most notably Whitaker (2003), to suggest that he was a secret Copernican at heart! Riccioli certainly presented an essentially balanced presentation of astronomical observation of every school in the Almagestum novum but the system he advocated was a Semi-Tychonic one as we see in this illustration which compares readings for Tycho Brahe’s geo-heliocentric system with his own variation on the same which agreed with Brahe in placing the Earth at the centre of the universe, and the planets Mercury, Venus, and Mars orbiting the Sun, but diverged from the Tychonic system by maintaining that Saturn and Jupiter orbited the Earth:

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Giambattista Riccioli, Almagestum novum Bologna, 1651), p. 429.

Vertesi (2007) points to other practical reasons why Riccioli’s lunar nomenclature system became more popular than that of Hevelius: he used the same style of map for his nomenclature as he did for his observations. Crucially his system was much easier to remember and, as Whitaker points out (2003), his system was divided into eight logical groups: names of ancient astronomers are in the same higher octant while more modern astronomers may be located in the lower quadrants, again grouped in a logical manner – for example, accompanying Copernicus in his ‘Ocean of Storms’ we find Rhaeticus, Maestlin and Reinhold and Galileo. The names Riccioli gave to the lunar landscape were much easier to remember and had an imaginative ring that is still apparent to this day. The differences between his own and Riccioli’s system are apparent in the following examples: for the beautifully named ‘Sea of Tranquillity’ which we owe to Riccioli, Hevelius gives us the rather more mundane ‘Euxine Sea; for the western part of Riccioli’s ‘Lake of Dreams’ Hevelius offers the more difficult to remember ‘Lake of Borysthenes’ – an ancient name for the Dnieper river.

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Giambattista Riccioli, Almagestum novum (Bologna, 1651), nomenclature of the Moon map.

Worth bought the works of both Hevelius and Riccioli. His adherence to Hevelius is perhaps related to his fellowship of the Royal Society for, as Vertesi relates (2007), the adherents of the Royal Society were usually advocates of the heliocentric system and they were attracted the Hevelius’s maps for this reason. But it should also be remembered that even the Fellows of the Royal Society might waver from the Hevelian to the Ricciolian system of nomenclature at times – or even use a combination of both together! Vertesi alludes to this when she refers to the practice of maps of both nomenclatures being sold together until the early eighteenth century and Whitaker (2003) points out that both were commonly used together for over 140 years after their creation. In any case Worth as a scholar of science and as a connoisseur of books could not ignore the texts of both men.

His status as a Fellow of the Royal Society explains the presence of another work on lunar observations in his library: Robert Hooke’s Micrographia (London, 1667). Hooke, 1635-1703, had been a severe critic of Hevelius’s observations and his attack on Hevelius had been keenly followed by the members of the Dublin Philosophical Society, especially William Molyneux. Molyneux, writing to Francis Aston on 12 November 1685, outlined the contribution of Hevelius as follows:

‘… it may deservedly be said of the noble and incomparable Hevelius, that he alone by his own labours and charges has done more towards the advancement of astronomy than the joint figures of all ages, both before and coeval with him, forasmuch as yet appears. What a number of noble curious volumes has he published to the world, his Selenography, Cometography, Organography to omit many others, are works of great learning, ingenuity, and corporal pains. But above all, his 2nd, 3rd, and 4th books of his Machina celestas, containing all his observations for almost 50 years, both of the fixed stars and planets, are an inestimable treasure.’*

Hooke, however, focussed on Hevelius’s (and indeed Riccioli’s) observational errors and included in his Micrographia, an excellent drawing of Riccioli’s ‘Hipparchus’ crater (called by Hevelius ‘Mons Olympus’:

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Robert Hooke, Micrographia (London, 1667), p. 245.

In this illustration of Hipparchus’s crater we may also see Hooke’s representation of the Pleiades and, at the bottom of the plate, his scale of brightness.


It is interesting to see that William Molyneux uses Hevelius’s nomenclature in the following table for a lunar eclipse, which he witnesssed at Dublin on 19 November 1686:

Eclipsis lunae observata Dublinii, Novembris 19, 1686.


Penumbra 9 15 00
Initium ob interpositas nubs praecise
Determinare haud licuit, ideoq’
Incertius pono ad * 9 25 00aut
Ad Paludem Maraeotim 9 38 20
Palus Maraeotis tecta 9 40 20
Mons Sinai Tegitur* 9 46 30
Mons Thambes Tectus* 10 01 20
Mons Audus Tegitur* 10 08 00
Neptunus Tectus* 10 14 00
Ad Montem Sypilum* 10 15 10
Insula Circinna tegitur* 10 16 40
Ad Montem Didymum* 10 19 20
Didymus tegitur 10 20 10
Emergit Audus 10 23 00
Ad Paludem Maraeotim. 10 48 30
Emergit Maraeotis 10 52 00
Ad Montem Sinai 11 38 00
Sinai Emergit 11 39 30
Finis Eclipsis 12 04 00
Tempus Horol. Oscillatorii ad stellas rectificati.*

This and other extracts from the Papers of the Dublin Philosophical Society demonstrates that members of the Royal Society and the Dublin Philosophical Society were very interested in the phenomenon of lunar eclipses – seen in diagrammatic form in this illustration from Jan Luyts Astronomica institutio (Utrecht, 1692):

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Jan Luyts, Astronomica institutio (Utrecht, 1693, p. 155).

Lunar Theory

Stephenson relates (1994) that ‘the Moon’s motion exhibited three inequalities: the evection (discovered by Ptolemy), the so-called variation (found by Tycho Brahe), and the annual equation (found independently by Tycho and by Kepler)’. According to Thoren (1967), before the later sixteenth century theories concerning the latitude of the Moon had not changed noticeably since Anaxagoras and Eudoxus. Lunar theory about latitude was primarily concerned with the problem of explaining why there wasn’t an eclipse every new and full moon. The solution lay in placing the orbit of the Moon on an incline from the ecliptic – which would insure that at times of syzygy (when the Moon was nearly in line with the Sun at either new or full moon) there would be no eclipse andlater astronomers adopted the value of 5o. Since eclipses did not take place at the same time each year it was further recognised that the nodes (its intersections with the ecliptic) must shift over time and a value of 1½o was accepted. For centuries the values were accepted by all – primarily because they fitted in with Ptolemaic figures for parallax. It was not until the sixteenth century that the inherent errors of the system became apparent.

Brahe spent many years working out the parallax of the moon before he arrived at his final theory in 1600. He could not apply the same methods of determining parallax to the Moon as he has so successfully used in determining the parallax of the nova of 1572 and the comet of 1577 because he could not be sure of the longitude of the Moon – and so he fell back on using latitudes. As Thoren states (1967), ‘on this scheme, the moon’s parallax was simply the different between the observed latitude (corrected for refraction) and its true latitude as computed from theory.’ By 1587 he had begun to realise that the accepted theories of lunar latitude did not match his observations and he adjusted Ptolemy’s latitude to 5o15′, publishing the new latitude as part of his De Mundi Aetherei Recentioribus Phaenomenis (Uraniborg, 1588) but as yet he had not understood that maximum latitude of the Moon was variable. His failure to correctly time a lunar eclipse in January 1591 raised questions in his mind about the velocity of the moon and by 1594 he had become convinced that the Moon moved faster when it was either full or new but more slowly during times of syzygy. In 1595-96 circumstances favoured a second approach to the problem of the Moon’s inclination since the Moon’s position was now at its most northerly point (in 1587 it had been at its most southerly). Tycho assumed that his findings for latitude would mirror his 1587 findings of 5o15′ but it soon became clear that the lunar latitude was variable throughout the synodic month. From his second discovery followed the nodal inequality of the Moon.

Kepler outlines his lunar theory in Books IV and VI of his Epitome of Copernican Astronomy (1617-21). His interest was primarily in explaining the physical reasons which were responsible for these in-equalities and in particularly in drawing attention to the magnetic power of both the Sun and Earth on the Moon. Galileo, on the other hand, paid relatively little attention to these arguments, preferring instead to draw attention to the rugged lunar landscape and the similarities between the Moon and Earth.

Selected Reading

*Quotations marked with an asterisk may be found in Hoppen, K. T. (ed.) (2008) Papers of the Dublin Philosophical Society 1683- 1709 (Dublin: Irish Manuscript Commission), 2 vols.
Ariew, R. (2001). ‘The Initial Response to Galileo’s Lunar Observations’, Studies in History and Philosophy of Science, 32. No. 3., 571-581.
Hoppen, K. T. (ed.) (2008) Papers of the Dublin Philosophical Society 1683- 1709 (Dublin: Irish Manuscript Commission), 2 vols.
Stephenson, B. (1994). The Music of the Heavens. Kepler’s Harmonic Astronomy. Princeton: Princeton University Press.
Thoren, V. E. (1967). ‘An Early Instance of Deductive Discovery: Tycho Brahe’s Lunar Theory’, Isis 58, no. 1, 19-36.
Thoren, V. E. (1990). The Lord of Uraniborg. A biography of Tycho Brahe. Cambridge: Cambridge University Press.
Vertesi, J. (2007). ‘Picturing the Moon: Hevelius’s and Riccioli’s visual debate’, Studies in History and Philosophy of Science 38, 401-421.
Whitaker, E.A. (2003). Mapping and Naming the Moon. A History of Lunar Cartography and Nomenclature. Cambridge: Cambridge University Press.
Wilson, F. (2001). ‘Galileo’s Lunar Observations: Do they imply the rejection of traditional lunar theory?’, Studies in History and Philosophy of Science, 32 no. 3, 557-570.
Winkler, M. G. and Van Helden, A. (2003). ‘Johannes Hevelius and the visual language of astronomy’, in J. V. Field and F. A. J. L. James (eds.) Renaissance and Revolution. Humanists, scholars, craftsmen and natural philosophers in early modern Europe. Cambridge: Cambridge University Press, pp. 97-115.
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