The Enigmatic Face of the Moon

THE INSPIRATION OF ASTRONOMICAL PHENOMENA VI ASP Conference Series, Vol. ???, 2010 E. M. Corsini The Enigmatic Face of the Moon Carlos D. Galles and Carmen J. Gallagher Facultad de Ciencias Exactas, Ingenierı́a y Agrimensura, Universidad Nacional de Rosario, Argentina Abstract. Whilst Man’s only way of observing the Moon was with the naked eye, attempts at explaining the spots on her surface remained highly speculative. The telescopic observation by Galileo of previously unknown spots, differing from the earlier ones by their variability in time, was to signify a radical change to the hereto medieval ideas on the material composition of the Moon. And curiously enough this new scenario was a revindication of Plutarch’s hypothesis construed more than a millennium before. The spots on the Moon’s surface, visible to the naked eye, have always been the object of admiration. Apart from the numerous myths and legends associated with their origin, there is a long history of explanations for their existence. Amongst Aristotle’s cosmological ideas, a paradox was to be found: the spots were apparent in a region that was supposed to be immaculate. The problem was approached from a wide range of hypothetical solutions. Plutarch, for example, considered the possibility of the spots owing their origin to the viewer being dazzled when looking at the moon. But Plutarch himself refuted this idea, pointing out that a similar effect should take place when observing the Sun, whose radiance is far greater than the Moon’s. He likewise rejected the idea that the spots were the reflection of the Earth’s surface in the vast mirror formed by the Moon, basing himself on geographical considerations. The hypothesis that Plutarch did espouse was the one according to which the Moon should be considered to be heterogeneous, sharing the nature of the earthly bodies, and therefore containing the four elements. Thus, on its surface there would be valleys, cliffs and precipices, and possibly also, water, which, amassed together to form vast seas, would be the origin of the spots. He refuted the opinion that held that the spots were due to the shadow of the mountains, considering that the former were immutable, even when observed during full moon with the Sun’s rays striking the Moon’s surface perpendicularly (Plutarch 1820). The eminent Arabic scientist, Ibn al Haytham, considered the question in two treaties entitled On the Light of the Moon and On the Spots that Appear on the Moon’s Face. The fundamental observational fact upon which the treaties hinged was that the Moon’s spots were unchanging, always presenting themselves in the same form to visual examination. It therefore ruled out the hypothesis that a body external to the Moon was interposed between the Moon and our vision, since if this were the case; the spot would be seen in multifarious guises by different observers at different points on the Earth’s surface. Nor would it be 3 4 Galles and Gallagher possible for the spot to have been produced by a transparent part of the body of the Moon since in this case the Sun would have appeared in the Sun’s eclipses in this zone. It is interesting to note how Ibn al Haytham likewise refutes the possibility of the spots being due to protuberances of the Moon’s surface projecting their shadow. He does so by arguing that were this to be the case, the spots would vary in accordance with the position of the Sun. This is the same situation that Galileo considered centuries later when observing variable spots on the Moon’s surface with a telescope, one that led him to postulate the existence of mountains and depressions. Other hypotheses are rejected by showing that if they were to be true, certain effects would be observed. For example, if the spots had been produced by an object located between the Earth and the Sun, they would demonstrate some form of parallelism when observed from different points on the Earth’s surface. This is clearly not the case. Ibn al Haytham’s explanation for the nature of the spot is as follows: The spot is in the body of the Moon itself. All the Moon’s parts are not identical. The place where the spot is located is markedly different from the rest. The spot appears in darkness since this part of the Moon does not receive as much as the rest. The satellite possesses a faculty for receiving light which is not uniform throughout it different parts. In these writings on the Moon, he makes a clear distinction between transparency, opacity and capacity for receiving light. He continues to view it as an Aristotelian star, perfectly spherical in shape, devoid of any deformities on its surface. The only alteration postulated by him is a difference in the receptive capacity of light (Hamedani 2006). In Medieval Europe, thinking was dominated by the position of Averroes, Aristotle’s famous commentator. According to his conception, the Sun excites the Moon’s entire surface, but since not all its component parts have the same response, they do not all appear to be equally luminous. In the words of Pierre Duhem, this explanation makes use of concepts which are close to what today we call fluorescence, namely, light is retained by the Moon’s surface and later emitted. The explanation of these observations was provided by considering the Moon’s surface to be composed of a single substance, the aether, which varied in terms of density, being either rare or dense (Duhem 1958). In the Convivio, Dante Alighieri had shown himself to be a partisan of the above-mentioned ideas. However, in the Paradise section of the Divine Comedy, Beatrice puts forward an explanation for the existence of spots on the Moon’s surface, which, although in keeping with Christian mysticism, is based on scientific experiment (Figure 1). It consists of considering the images of a torch in a series of nearby mirrors, as well as in a faraway one. The conclusion is that all the mirror images are equally bright (Alighieri 1965). In modern idiom, this means that the brightness of the image does not depend on the position of the mirrors, and therefore the difference between “the rare and the dense” would not produce changes in the reflection of the Sun’s light. The experiment presented by Beatrice, has an additional merit: an experiment carried out on the Earth’s surface is called upon to solve a situation The Enigmatic Face of the Moon 5 Figure 1. The experiment presented by Beatrice in a miniature by Giovanni di Paolo, 15th century, British Museum, Ms. Yates Thompson 36-f.132 r. in the celestial spheres. This signifies a step towards considering a single physical theory as being valid for both regions. At the beginning of the 16th century, Leonardo da Vinci formulated a hypothesis whereby the Moon’s surface was covered in water except in those regions where the spots are to be observed, regions he considered to be continents. He based it on his observations of the reflection of the Sun’s light on the sea. Furthermore he utterly rejected the idea that the Moon was a polished mirror, flat or spherical, since were this to be the case, only a small portion of it would be seen from the Earth. He considered the surface of a rough sea to be made up of innumerable small mirrors reflecting the Sun’s light at all angles. In the same manner, a sea on the Moon’s surface would enable the Sun’s light to be diffused in all directions. In this conception, the spots visible to the naked eye would be the continents. A few decades ago, some excellent representations of the Moon’s spots were found among Leonardo’s manuscripts. They consist of two pen and ink sketches, from between 1505 and 1508, and a charcoal drawing by the artist from 15131514 depicting the Western portion of the Moon. The Moon’s seas can be clearly made out in these works (Reaves 1987). Curiously enough, the Moon’s spots appear very rarely in works of art before the 17th century. As a rule, Van Eyck’s The Crucifixion painted around 1430 is held to be the earliest naturalistic representation in European painting where the Moon’s spots can be clearly distinguished (Montgomery 1994). However, another representation of the subject is a Neapolitan miniature from the 14th century by an unknown artist which shows a mirror image of the Moon with her characteristic spots, the Nectaris and Fecunditatis seas, clearly visible. The image is part of a painting which shows Dante and Beatrice ascending into the Heavens against a starry background. It is conjectured that the artist may not have been altogether satisfied with the work, hence his addition of the word “Luna” identifying the satellite (Alighieri 1958). The original is in the Lord Leicester Library, Holkham Hall, Ms. Holkham 514-f.49 r (Figure 2). 6 Galles and Gallagher Figure 2. Beatrice and Dante looking at the Moon’s spots, in a Neapolitan miniature, 14th century, Holkham Hall, Lord Leicester Library, Ms. Holkham 514-f.49 r. At the turn of the century, the English scholar, William Gilbert, also produced an excellent drawing of the Moon’s spots. Likewise, Kepler, who espoused a theory similar to Leonardo, executed a drawing by using a device fitted with a pin-hole opening for the purpose. The result is undoubtedly inferior to what could have been obtained by observation with the naked eye (Kepler 2000). In the year 1609, knowledge of the Moon underwent a radical change when Galileo began his astronomical observations using a telescope. In his Sidereus Nuncius, published in 1610, Galileo made mention of certain spots “visible to all” which darken the visible face of the Moon, dismissing them as “ancient”, before turning to focus on other spots “smaller in scope”, but which had not been observed “by anyone previous to ourselves”. He dedicates a number of pages to his observations which have enabled him to reach the certainty that the Moon is far from being a perfect sphere. Quite the contrary, on its illuminated side it has a varied topography of mountains, valleys and craters. He thus demolishes the hypothesis shared by Leonardo and Kepler according to which this region was covered in water (Galileo 1976). Galileo reserved a thorough treatment of the Earth’s satellite for his work Dialogue on the Great World Systems (Galileo 1957). At a certain point of the debate, Simplicio, who sums up the position of medieval Christianity, states the Moon is perfectly spherical, like the rest of the celestial bodies, with parts “being unequally opaque and perspicuous”. Although this hypothesis could provide a reasonably convincing answer to the origin of the existence of the ancient spots, it was unable to account for the “one by one appearances to be seen on the Moon” as Salviati, Galileo’s alter ego, was to observe in his reply to Simplicio. The Enigmatic Face of the Moon 7 Obviously it is always possible to conceive ad hoc hypotheses to support Simplicio’s position, for example changes in the density of the Moon’s substance in relation to the incidence of the Sun’s rays, but such hypotheses would not withstand the slightest use of Ockham’s razor. It is very interesting to see the works of Wilson (2001) and Ariew (2001) on this subject. Finally, with regard to the ancient spots on the Moon, Galileo states: Thus the Moon’s Spots could have been caused by enormous woods, but they could also have resulted from seas, if these were to have existed on its surface. And finally nothing is to prevent these spots from being a darker color than the rest of the Moon’s surface. It is somewhat surprising that even for Galileo the issue should have remained open. But it must be remembered that he was always very respectful of observation and this had not produced any conclusive results at the time. However, little by little consensus was to grow to support his hypothesis of a col-our differentiation. 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