Ignoring the occasional pre-telescopic appearance of exceptionally large sunspots, the Moon is the only heavenly body which shows features to the naked eye--the Man in the Moon. These features are permanent, and it was therefore obvious that the Moon always keeps its same face turned to us (eventhough there are minor perturbations that were not noticed until later). In the philosophy of Aristotle (384-322 BCE), these features presented somewhat of a problem. The heavens, starting at the Moon, were the realm of perfection, the sublunary region was the realm of change and corruption, and any resemblance between these regions was strictly ruled out. Aristotle himself suggested that the Moon partook perhaps of some contamination from the realm of corruption.

Eventhough Aristotle's natural philosophy was very influential in the Greek world, it was not without competitors and skeptics. Thus, in his little book On the Face in the Moon's Orb, the Greek writer Plutarch (46-120 CE) expressed rather different views on the relationship between the Moon and Earth. He suggested that the Moon had deep recesses in which the light of the Sun did not reach and that the spots are nothing but the shadows of rivers or deep chasms. He also entertained the possibility that the Moon was inhabited. In the following century, the Greek satirist Lucian (120-180 CE) wrote of an imaginary trip to the Moon, which was inhabited, as were the Sun and Venus.

The medieval followers of Aristotle, first in the Islamic world and then in Christian Europe, tried to make sense of the lunar spots in Aristotelian terms. Various possibilities were entertained. It had been suggested already in Antiquity that the Moon was a perfect mirror and that its markings were reflections of earthly features, but this explanation was easily dismissed because the face of the Moon never changes as it moves about the Earth. Perhaps there were vapors between the Sun and the Moon, so that the images were actually contained in the Sun's incident light and thus reflected to the Earth. The explanation that finally became standard was that there were variations of "density" in the Moon that caused this otherwise perfectly spherical body to appear the way it does. The perfection of the Moon, and therefore the heavens, was thus preserved.

It is a curious fact that eventhough a number of symbolic images of the Moon survive in medieval and Renaissance works of art (commonly a crescent), virtually no one bothered to represent the Moon with its spots the way it actually appeared. We only have a few rough sketches in the notebooks of Leonardo da Vinci (ca. 1500) and a drawing of the naked-eye moon by the English doctor William Gilbert. None of these drawings found its way into print until well after the telescope had come into astronomy.

The telescope delivered the coup de grace to attempts to explain away the Moon's spots and to the perfection of the heavens in general. With his telescope, Galileo saw not only the "ancient" spots, but a number of smaller ones never seen before. In these smaller spots, he saw that the width of the dark lines defining them varied with the angle of solar illumination. He watched the dark lines change and he saw light spots in the unilluminated part of the Moon that gradually merged with the illuminated part as this part grew. The conclusion he drew was that the changing dark lines were shadows and that the lunar surface has mountains and valleys. The Moon was thus not spherical and hardly perfect.



Posted by: Sean    Source

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