1.8 The moon  (Page 2/5)

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

Those who wished to defend the perfection of the heavens brought out the old argument about rarity and density. In the letter ofthe Collegio Romano mathematicians to Cardinal Bellarmine of April 1611, Christoph Clavius (74 years old) expressed a minority opinion: "But it appears to Father Claviusmore probable that the surface is not uneven, but rather that the lunar body is not of uniform density and has denser andrarer parts, as are the ordinary spots seen with the natural sight." The other three Jesuit mathematicians on the faculty of the college, however, believed that the lunar surface was indeed uneven. Inthis case the opposition faded away over the next few years.

Galileo wrote in a letter, 1610, that he would like to make a series of representations of the Moon showing its changingphases. Presumably his purpose was to show how the shadows of individual features changed with the illumination. It appearsthat he abandoned this plan when he saw that there was no need for such an ambitious and expensive project: even the Jesuitfathers in Rome were convinced that the Moon's surface was uneven. Indeed, Galileo never returned to the task ofrepresenting the Moon. (In the 1630s he did, however, observe lunar librations    , which show that the Moon does not always keep exactly the same face turned towardthe Earth.) Others did little better. Thomas Harriot did make a rough map of the full Moon but never published it. Representations by Christoph Scheiner , Giuseppe Biancani, and Charles Malapert were little more than diagrams, useful only forsupporting the verbal argument that the Moon's surface is rough and uneven. These were, so to speak, generic moons, notportraits of our nearest neighbor.

If early observations and representations of the Moon were designed to address the issue of its mountainous nature andaffinity with the Earth, by the 1630s the accent was shifting. The rough lunar surface was now accepted byastronomers and they turned their attention to how telescopic observations could help them solve the problem of longitude . A lunar eclipse is an event that appears the same to all observers for whom the Moon isabove the horizon (which is, of course, not the case with solar eclipses). As the Moon enters the Earth's shadow cone, one canmark the times at which the shadow crosses a particular feature and later compare this time with the (local) time at which adistant colleague observed the same event. The difference in local times translates directly into their difference inlongitude.