Galileos drawing of the Moon 1610

Galileo's drawing of the Moon includes a large round crater on the terminator (the dividing line between lit and unlit hemispheres), a nearby field of smaller craters on the bright side of the terminator, and three bright arms crossing the terminator into the dark hemisphere.

These features are not found in modern photographs of the Moon. Perhaps Galileo was obliged to depict in his small engraving the crater much larger than it actually is so he could show the contrasting illumination of the rim of the crater at first and last quarter. Or perhaps Galileo's drawing was not entirely an artifact of observation free of mental assistance. The size of the enormous round crater near the terminator may be commensurate with the psychological impact on Galileo's thinking of Albategnius, an actual and considerably smaller crater.

Early telescopes rarely were powerful enough for naïve, unbiased observers to recognize what they were seeing. The earliest extant record of an astronomical observation made with a telescope is the Englishman John Harriot's lunar drawing of July 26, 1609. The shading is haphazard, with no suggestion of craters and mountains, and the terminator curves in an impossible manner. Correspondence between Harriot and his friends reveals their initial confusion; they had no idea that they were seeing shadows cast by craters and mountains.

The ability to see often was dependent less on visual acuity and more on the enhancing effect of theoretical preconception. By the summer of 1610 Harriot and his friends were discussing Galileo's reported observations, and Harriot could have seen a copy of Galileo's Sidereus nun-cius, or Starry Messenger, by early July. Harriot's July 1610 drawing of the Moon contains several idiosyncratic features (an enormous crater, three jagged protrusions, and a field of craters), all of which also are found in Galileo's drawing, and none of which are found in modern photographs of the Moon. Belief was transformed into sight; believing had become seeing. Image copyright History of Science Collections, University of Oklahoma Libraries.

On January 7 in the year 1610, Galileo noticed three objects, small but very bright, near Jupiter—two on the eastern side of the planet, and one to the west. He believed them to be stars, but his curiosity was aroused by the fact that they lay in a straight line parallel to the ecliptic. The next night, all three were to the west of Jupiter. Galileo wrote: "Hence it was with great interest that I awaited the next night. But I was disappointed in my hopes, for the sky was then covered with clouds everywhere" (Drake, Discoveries, 52). On January 10, two of the objects were to the east, and one was presumably hidden behind Jupiter. On the next night, there were two again to the east, but one much brighter than the other and farther from Jupiter than on the previous night. Galileo wrote: "I had now decided beyond all question that there existed in the heavens three stars wandering about Jupiter as do Venus and Mercury about the Sun" (Drake, Discoveries, 53). On January 13, Galileo saw four stars for the first time. He concluded:

Above all, since they sometimes follow and sometimes precede Jupiter by the same intervals, and they remain within very limited distances either to east

or west of Jupiter, accompanying that planet in both its retrograde and direct movements in a constant manner, no one can doubt that they complete their revolutions about Jupiter . . .

Here we have a fine and elegant argument for quieting the doubts of those who, while accepting with tranquil mind the revolutions of the planets about the Sun in the Copernican system, are mightily disturbed to have the Moon alone revolve about the Earth and accompany it in an annual rotation about the Sun. Some have believed that this structure of the universe should be rejected as impossible. But now we have not just one planet rotating about another while both run through a great orbit around the Sun; our own eyes show us four stars which wander around Jupiter as does the Moon around the Earth, while all together trace out a grand revolution about the Sun in the space of twelve years. (Drake, Discoveries, 56—57)

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