In the Sidereus
Nuncius (or Starry Messenger), first published in
1610, Galileo Galilei (1564-1642) reported his observations made
with a new astronomical instrument, the telescope. Galileo did not
invent the telescope, but hearing that such an instrument was being
sold in France and the Netherlands, he tried to obtain one. When
he failed, he undertook to make it himself. Spyglasses, as they
were called, were made by placing a convex and a concave lens in
a tube. The earliest instruments magnified three or four times.
Galileo figured out that the magnification was determined by the
ratio of the focal lengths of the two lenses. He set about to improve
the instrument and in the process, learned to grind and polish lens.
He first made an instrument that magnified eight or nine times,
and eventually fabricated one that magnified twenty times.
His first astronomical
observations with the new instrument were of the moon. Galileo's
observations of the moon and other heavenly bodies such as the moons
of Jupiter and the stars of the Milky Way revolutionized astronomy
because they provided evidence that the heavens were not perfect
and unchanging, as the Aristotelian geocentric model assumed. For
example, Galileo showed that the moon was not smooth, but rough
and uneven, similar to the earth. He argued that the surface of
the moon contained structures such as mountains and valleys. He
observed that the line on the moon that separated light and darkness
(the terminator) was not a smooth curved line that would be found
on a uniform surface, but jagged and irregular as was expected for
a rough, uneven surface. Rather than a sharp division between the
sublunary and supralunary spheres, Galileo's observations supported
the view that the moon was like the earth. It followed that the
earth was similar to the other heavenly bodies (and perhaps orbited
the sun as the moon orbited the earth and the moons of Jupiter revolved
around that planet). Galileo's observations provided powerful support
for the Copernican heliocentric model of the cosmos.
described the moon's earth-like surface but he also provided drawings
that served as strikingly persuasive images. This is the second
of a series of four moon drawings that appeared in the Sidereus
Nuncius. Galileo's drawings are not accurate maps of the moon;
it is difficult to correlate his structures with modern moon maps.
Rather, his drawings were meant to persuade his readers of the earth-like
nature of the moon. Perhaps in his use of images he was influenced
by William Gilbert's drawings of the terrella or "little
earth," that appear throughout the De magnete (1600)
and help to convince the reader of Gilbert's argument that the earth
is a magnet.
Drake, Stillman. "Galileo's First Telescopic
Observations." Journal for the History of Astronomy
7 (1976): 153-168.
Van Helden, Albert. The Invention of
the Telescope. American Philosophical Society, Transactions
67.4 (1977): 1-67.
---. Introduction to Sidereus
Nuncius or The Sidereal Messenger, by Galileo Galilei, trans.
by Albert Van Helden. Chicago: University of Chicago Press, 1989.