Plate 2 from Nova Reperta
by Johannes Stradanus

Astronomical Observations


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.

Galileo 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.

Pamela O. Long
Washington, DC

Suggested Reading

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.