Galileo's Message of the Stars

by Joe Orman

[Originally published in the newsletter of the East Valley Astronomy Club, April 1999]

Wouldn't it be wonderful to read the observing notebook of Galileo Galilei, the first person to systematically observe the heavens with the recently-invented telescope? Well, you can! Galileo announced his first observations in the 1610 book "Siderius Nuncius" ("The Sidereal Messenger"), which has been called "perhaps the greatest scientific popularization ever written." A 1989 translation from the original Latin by Albert Van Helden is completely annotated and very readable. While Galileo's 20-power telescopes were small and crude by today's standards, his observing process and thrill of discovery should seem familiar to any modern astronomer.

Galileo's observation of mountains on the moon proved that heavenly bodies were not perfectly smooth and spherical: "Not only are the boundaries between light and dark on the Moon perceived to be uneven and sinuous, but, what causes even greater wonder, is that very many bright points appear within the dark part of the Moon, entirely separated and removed from the illuminated region and located no small distance from it. Gradually, after a small period of time, these are increased in size and brightness. Indeed, after 2 or 3 hours they are joined with the rest of the bright part, which now has become larger. ... Now, on Earth, before sunrise, aren't the peaks of the highest mountains illuminated by the Sun's rays while shadows still cover the plain? Doesn't light grow, after a little while, until the middle and larger parts of the same mountains are illuminated, and finally, when the Sun has risen, aren't the illuminations of plains and hills joined together?"

And Galileo's telescope revealed the true nature of the Milky Way: "For the galaxy is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view, of which many appear rather large and very conspicuous but the magnitude of small ones is truly unfathomable."

Here Galileo describes his first look at the moons of Jupiter: " ... I saw that three little stars were positioned near him [Jupiter] -- small but yet very bright. Although I believed them to be among the number of fixed stars, they nevertheless intrigued me because they appeared to be arranged exactly along a straight line and parallel to the ecliptic, and to be brighter than others of equal size." His later observations revealed that there were actually four bright moons, which were later named the Galilean moons in his honor. It would be interesting to match Galileo's almost-daily sketches of the moons for the period January 7 to March 2, 1610 with modern astronomical software. And what observer cannot relate to this emotion: "...I waited eagerly for the next night. But I was disappointed in my hope, for the sky was everywhere covered with clouds."

Galileo recognized that these observations, which confirmed the Copernican model of the solar system and showed that the heavens were not perfect and unchanging, would be controversial, and indeed in later years he suffered great persecution. But the truth of Galileo's message proved itself: the Universe contains real wonders beyond the grasp of our normal senses that require new ways of thinking; by extending our senses we expand our mind. By bringing this message to mankind, he was opposing the forces of ignorance and pseudoscience. In this sense, modern astronomers continue to spread Galileo's message.

Revised: May 19, 1999
Copyright © 1999 Joe Orman
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