Galileo's Moon-- Then and Now

Galileo's Work

When Galileo first turned his telescope to the moon, he had no idea that he was about to see something that would cast doubt upon centuries-old beliefs. At the time of Galileo, science was based upon the teachings of Aristotle. According to Aristotle, the Earth was the center of the universe. All heavenly bodies, including the moon, were perfect spheres that moved around the Earth with circular motions (or combinations of circular motions), and the Earth was the only source of imperfection in the universe. The dark spots on the moon that been visible to man throughout the ages were explained away as parts of the moon that absorbed and emitted light differently than other parts-- the surface itself was perfectly smooth.

Copernicus's theory that the sun rather than the Earth was the center of all motion had moved the Earth into the realm of heavenly bodies. However, Copernicus's theory went against fundamental beliefs of the time, and therefore was dismissed by most learned people. One of the unacceptable notions was that of the imperfect Earth existing in the realm of the perfect heavens.

However, in November 1609 Galileo pointed his improved telescope with 20x magnification to the moon for the first time. He had decided to make an in-depth study of the moon. From November 30 until December 18 of that year, he examined and drew the moon and described that research in his book Sidereus Nuncius. Galileo's observations led him to the startling conclusion that the moon is anything but perfect. With his telescope, he noticed small dark spots that had never been seen before on the illuminated part of the moon's surface, along with similar light spots in the dark area. He observed that as time passed, these and other spots changed, either getting lighter and eventually disappearing or getting darker and more distinct. The interface between the light and dark sides of the moon was rough and uneven, rather than smooth as would be expected on a perfectly smooth sphere. Galileo also observed that the spots all "have a dark part on the side toward the Sun while on the side opposite the Sun they are crowned with brighter borders like shining ridges." (Sidereus Nuncius, p 41)

From these and many other observations, Galileo concluded that the moon's surface consists of valleys, plains and mountains much like the surface of the Earth (Sidereus Nuncius p 48-49). The dark spots are shadows cast by these mountains and valleys as the sun falls on them. As the moon's position relative to the sun changes, the shapes and intensities of these shadows change. Galileo's conclusion was a shocking one-- how can the moon, a heavenly body, not be perfect and spherical? If the moon is imperfect, could there be other imperfect heavenly bodies as well? If heavenly bodies can be imperfect, why can the Earth not be a heavenly body? Galileo's conclusions about the moon did not give solid proof of Copernicus' theory, but his observations paved the way for the theory's eventual acceptance.

[Note: The page numbers in Sidereus Nuncius cited above refer to Sidereus Nuncius or the Sidereal Messenger. Translated with introduction, conclusion, and notes by Albert Van Helden. Chicago: University of Chicago Press, 1989. AVH]

Galileo's Work Today

I am a member of the Spring 1995 History 333 ("Galileo in Context") class at Rice University. Our assignment was to study Galileo's work in a particular field and then recreate his work as closely as possible. The Astronomy Group built Galilean telescopes and used them to study the same heavenly bodies that Galileo did 375 years ago.

Our group experienced many of the exciting discoveries and frustrating problems that Galileo himself may have encountered. We used modern materials to make replicas of Galileo's telescopes (click here for more information on our Galilean telescopes). One fundamental difference between our telescopes and Galileo's is the fact that ours had only about 9x magnification while Galileo used telescopes with magnifications of up to 20 to study the moon. Our attempt to rigorously recreate Galileo's work was also thwarted by the uncooporative Houston spring weather. The skies were clear for only about five nights during the semester, making it impossible to observe all the phases of the moon as Galileo did.

During my first attempt at observing the moon, I encountered problems, both expected and unforeseen. I was initially frustrated in trying to find the moon due to the small field of view the telescope. The best technique, I found, was to search until a bright light (the light of the moon) was visible on the interior surface of the tube. By moving the telescope so that this light moved down the length of the tube towards my eye, I was able to bring the moon into view. After securing the mounting so that the telescope was fixed on the moon, I was ready to begin viewing.

Before my work on this project, I had had very little experience observing the heavens with a telescope. Therefore, on my first night of observing, I felt much of the same amazement and excitement that Galileo must have felt when he first saw the detail of the moon that is invisible to the naked eye.

This is my drawing of the moon as seen through a Galilean telescope at 11pm on April 26, 1995.

The 9x magnification of our telescope brought out numerous small shadows in the lower left face of the moon, along with a distinctly rough interface between the dark and light sides of the moon. The telescope also made it possible to see the variations in the darkness of the shadows, which gave the moon the distinct three-dimensional look. Galileo's conclusions about the imperfect surface of the moon may have been revolutionary, but they are not surprising once one has looked at the moon through a telescope. Before Galileo made his 20x telescope, the strongest telescopes people had been able to make had only about 3x or 4x magnification. Presumably these telescopes were not strong enough to bring out the details of the moon as Galileo's did; It was this high magnification that allowed Galileo to make his discoveries before others. Galileo's effort to examine the moon through all its phases also played a big role in enabling him to discover and interpret the changing shadows.

As I continued my observations, a number of other problems with our telescope and mounting became obvious. At least one of these problems was one with which Galileo probably had to deal. The small field diameter of our telescopes (my telescope had a calculated field diameter of 964.8 arcseconds, or 16 arcminutes) made it impossible to see the whole face of the moon at once. In the picture above, the concentric circles indicate the field of view visible at one time. The outer circle shows the total view possible with the telescope, while the inner circle roughly indicates the maximum field I was able to get in focus. The small field of view made it necessary to move the telescope around in order to see the whole moon when observing and drawing it. Many of Galileo's drawings of the moon indicate that he had to deal with the same problem. His pictures seem to have been drawn more as representations of the moon than as accurate replicas of what one sees through a telescope. For example, in the picture below, the size of the circular crater on the terminator (perhaps Albategnius) is greatly exaggerated. It is speculated that Galileo represented it this way in order to emphasize the effect of shadowing in creating a three-dimensional image.

From Galileo's Sidereus Nuncius (p 44)

Keeping the image withing the field of view also became a problem. In spite of our efforts to make the mounting as stable as possible, we could not entirely eliminate the wobble from the telescope. Any slight wind or shakiness of the viewer's hand, coupled with the tiny field of view, would move the telescope enough for the object under study to move out of the field. This effect made observing somewhat more difficult, although it had a more detrimental impact on observations of the more distant (and therefore smaller) planets than on observations of the moon.

Full moon observed and drawn March 16, 1995 by Nicole Peterson.

One final problem that future groups will find easily correctable is the height of the eyepiece of our telescope. Once I got the telescope angled at the moon, I discovered that the eyepiece was too high for my eye to reach while I was sitting, yet too low to see into while kneeling. The half-crouch position that I finally had to settle for became uncomfortable and made the observing much more difficult. A suggestion for the future is to design each person's mounting so that the eyepiece sits where it is most comfortable for that person in order to eliminate this annoying problem.

Ann Zumwalt

Galileo || Bibliography || Sunpots || Orion & Pleiades || Moon || Saturn || Jupiter || Venus || Mars || Telescope

We appreciate any comments or suggestions.