Twenty years ago this month I had my first history of astronomy publication!
Sky & Telescope magazine published a letter of mine in their May 2006 issue, under the heading “Galileo’s Pride and Prejudice” (above). I wrote the letter in response to a 2004 article in S&T. That article would change my life.
Here is what I wrote:
“A New View of Mizar” by Leos Ondra (July 2004, page 72) discusses how Galileo, in a search for direct evidence of Earth’s motion, observed the double star Mizar in Ursa Major in an attempt to measure its parallax. However, Ondra inadvertently raises interesting questions regarding Galileo’s assumptions about the universe and the conclusions that he drew from his observations.
Ondra dates Galileo’s observations of Mizar to 1617. Certainly Galileo would have been very interested in parallax at the time, for Cardinal Robert Bellarmine had recently (1615) written that Catholic authorities would accept the heliocentric theory if direct evidence for Earth’s motion were obtained. Ondra finds that Galileo measured the apparent diameters of Mizar’s component stars to be 6 and 4 arcseconds, separated by 15 arcseconds [the apparent diameter of the full moon is about 1800 arcseconds]. To determine their distances, Galileo assumed that all stars were roughly the same size as the Sun, then calculated that since Mizar A was 1/300 the apparent size of the Sun, it must be 300 a.u. distant (Mizar B would be 450 a.u. distant).
Galileo knew nothing of light from a point source diffracting through a circular aperture and couldn’t know that the sizes he measured were due to wave optics and did not reflect the stars’ dimensions. His size measurements would seem good, and his distance calculations would seem as good as his assumption that the stars were suns.
Galileo must have expected the components of Mizar to swing around each other dramatically as he observed them over a period of weeks and months. Based on his calculations he would have expected A and B to have parallax angles of ±11.5 and ±7.6 arcminutes, respectively. Their relative motion would dwarf their separation. But in fact Mizar A and B do not budge. Since no parallax is seen, Galileo logically had to conclude either that the Earth was stationary or that his assumption regarding stars being suns at differing distances from Earth was wrong.
Yet Galileo asserts both these things in his Dialogue Concerning the Two Chief World Systems (1632). In the Dialogue, Galileo argues that 1st-magnitude and 6th-magnitude stars have apparent sizes of 5 and 5/6 arcsecond, respectively, that stars are the same size as the Sun, and that since 5/6 arcsecond is 1/2160 the size of the Sun, 6th-magnitude stars are 2160 a.u. distant — arguments in line with his work on Mizar. He states, “If some tiny star were found by the telescope quite close to some of the larger ones, and if that one were therefore very remote, it might happen that some sensible alterations would take place among them,” and he goes on to suggest that the “sensible alterations,” or parallax, would provide proof of Earth’s motion.
The Dialogue conflicts with Galileo’s earlier work on Mizar, raising interesting points. Had Galileo published his Mizar observations, they would have influenced the ongoing debate regarding Earth’s motion, likely prolonging the time before the geocentric theory was finally overturned. It would seem that at the time the Dialogue appeared, Galileo was sitting on results that strongly challenged the Copernican theory he was championing!
Once I read Ondra’s article I figured out pretty quickly that Galileo’s measurements pointed to a problem for Copernicus. I thought Ondra’s article would create a big stir. Certainly some historian would jump on it and make hay with it. But no one did. Thus, I wrote the letter. In time, I would figure out that I would have to be the historian to make the hay. That would require me to become an historian, rather than a community college physics and astronomy professor. It would eventually lead to my affiliation with the Vatican Observatory.
In one sense I have learned very little in the last two decades beyond what is in this May 2006 letter. In the seventeenth century, telescopic observations of the stars were a problem for the ideas of Copernicus. Stars could not be suns at differing distances from Earth. That has sat at the heart of the vast majority of the research I have done, and the papers and books I have published in the past twenty years. Even my new book with Dennis Danielson, A Universe of Earths, is closely tied to this this idea. If stars could not be suns, then the idea that they were indeed suns (with earths orbiting them), an idea that people like Giordano Bruno tacked on to the Copernican theory, was not based on science, but on something else.
Interestingly, what I wrote was not quite enough for S&T in 2006. They edited my letter a little. Some of the editing was fine. However, they edited the first sentence of the last paragraph to include three extra words (boldfaced here): “The Dialogue conflicts with Galileo’s earlier work on Mizar, raising some very interesting ethical points.” I was only comfortable with “raising interesting points”, and I asked for a correction. Eventually one appeared in the July 2007 S&T.
Even today I am not sure what I think about Galileo’s ethics here. I understand why S&T would jump to the presumption that Galileo’s Mizar measurements indicated an ethical problem, and no doubt Galileo did some weird things that do not appear fully “according to Hoyle”. But, Galileo did some weird things that made his own life harder, things that I think were owed to oversight more than ethical lapses — such as some things with his tidal theory (click here for more on that). So, I am still happy to have gotten the retraction regarding that letter of twenty years ago.
Not long after the letter came out, I posted a fuller discussion on the ArXiv pre-print server: “Galileo’s Double Star: The Experiment That ʻProved’ the Earth Did Not Move”. The discussion was from the standpoint of teaching, but I was starting the process of becoming an historian of astronomy.
