Twenty years ago this month Sky & Telescope magazine featured an article about Mizar, the star in the bend of the handle of the Big Dipper: “A New View of Mizar” by Leos Ondra was on page 72 of the July 2004 issue. Mizar is a double star — to the naked eye it appears to be a single star, but looking at it through a telescope reveals two component stars that the eye cannot distinguish from each other. Ondra’s article about this double star would change my life.
Ondra was interested in the question of who first observed a double star. He writes:
Almost all astronomy books tell the same tale. Double stars were discovered around 1650, a generation after Galileo died, when Giambattista Riccioli (1598-1671), a Jesuit astronomer and geographer at Bologna, Italy, turned his primitive telescope on Mizar and saw that it was not one star, but two.
For a long time I searched for a primary source to verify this much-repeated story. I asked historians of astronomy and even managed to check Riccioli’s two monumental surveys of the astronomy of his time, Almagestum Novum (1651) and Astronomiae Reformatae (1665). I failed to find anything about any double star at reasonable places in these books. I began to wonder: was this another of the astronomy-history myths that have been blindly copied from one book to the next?
Then I unexpectedly chanced upon the elusive reference: Almagestum Novum, volume 1, part 1, page 422. I had overlooked the note because it was so brief, merely an aside: “There appears to be one star in the middle of the Great Bear’s tail, when there are actually two, as the telescope reveals.” That is all Riccioli wrote on Mizar being double. It didn’t quite sound like someone announcing a great discovery. Was something missing?
Ondra goes on to relate how he asked for help on “H-ASTRO, an Internet forum for the history of astronomy”. A member of the forum pointed him to a 1949 paper from Umberto Fedele of the Bologna observatory. Ondra continues:
Entitled “The First Observations of Double Stars,” [the Fedele paper] included precious references to primary sources: manuscripts of Galileo’s that are preserved in the National Library in Florence.
And from these one can learn that Galileo himself unquestionably resolved Mizar in a telescope in [the year 1617]. Moreover, he apparently hoped to use the discovery to prove his case that, contrary to traditionalist academics and the decree of the Catholic Church, the Copernican theory was correct and the Earth moved around the Sun.
Ondra relates how Galileo’s friend and former student, Benedetto Castelli (a Benedictine priest who, among other things, would go on to become abbot of the Benedictine monastery at Monte Cassino), first noticed Mizar’s unusual nature when viewed through a telescope. In January 1617 Castelli urged Galileo to take a look at the star himself. Galileo did, as Ondra relates:
Notes in Galileo’s own handwriting… present a detailed record of him splitting Mizar into an unequal pair of stars that he measured to be 15 arcseconds apart. This is remarkably close to the modern value of 14 [arcseconds], considering the crudeness of Galileo’s instrument.
Galileo then attempted to calculate the two stars’ distances from Earth. He estimated the angular radius of the brighter of Mizar’s components to be 3 arcseconds and wrongly assumed that this represented the star’s actual disk. He compared this with the angular radius of the Sun [900 arcseconds] and, by presuming that the Sun and the star were the same physical size, got a distance of only 300 times the Sun’s distance from Earth.
Ondra also notes that Galileo put the radius of the lesser of Mizar’s components at 2 arcseconds.The image below shows both a copy of the notes Ondra mentions (left), and a representation of how Galileo saw Mizar, according to those notes (right).
Ondra relates how Castelli then found another double star in Monoceros in late January, while in early February Galileo observed the close stars of the Trapezium in what we now recognize to be the Orion nebula. Ondra continues:
According to Fedele’s research, Castelli and Galileo became interested in double stars because they wanted to prove Earth’s motion around the Sun. As Galileo explained later in his Dialogue Concerning the Two Chief World Systems… the method was simple. If one finds with a telescope a close pair of stars, one bright and thus presumably closer to Earth, and the other fainter and farther… the revolution of Earth around its orbit should show itself in an annual change in their relative position. Castelli wrote to Galileo… saying of Mizar, “I don’t believe that in our pursuit one could desire better”.
Their “pursuit” was most likely pursuit of telescopic stellar evidence of Earth’s motion. See the diagram above. If Earth moves around the sun, then when it is on one side of the sun, at A, our view of two sun-sized stars that appear close together in the sky but that are at differing distances will be different than when Earth is on the other side of the sun, at B.
See the diagram below. When Earth is at A (left), the two stars appear more widely separated; when Earth is at B (right), they appear closer together. Note that the more distant star will always appear smaller.
This change in the relative position of the stars between A and B is called “differential parallax”. Given Galileo’s assumptions about what he was seeing, it should have been a very sensitive method for detecting Earth’s motion. However, as Ondra put it,
The first double-star observers detected no credible relative motion…. Today it’s clear that the attempt to use Mizar to support the Copernican view of the universe was doomed from the beginning. Mizar’s components…. show no parallax at all with respect to each other.
I thought Ondra’s article was pretty cool. But something immediately hit me…
Ondra said that Mizar’s components show no parallax at all with respect to each other. But given Galileo’s assumptions about what he was seeing, and given his ability to record the separation of Mizar’s component stars so accurately, basic trigonometry indicates that Mizar’s components would have to show differential parallax if Earth was moving, and show it very quickly. Since they did not, the logical conclusion that follows is that Earth is not moving.
Galileo and Castelli did not merely fail to find evidence in Mizar to support Copernicus. They found evidence against Copernicus. The telescope was providing evidence against Copernicus.
Now there was something interesting. We always hear that the telescope provided evidence in favor of Copernicus. But none of Galileo’s famous telescopic discoveries actually gave evidence for Earth’s motion — not the phases of Venus that showed it to circle the sun; not the moons of Jupiter that showed that it was a center of motion; not the craters on the moon or the spots on the sun or the many stars visible only telescopically. These were all important discoveries that showed that ancient ideas about the heavens were wrong, but they did not really bear on the matter of Earth’s motion. By contrast, Galileo and Castelli’s telescopic discovery of the double nature of Mizar did directly bear on the matter of Earth’s motion — and suggested that Earth did not move.* It seems that there was more standing against Copernicus in Galileo’s time than merely rigid traditionalist academics and decree-issuing Church prelates.
But wait! There’s more…
Recall how Ondra discussed that As Galileo explained later in his Dialogue Concerning the Two Chief World Systems, if one finds with a telescope a close pair of stars, one bright and thus presumably closer to Earth, and the other fainter and farther, the revolution of Earth around its orbit should show itself in an annual change in their relative position.
“If one finds”? If?
Galileo published the Dialogue in 1632. Galileo was feeding his readers bogus information in the Dialogue when he claimed that a double star observation could reveal Earth’s motion, while he did not disclose that he and Castelli had already found several such stars in 1617, and none of them showed any such thing. That’s the kind of scientific no-no that will up-end a career today.
And wouldn’t Riccioli, who knew Mizar was double, have realized this? And since, as Ondra said, Riccioli mentions Mizar so casually, isn’t it likely that others realized this as well?
Leos Ondra’s article raised many interesting and important questions.
In July 2004, I figured someone would be sure to jump on those questions. I was a community college physics and astronomy professor on summer break. I was interested in the history of astronomy, yes. I had been learning more about the history of astronomy in an attempt to be able to better answer student questions. Through the work of the late Owen Gingerich of Harvard, in particular, I was learning that there was a lot more to the history of science and especially of the Copernican Revolution than I had thought. But I was not doing anything like my own research in the field. I was doing some research and publishing some papers, but on things like the physics of ping-pong paddles and automobile acceleration, and on using video capture in student physics laboratories. I figured some historian of astronomy would be the one to jump on the questions Ondra’s article raised.
I was wrong. It took me a while to realize that the person who would have to jump on those questions was me. Jumping on those questions has led to me writing two books (one on Riccioli, who, yes, realized the problems that star sizes posed for Copernicus), editing a third book, and co-authoring a fourth, as well as writing many scholarly papers published in peer-reviewed journals and many articles published in more general media outlets. The question of Galileo and Castelli and their double stars, and the whole star size issue, has in turn led to questions about the nature of science and even the nature of our search for life on other worlds. I have talked to NASA people, been flown to other countries to lecture, been invited to be a part of the Vatican Observatory. I got to know Owen Gingerich and other accomplished historians in person, and I have spent a lot of time on H-ASTRO. I’ve become that historian of astronomy.
All because of an article by Leos Ondra that Sky & Telescope published twenty years ago this month.
Thanks, S&T. Thanks, Mr. Ondra.
*How can this be? The problem lies in Galileo’s 3 arcsecond star radius that he measured. He could use the telescope to measure the sun and determine its apparent radius to be 900 arcseconds, and that measurement does indeed reflect the size of the sun. But the radius of the star is spurious, and wildly too large. It is a product of the optics of the telescope, not a product of the size of the star. However, it would take a long time, long after Galileo’s death, before astronomers figured out that the sizes of stars seen through telescopes are spurious, whereas the size of the sun (or moon, or Jupiter) is not. Until they figured that out, star observations would be a problem for the Copernican system.
