In the days before photography, astronomers had to use their eyes. If they saw something new though their telescope, they would record it and tell others, who would then attempt to see the same object. What happens, however, if one sees something that the other doesn’t?
The Moon that Wasn’t discusses in great detail a series of such observations. In the seventeenth and eighteenth centuries, several astronomers claimed to have seen a satellite of Venus. Others did not see it. In the resulting debate, most astronomers thought it was unlikely that Venus had a moon (sometimes for what sound like a priori rather than observational reasons), but the matter remained unsettled for a long time. Only in the nineteenth century did a consensus conclusion develop: there was no such moon. So the matter has stood since then; photographic observation and spacecraft have confirmed that the moon is not there.
The question that animates Kragh’s book is both historical and philosophical: how did scientists come to the conclusion that there was no moon? Were they right to do so? After all, the standard line is that “absence of evidence is not evidence of absence”… but if so, doesn’t that mean it is impossible to establish the non-existence of something?
Most of the book follows the history of the spurious moon in detail, in the process clarifying or correcting much of what has been said about it. The last chapter brings out the implications for the philosophy of science by contrasting this story with the story of the moons of Uranus as “discovered” by William Herschel.
Having discovered the planet Uranus, Herschel soon after observed two moons, then four more. No one else saw those four satellites. Today, the consensus is that Herschel did see Oberon and Titania, the two large moons of Uranus, but that the other four moons he claimed to have seen are not there. (Uranus has several other satellites, but none of them can be the objects Herschel described.) Nevertheless, for decades astronomers listed six moons of Uranus, apparently because of Herschel’s reputation as an observer.
So the question is: why was everyone so certain about Herschel’s alleged satellites, but so dubious about Venus’s moon, which was observed repeatedly by different astronomers? There might have been, for example, a hidden assumption that the farther one went from the sun, the larger the number of moons. What role did that play? If it played a role, this seems to cast serious doubt on the often-proclaimed primacy of observation over theory. The last chapter of The Moon that Wasn’t calls attention to all of these questions, but doesn't really propose answers; the book seems intended to serve, then, as historical raw material for philosophers of science.
Kragh’s book is clearly written, though at times one sees a sign or two that the author is not a native speaker of English. There are useful illustrations throughout. An appendix gives brief biographies of some of the major players. Though there might be a little more detail here than most of us really want to know, it is good that someone went to the trouble of investigating the history carefully. As a result, Kragh has given us both a detailed history of the spurious satellite of Venus and much food for thought.
Fernando Q. Gouvêa is Carter Professor of Mathematics at Colby College in Waterville, ME.