An historical mistake

We highlight an instance in which not knowing one’s history led to mistakes in astrophysics.

Our astronomer has continued his recreational reading in nineteenth-century astronomy books, mostly those written for non-professionals.  Two things have struck him in particular recently.  The first is the descriptions of colors of stars, apparently of wonderful variety; we may go into that someday.  The other is the deduction, from Bayer letters, that stars have changed their brightness.

This requires some background.  In making lists of stars (vital for saying almost anything about them) the earliest astronomers described each by its place in the mythological constellation figure (“right shoulder,” or “on the hip,” or suchlike).  As we’ve noted, this concern continued even through the end of the nineteenth century, when the animals, humans and other figures had long since ceased to have any meaning.  But it depends on everyone having the same figure, which was never quite true, and is inadequate once you concern yourself with more than limited number of stars.  A few dozen stars have accepted names that are unambiguous.  Somehow you have to come up with “names” of some sort for the rest.

In making his star-map in 1603, Johann Bayer hit upon one solution.  He would assign each star a Greek letter.  For the most part (the qualification is important) he went in alphabetical order of brightness within each constellation, the brightest star being alpha, the second-brightest beta, and so on through the twenty-four letters of the Greeks.  Then one could refer to a star as “delta Orionis” without worrying about getting the figure right, and everyone would know what you’re talking about.  Sometimes he was influenced by a star’s place in the figure, and in any case had no way of measuring brightness accurately, so sometimes the order is not exact.

[We note that this requires your readers not only to be familiar with the Greek alphabet but also with the use of the Latin genitive case, that is, to have at least a nodding familiarity with both Classical languages.  Such was actually the case for most serious astronomers through the nineteenth century.]

Our astronomer notes that the authors he is reading have taken the general method of Bayer and assumed it as a strict rule.  As a consequence, when they found that (for instance) gamma Aquilae was now brighter than beta, they concluded that it had actually become brighter (or beta had become fainter) in the three centuries since Bayer.  Even such a scholar as Admiral William Smyth made this mistake a number of times.

Now, there are stars that vary in brightness, quite a few of them; but most are very steady.  All will change as they evolve, but three hundred years is an eyeblink for stellar evolution.

How did our authors come to mistake Bayer letters for strict photometry?  That would probably take some deep research into sources and their uses.  At present our guess is that Bayer’s atlas itself was too old and inaccessible for them to consult, and they had only the letters themselves, passing through several intermediate hands, to work with.  The lesson, we think, is that any scientist must be utterly familiar with his data before he bases a conclusion on it.