How Many Do We Need?

We have once again come across a mention of al-Sufi’s Book on the Shapes of Stars (Kitāb suwar al-kawākib, 10th century), considered by many as the the most important update on Ptolemy’s enormously influential Almagest (originally entitled Μαθηματικὴ Σύνταξις; the Syntaxis Mathematica, 2nd century).

The fascinating history of this relation has been explored time and again from many angles. Briefly, as regards uranography—the “cataloguing of stars” contained in books 7 and 8 of the Almagest—al-Sufi was the pivotal figure who revised the Alexandrian material and prepared it for further developments, culminating with Tycho Brahe’s Rudolphine Tables (1627).

Now, coming to our nautical context: when Ibn Mājid mentions al-Sufi, he invariably adds “and his forty-eight constellations,” a number which seems to go back to Ptolemy. Studying the lunar mansions has made palpable how arbitrary the division of the ecliptic is: you have a rotating circle dotted with shiny specks, you introduce some boundaries and project your lines. You can have 28 mansions or 12 signs or 36 decans or even 144 dodecatemoria. The division in 36 is easy to understand if we think of the rationale of the Babylonian “Three Stars Each” catalogues: for each twelfth of the ecliptic you select one equatorial, one southern and one northern star or asterism, so you cover a wider area and thus give more parameters for orientation (we are not going to waste all those stars!).

In a very useful summary of Ibn Mājid’s uranography, Ibrahim Khoury speaks of 24 major stars used in Indian Ocean navigation. Why 24? and why 48? These figures might be a Greek alphabetic reminiscence: the Greek alphabet has 24 letters, and using this number meant you were speaking of “the alpha and omega” of the stars, the “heavenly ABC.” It meant it was enough, that no other element was needed. I suspect some earlier source for this, perhaps Hipparchus… but the abiding and concrete question is how many stars do we really need to find our way. Right now, dear reader, when it is dark, how many stars in the sky do you need to know in order to find your way home? [JA]