A question suddenly struck me: when was it that the twelve constellations of the zodiac were “invented”? I mean, anyone can divide the Sun's yearly course in twelve parts, with the equinoxes and solstices at cardinal points—but when was it that this and that groupment of stars were identified as a Ram, a Bull, Twins, a Crab, and so on?
Of course, I could ask more generally when it was that all the constellations got their names, not just the twelve (or thirteen, as the boundaries are now set) that lie on the ecliptic. Manifestly the names were set by a people living well in the northern hemisphere, since the southern hemisphere constellations have evidently “modern” names (like “the telescope”); but then, some northern hemisphere, or high southern hemisphere, constellations also have funny names (however did the “Shield of Sobieski” come to fill that little gap between the Eagle and the Serpent? Jan Sobieski lived in the XVIIth century, so this doesn't date from the Babylonians).
While I'm at it, how come is it that Dante, in the early 1300's, was able to describe the constellation of the Southern Cross near the southern celestial pole? (Purgatory, canto I, verses 22–24:)
I' mi volsi a man destra, e puosi mente
a l'altro polo, e vidi quattro stelle
non viste mai fuor ch'a la prima gente.
Probably it is a mere stroke of luck, and not a very unlikely one at that, that there are, indeed, four first-magnitude stars forming a kite or cross not too far from the south pole. It may have been that this was a common conception at Dante's time and that when the stars were indeed first seen by Europeans, they were immediately associated with a cross (and not seen as part of a larger whole, like they could have). Or could it have been that someone who had seen these stars had told someone who had told someone who…had told Dante? But then, Dante says, just there, that the stars had never been seen except by the First People. Which is stupid anyway, as one doesn't need to go so very far south to see the Southern Cross: currently all four stars are visible from latitutes below 26°54′N; in 1300, taking into account the precession of the equinoxes, if my back-of-envelope computation with spherical trigonometry is correct, it could be seen up to a latitute of 30°47′N—and the latitude of Cairo is 30°03′N, so I guess at the deadest of a spring night in Cairo around 1300 all four stars of the Southern Cross could be seen on the southern horizon.
But as to the constellations of the Zodiac, who can say? Was it the Greeks who gave them names, or was it the Babylonians perhaps? Certainly anyone not as far North as the arctic circle (or as far South as the antarctic circle) can see them. We can get a hint as to when they were named, however, because the presence of the Sun in the constellation of the Ram has always been associated (by astrologers) with the first month of spring, and this was true roughly 1800 years ago, whereas 3800 years ago (always because of the precession of the equinoxes) it was the Bull that marked the first month of spring. Not a very conclusive interval, but it tends to indicate that it was probably not in the very first days of Egyptian antiquity that the constellation of the Ram was marked as the first sign of the zodiac.
Incidentally, the “New Age” or “Age of Aquarius” gets the latter name from the time, around five centuries from now or so, when the vernal point enters the constellation of Aquarius, the Water-Bearer. The present age should be known as the Age of Pisces (and before that we had the age of Aries, and even before that, Taurus). Of course, the limits of the constellations are completely arbitrary.
But enough astronomy. Another thing I can think of are the twenty-two major arcana of the tarot: it would be interesting to know what their story is (barring all mumbo-jumbo mysticism).
I am not at all a mystic, but I find that mystical constructions and correspondances form a nice template for literary work. This idea was very much to the taste of the “Oulipo” group and related writers. In fact, Italo Calvino, in Il Castello dei destini incrociati imagines a number of stories from an arrangement of tarot cards, read in various directions and orders. So I thought of the following idea (which quite possibly has been already used): write a novel consisting of twenty-one chapters, of roughly equal length, named The Magician, The High Priestess, The Empress, and so forth until Judgement and The World, each inspired by the figure of the corresponding major arcana; and the book should start with an introduction and conclusion, both named The Fool, and possibly word-for-word identical (but interpreted in a very different way at both extremes of the book). It might also be interesting (but probably far more difficult) to write a book with twelve chapters, The Ram, The Bull and so forth, given that the reader should be able to start with any of them and proceed cyclically. And yet another idea I had (less constraining, I guess) was to write a story with twelve characters in it, somehow clearly (but freely) identified as the twelve courts on a usual (non-tarot) deck of cards, perhaps divided in twelve chapters each one focusing on one of the characters (probably having a complex graph of relations with the others). In the same line of thought, it might be possible to conceive a book that follows the lines of a chess game, where each piece moved corresponds to an action of some character, but I'm not sure that would be very interesting, actually. (To be sure, Lewis Carroll's Through the Looking-Glass does this, but it takes considerable liberty with the rules of chess.)
Update: This later entry (in French) expands upon the same ideas.