Where the wind blows
A poetic phrase has an unexpected scientific source.
We’ve mentioned that one of our consultants has recently moved from one apartment to another in Alexandria. He is now attempting to restore order to his library of several hundred books. One of the joys of this tedious process is discovering forgotten works, some to be read and others reread. The converse is confirming that some books are indeed absent. A book that has, alas, been finally lost has the title The Wind’s Twelve Quarters. Another time we may say something about the stories it contains. For now, notice that the title itself, as a piece of mathematics, is puzzling. Mathematically, twelve quarters equals three; are we to have the picture of winds shifting thrice around the compass? Our navigator considers that unlikely; even in the most violent storms the wind hardly backs or veers through as much as half the sky.
Neither is it hyperbole. The key is that one meaning of “quarter,” perhaps the oldest, is “direction,” or “source” from which something comes. In this sense it doesn’t imply a piece of which four make a whole. So the wind’s twelve quarters are the twelve directions from which it can blow.
This is still puzzling. Our navigator is familiar with the four cardinal points, North, South, East and West; and their subdivisions, NW, NE, SE, SW, giving eight; even down to the 32-point subdivision, with such directions as east northeast (ENE) and northeast by east (NExE). Transferred from the compass to the ship one comes up with the undeniably nautical “three points abaft the starboard beam” and similar phrases. Resisting the temptation to brail the mizzen tops’l (which none of his ships actually possessed, in any case), he normally expressed direction with a 360-degree compass. But he never used a twelve-fold system. Who did?
After tracking down a number of references, we found it was Aristotle, in chapter VI, book II of Meteorologica. There he gives the names of each wind, the direction it comes from and the characteristic weather it brings. There are twelve of them: the expected north, south, east, west, and two between each of the first four. Why divide each quarter of the compass into three, instead of two? The clue is that the wind Caecias, for instance, blows from the direction of the midsummer sunrise and Eurus from the midwinter sunrise. At the latitude of Greece these are very close to thirty degrees north and south of due east, respectively. With these markers it apparently seemed logical to divide the compass into thirty-degree segments, for a total of twelve. So it was an accident of astronomy and the latitude of Aristotle’s home that gave us the wind’s twelve quarters.
This was probably also practical, in that the twelve-fold wind rose gave better precision than an eight-fold one, while in dealing with weather there was no real gain in going to sixteen directions. For other purposes, anyone seriously measuring angles would use the 360-degree system inherited from Babylonian mathematics. Are twelve winds enough for you?
