Now, whatever Aristotle and his disciples may say, the firmament is not incorruptible (ἀφθαρτός) and immovable; even the _fixed_ stars, as we call them, change their place in time. We have seen that the whole celestial vault or "right sphere" runs, from east to west, around the poles of the world; we have seen also that the sun moves, on his own account, from west to east, around the poles of the oblique sphere, or the Ecliptic. Well, this does not suffice; there is a third movement to be observed,--that of the right sphere itself round the poles of the Ecliptic; and this, not like that of the sun, from west to east, but inversely, from east to west. Only, this movement of the starry sphere in longitude, or parallel to the plane of the Ecliptic, is extremely slow, compared with the movement of the same sphere in right ascension, or parallel to the Equator of the world. While the former traverses in twenty-four hours the 360° of the circle, the latter occupies (in round numbers) 25,000 years. Who was the first discoverer of the slow movement of the heavens? Hipparchus. This great astronomer, on comparing his own observations with the more ancient ones of Aristillus and Timocharis, succeeded in ascertaining that the constellation which, 150 years before him, corresponded to the spring equinox, did not, in _his_ time, any longer coincide exactly with the same equinoctial point, but had outstripped or preceded it about 2°. This is what we mean by the _precession of the equinoxes_. Hipparchus was at first of opinion that this movement affected only the constellations of the Zodiac; but he soon became assured of its universality. He perceived that if it does not alter the parallels of latitude; because it has occurred parallel with the Ecliptic, it makes the position of the equinox retrograde from east to west, and the sun pass slowly through the same constellations in the reverse of the order in which he annually traverses them. We know this movement now to nearly the fraction of a second. By an inappreciable _daily_ quantity, it rises, at the end of the year, to 50".3,--in a century, to about 1-1/2°,--in twenty centuries, to 30°, or the twelfth part of the Zodiac. It is for this reason the Ram, which, in the days of Hipparchus, was occupied by the sun in spring, has no longer any value as a commemorative sign; it gives place now-a-days to the constellation of the Fishes, and corresponds to the constellation of Taurus, or the Bull; the constellation of Taurus to Gemini, or the Twins; the constellation of the Twins to Cancer, and so on. But little more than a month, then (a month of 2000 years!), of the _great year_ (a year of 25,000 years!) has elapsed since the epoch of Hipparchus. It is to astronomy especially that, with a slight variation, we may apply the aphorism of Hippocrates--"_Brevis vita, ars longa_" (Life is short, and art long). * * * * *