Precession of the Equinoxes

PRECESSION OF THE EQUINOXES, in astronomy, an effect connected mainly with a gradual change of the direction of the earth's axis of rotation. There is a general resemblance between the motion of the earth and that of a spinning top. It is well known that when a top is slightly disturbed its axis precesses round the vertical so that it traces out a cone ; the earth's axis similarly describes a cone at the rate of one revolution in about 26,00o years. In applying this analogy we must take the ecliptic (i.e., the plane of the orbit of the earth round the sun) to corres pond to the horizontal; the axis about which the earth spins is inclined at to the "vertical," and keeping this inclination it turns slowly round the "vertical." It must be emphasized, how ever, that this correspondence between the earth and a top is superficial, the cause of the precessional motion being upon different principles.

In this way the north pole of the celestial sphere describes among the constellations a circle of radius, making a revolu tion in 26,000 years. At present it is near the star a Ursae Minoris, which is therefore called the Pole Star ; but it has trav elled a considerable distance within historic times. About 3000 B.C. the star a Draconis would have served as pole star ; in 13000 B.C., also in A.D. 1300o Vega would be near enough to the pole to mark roughly its position. By this displacement the part of the sky visible from a particular terrestrial station gradually changes; certain constellations cease to rise above the horizon and others appear for the first time. In the time of the early Chaldean astron omers it was not necessary to travel so far south to see the South ern Cross as it is now.

Cause of Precession.

This was first explained by Isaac New ton. It is due to the attraction of the sun and moon on the equa torial protuberance of the earth, the moon being responsible for about 3 and the sun for 4 of the motion-the same proportion as the lunar and solar tides. Treating the equatorial bulge as an extra ring of matter surrounding a spherical earth, the attraction of the sun and moon on this ring gives a couple tending to turn the ring into the plane of the ecliptic, since both disturbing agents are in or near the ecliptic. If the earth were not spinning this would turn the earth over until the equator coincided with the ecliptic, but the spinning earth behaves like a gyrostat, so that its axis moves at right angles to the plane of the couple-just as the couple which would upset a top at rest gives the axis of the spinning top a conical motion.

The moon's orbit is inclined at about to the ecliptic, but it does not remain still; its nodes travel round the ecliptic in 18.6 years. Averaged over a long period of time the deviations of the moon from the ecliptic cancel out ; but at any moment the precession caused by the moon may be greater or less than the average, according to the position at the time of the lunar orbit. In fact the path of the pole among the stars is a slightly sinuous curve. Astronomers distinguish the average secular motion as precession and the periodic fluctuations or sinuosities as nuta tion (q.v.).

As the pole (corresponding to the equator) moves round the pole of the ecliptic, so the equinox or intersection of the equator and ecliptic moves round the ecliptic once in 26,000 years. Both right ascensions and longitudes are reckoned from the equinox as zero-point; stellar longitudes on this account increase steadily by nearly a minute of arc every year ; the effect on the right ascensions is more complicated, but these also continually in crease. The vernal equinox is commonly called the First Point of Aries, but it has already moved away from that constellation and is now in Pisces. It should be understood that the precession of the equinoxes has no effect on the seasons, and, for example, has no connection with the gradual departure of the spring equinox from March 21 which occurred in the old Julian calendar.

Planetary Precession.

Besides the foregoing luni-solar pre cession, a phenomenon of much smaller magnitude, known as planetary precession, is recognized. It is due to perturbations by the planets which cause slow changes in the plane of the earth's orbit. Planetary precession changes the position of the ecliptic, whereas luni-solar precession changes the position of the equator; either change affects the equinox, which is the intersection of the two planes. Coirections for precession and nutation are of great importance in most branches of positional astronomy, and their computation is very burdensome. For details of the theory of the precessional motions see Newcomb's Spherical Astronomy, ch. ix.