Precession, the name given to a slow motion of the earth, under the action of the sun and moon, which causes the poles of the heavens (which must remain always vertically above the poles of the earth) to describe circles on the sphere of the heavens about the poles of the ecliptic as centres. As the places of stars on celestial charts are marked with reference to the celestial poles, this motion of these poles causes all such charts to become less and less accurate with the lapse of time. A correction for precession has therefore to be applied to such charts in order to find the true places of stars at any epoch other than that for which they
(4) Peers of Ireland; (5) Peers of the United Kingdom, and Peers of Ireland created subsequently to the Irish Union. A similar order is understood to obtain in regard to baronets, though in Ireland it seems lately to have become the practice to allow all baronets to rank according to the respective dates of their patents.
The following is the table of precedence in Scotland, as recorded in the Lyon Office. It is founded partly on usage and partly on the statutes of 1623 and 1661. are constructed. This motion of the earth also causes the Equinoxes (q.v.) to recede slowly along the ecliptic, so that the sun comes to them, in his annual course, a little earlier each year. Hence the name, 'Precession of the Equinoxes.'
The physical cause of this motion is the attraction of the sun and moon for the protuberant part of the earth around the Equator (see EARTH). This causes the earth slowly to turn on itself, as a spinning top gyrates when its speed slackens before it falls. As this disturbing force on the earth is small relatively to its mass, this turning takes place at the mean rate of only 50"1 per annum. It requires, therefore, 25,868 years for the equinoxes to describe a complete circle on the ecliptic. For a very interesting case of the effect of precession, see POLE-STAR. In actual observation the effects of precession are complicated with those of Nutation (q.v.) and of change of inclination of the ecliptic. The subject is pretty fully discussed in a popular manner in Herschel's Treatise on Astronomy. For the suggested influence of precession, along with the increased eccentricity of the earth's orbit, or great climatic changes on the earth, see PLEISTOCENE, p. 236.