397
as a consequence, either that Arcturus was subjected to somelotion to which all other stars were not, or that some or all ofose stars were subject to motions from which Arcturus was
Such motions, not generally affecting all stars, are called byatronomers, Proper Motions , and are to be assigned to starsn °t from any theory but solely by observation,
If is clear, however, that if there exists no other method of®tectmg these proper motions than what has been just described,at they can never be entirely disengaged from the effect of pre-cession and exhibited separately ; since they themselves enter intoe composition of precession. AH that can be done is to deter-n>Ble much the annual changes of the mean right ascensionatl d mean north polar distauce of each star differ from its meanP re cession in right ascension and mean precession in north polar•stance; understanding, thereby, those values which are com-Pcted, by the formulae of pp. 340, 341, and from a quantity' 0 A) held to be the mean quantity of the precession.
I iiese annual changes, which are compounded of the pre-cession and certain proper motions, are technically denominatede Annual Variations in right ascension and north polar•stance, and are inserted as such in the catalogues of stars.
."e annual precessions in north polar distance and right ascen-® l0 n are then subjected to a certain law (see pp. 190, &c. 340,hut the annual variations are altogether irregular, never•neruig, however, from the former, except by minute quantities.
^ We have seen, then, that the precession, as its results fromstrouomical methods, is not the actual retrogradation of the in-I ersec tiou of the equator and ecliptic on the ecliptic. We will
I)0 'v consider another point. Is the retrogradation (supposing itPahle of being determined) produced solely by the influence ofj^ e ^ ur * a «d Moon on the excess of the Earth above a sphere ?
•b in fact, a luni-solar precession ? This is a question which' e must go out of the precincts of Plane Astronomy to find an
a »swer to.