. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. The latitudes of all the fixed stars remain very nearly thesame from year to year, and even from century to century: andwe therefore conclude that the position of the ecliptic withreference to the celestial sphere remains very nearly the longitudes of the stars are all found to increase by anannual amount of : and hence the line of the equinoxesmust have an annual westward motion of the same westward motion is called the precession of the the ecliptic remain
. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. The latitudes of all the fixed stars remain very nearly thesame from year to year, and even from century to century: andwe therefore conclude that the position of the ecliptic withreference to the celestial sphere remains very nearly the longitudes of the stars are all found to increase by anannual amount of : and hence the line of the equinoxesmust have an annual westward motion of the same westward motion is called the precession of the the ecliptic remains stationary in the heavens (or at leastso nearly stationary that the latitudes of the stars only varyby half a second of arc in a year), this precession must be con-sidered to be a motion of the equinoctial on the ecliptic. In Fig. 46, let EQ represent the equinoctial, and LC the eclip-tic. B V is the line of the equinoxes, which moves about in theplane of the ecliptic, taking in course of time the new positionKVf. Perhaps the clearest conception of this motion is ob- FRECESSION. 113. taiued by considering P, the pole ofthe equinoctial, to revolve about A>the pole of the ecliptic, in the circlePG (the polar radius of which, AP,is 23° 27), moving westward in thiscircle with the annual rate of ,and completing its revolution in25,817 years. A general explanation of thecause of precession may be given bymeans of Fig. 47. The earth mayLe regarded as a sphere surrounded by a spheroidal shell, asrepresented in thefigure by EPQp,and the matter inthis shell may beconsidered to forma ring about theearth in the planeof the equator, asshown in is to the attrac-tion of the sun andthe moon on thisring, combined withthe earths rotation,that the precessionof the equinoxes isdue. In the figurelet S be the sun, thecircle ABV the ecliptic, and EQ this equatorial ring of theearth. Let ACVhe the plane of the equinoctial, meeting theplane of the ecliptic in the line of equinoxes A V
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectastrono, bookyear1901
