76 J!f'^A YENS. 



Nuta'tion. — A vibratory motion of the earth's axis, arising 

 from j)(!rio€lical fluctuations in the obliquity of the echptic. 



Occulta'tion. — The hidintJi; of a lieavciily body from our si^ht 

 by the intervention of some other of the heavenly bodies. 



Orbit. — The path described by a heavenly body in its periodical 

 revolution. 



Parallax. — The change of place in a heavenly body in conse- 

 quence of being viewed from different points. 



Parasele'ne. — A luminous ring or circle encompassing the 

 moon, in which sometimes are other bright spots bearing 

 some resemblance to the moon. 



Penum'bra. — A partial shadow or obscurity on the margin of 

 the perfect shadow in an eclipse, or between the perfect 

 shadow, where the light is entirely intercepted, and the 

 full light. 



Per'igee. — That point in the orbit of the sun or moon in which 

 it is at the least distance from the earth. 



Perihe'lion. — That part of the orbit of a planet or comet in 

 which it is at its least distance from the sun. 



Pis'ces (Lat., Fishes). — The twelfth sign or constellation of the 

 zodiac. 



Plan'et. — The name given to a few bright and conspicuous 

 stars which are constantly changing their apparent situa- 

 tions in the celestial sphere. 



Ple'iades. — A cluster of seven stare in the neck of Taurus, 

 assigned by jMiidler as the central point of the sidereal 

 system. 



Precession of the Equinoxes. — A continual shifting of the 

 equinoctial points from east to west. 



Retrocession of the Equinoxes. — The going backward of 

 the equinoctial points. 



Sagittarius (Lat., the Archer). — One of the twelve signs of 

 the zodiac, which the sun enters about November 22. 



Sat'ellite. — A small planet revolving round another planet. 



Scorpio (Lat., the Scorpion). — The eighth sign of the zodiac, 

 which the sun enters about October 23. 



Sec'ondary. — A great circle of the sphere passing through the 

 poles of another great circle, perpendicular to its plane, as 

 secondaries to the ecliptic. 



Selenog'raphy (Gr. seUne, the moon, and graphs, descrip- 

 tion). — The description of the surface of the moon. 



Sign. — The twelfth part of the ecliptic. 



