MADE USE OF IN ASTRONOMY. 429

Northern Signs of the ecliptic, are those fix which lieto the north of the equinoctial; as Aries, Taurus, Ge-mini, Cancer, Leo, and Virgo.

Nucleus, a term used by some Astronomers for thehead of a comet, and by others for the central parts ofthe planets.

O.

Oblique Ascension, is an arc of the equinoctial con-tained between the first degree of Aries, and that pointof it which rises with the center of the fun or a star.

Oblique Sphere, is that position of the globe, in whicheither of the poles are elevated above the horizon anynumber of degrees less than ninety.

Occultation, is when a star or planet is hid fromour sight by the interposition of the moon, or some otherplanet.

Octant, an aspect of the planets when they are forty-five degrees distant from each other.

Ophiuchus, or Serpentarius, a northern constellation,consisting of sixty-feven stars.

Opposition, an aspect of the stars or planets when theyare a hundred and eighty degrees distant from eachother ; which in the Ephemeris is marked §.

Orbis Magnus, the orbit of the earth, which is madeby its annual revolution round the fun.

Orbit of a planet, the curve or path which it describesin its revolution round the son.

Orion, a southern constellation, consisting of ninety-three stars.

P.

Parallax, the difference between the places of any ce-lestial object, as seen from the surface of the earth andfrom its center.

Parallax