43Q explanation of the principal terms.

Parallax of the earth’s annual orbit, is the angle at anyplanet which is subtended by the distance between thesun and earth; or it is that change of place in theplanets, which arises from their being seen from diffe-rent points of space, as the earth moves round the fun.

Parallels of latitude, small circles of the sphere, whichare drawn parallel to the equator.

Pavo, the Peacock, a southern constellation, composedof fourteen stars.

Pegasus, a constellation in the northern hemisphere, con-sisting of sixty-seven stars.

Pendulum, a body that swings backwards and forwardsabout a fixed point; and which, on account of itsequal vibrations, is made use of for measuring time.

Penumbra, a faint shadow which accompanies an eclipse,and occasions a partial obscurity of the body to thatpart of the earth on which it falls.

Perioeci, those inhabitants of the earth who live underthe fame parallels of latitude, but on opposite sides ofthe meridian.

Perigeon, that point of a planet’s orbit in which it is atits least distance from the earth.

Perihelion, that point of a planet’s orbit in which it isat its least distance from the fun.

Period, a certain length of time after which eclipses, andother celestial phænomena, return again in the famemanner as before.

Periphery, the circumference of a circle, ellipse, orany other regular figure.

Periscii, the inhabitants of either of the frozen zones;so called, because their shadows go round them for sixmonths, or fall towards opposite points of the compass.

Perseus, a constellation in the northern hemisphere,consisting of sixty-seven stars.

Phase?,