in Geometry, a plane figure comprehended by a single curve line, called its circumference, to which all right lines drawn from a point in the middle, called the centre, are equal to one another.

Circles of the Sphere are such as cut the mundane sphere, and have their periphery either on its movable surface, or in another immovable, conterminous, and equidistant surface. Hence arise two kinds of circles, movable and immovable. The first are those whose peripheries are in the movable surface, and which therefore revolve with its diurnal motion, as meridians, and the like. The latter, having their periphery in the immovable surface, do not revolve, as the ecliptic, equator, and its parallels.

Circles of Altitude, otherwise called almucantars, are circles parallel to the horizon, having their common pole in the zenith, and still diminishing as they approach the zenith.

Diurnal Circles are immovable circles, supposed to be described by the seven stars, and other points of the heavens, in their diurnal rotation round the earth, or rather, in the rotation of the earth round its axis. The diurnal circles are all unequal; the equator is the largest.

**Horary Circles**, in Dialing, are the lines which show the hours on dials; though these be not drawn circular, but nearly straight.

**Circles of Latitude**, or **Secondaries of the Ecliptic**, are great circles parallel to the plane of the ecliptic, passing through the poles thereof, and through every star and planet. They are so called, because they serve to measure the latitude of the stars, which is nothing but an arch of one of these circles intercepted between the star and the ecliptic.

**Circles of Longitude** are several lesser circles, parallel to the ecliptic; still diminishing, in proportion as they recede from it. On the arches of these circles the longitude of the stars is reckoned.

**Circle of perpetual Occultation**, one of the lesser circles, parallel to the equator, described by any point of the sphere touching the northern point of the horizon, and carried about with the diurnal motion. All the stars included within this circle never set, but are ever visible above the horizon.

**Circle of perpetual Occultation** is another circle at a like distance from the equator, and contains all those stars which never appear in our hemisphere. The stars situated between these circles alternately rise and set at certain times.

**Polar Circles** are immovable circles, parallel to the equator, and at a distance from the poles equal to the greatest declination of the ecliptic. That next the north pole is called the Arctic, and that next to the south pole the Antarctic.

**Druidical Circles**, in British topography, a name given to certain ancient inclosures formed by rude stones circularly arranged. These, it is now generally agreed, were temples, and, as many writers think, also places of solemn assemblage for councils or elections, and seats of judgment.