in Geometry, a plane figure comprehended by a single curved line called its circumference, every part of which is equally distant from a point called the centre. Consequently all straight lines drawn from the centre to the circumference are equal to each other.

Circles of the Sphere are such as cut the mundane sphere, and have their periphery either on its movable surface, as the meridians; or in another immovable, continuous, and equidistant surface, as the ecliptic, equator, and its parallels.

Circles of Altitude or altucentors, are circles parallel to the horizon, having their common pole in the zenith, and diminishing as they approach the zenith.

Diurnal Circles are immovable circles, supposed to be described by the several 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 Dialling, 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 its poles, 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 arc 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 as they recede from it. On the arcs of these circles the longitude of the stars is reckoned.

Circle of Perpetual Apparition, 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 are always visible above the horizon.

Circle of Perpetual Occultation, another lesser circle at a like distance from the equator, containing 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 the south pole the Antarctic.

Druidical Circles, a name given to certain ancient inclosures formed by rude stones circularly arranged; as that of Stonehenge near Salisbury. These, it is now generally supposed, were temples, and also places of solemn assemblage for councils or elections, and seats of judgment.

Circles of the Empire, the provinces or principalities of the German empire, which had a right to be present at the diets.