PLANISPHERE

, a projection of the sphere, and its various circles, on a plane; as upon paper or the like. In this sense, maps of the heavens and the earth, exhibiting the meridians and other circles of the sphere, may be called Planispheres.

Planisphere is sometimes also considered as an astronomical instrument, used in observing the motions of the heavenly bodies; being a projection of the celestial sphere upon a plane, representing the stars, constellations, &c, in their proper situations, distances, &c. As the Astrolabe, which is a common name for all such projections.

In all Planispheres, the eye is supposed to be in a point, viewing all the circles of the sphere, and referring them to a plane beyond them, against which the sphere is as it were flattened: and this plane is called the Plane of Projection, which is always some one of the circles of the sphere itself, or parallel to some one.

Among the infinite number of Planispheres which may be furnished by the different planes of projection, and the different positions of the eye, there are two or three that have been preferred to the rest. Such as that of Ptolomy, where the plane of projection is parallel to the equator: that of Gemma Frisius, where the plane of projection is the colure, or solstitial meridian, and the eye the pole of the meridian, being a stereographical projection: or that of John de Royas, a Spaniard, whose plane of projection is a meridian, and the eye placed in the axis of that meridian, at an infinite distance; being an orthographical projection, and called the Analemma.

PLANO-Concave glass or lens, is that which is plane on one side, and concave on the other. And

Plano-Convex glass or lens, is that which is plane on one side, and convex on the other. See Lens.

PLAT-Band, in Architecture, is any flat square moulding, whose height much exceeds its projecture. Such are the faces of an architrave, and the Platbands of the modillions of a cornice.