Octagon. A figure with eight sides, and consequently with eight angles.
Opaque. Not transparent; refusing a passage to the rays of light.
Optics. That branch of science which treats of light, and vision. It is generally divided into two parts. Catoptrics, which treats of the reflection of light, and Dioptrics, which treats of its refraction.
Orbit. The line in which a primary planet moves in its revolution round the sun; or a secondary planet, in its revolution round its primary. These orbits are all elliptical, or oval.
Parabola. A particular kind of curve; that which a body describes in rising and in falling, when thrown upwards, in any direction not perpendicular to the horizon.
Parallelogram. A figure with four sides, having those which are opposite, parallel to each other. A square, an oblong square, and the figure usually called a diamond, are Parallelograms.
Parallel Lines. All lines, whether straight or curved, which are every where at an equal distance from each other, are parallel lines.
Parallel of Latitude. See [Latitude].
Perihelion. That part of the orbit of a planet, in which it approaches the sun most nearly.
Pendulum. A body suspended by a rod, or line, so that it may vibrate, or oscillate, backwards and forwards. Pendulums of the same length, perform their vibrations in the same time, whatever may be their weight, and whether the arc of vibration, be long or short.