Parabola. [See Fig. 129].
Parallel. [See Appendix B].
Particle. A bit of matter. A particle may be a bit of matter which can be seen or it may mean an amount so small that it cannot be seen.
Pencil. A group of rays from the same source of light. See Beam. See Ray.
Pendulum. A body suspended from a fixed point and free to swing.
Perihelion. The point where a planet or a comet is nearest the Sun. As the orbit of a planet is an ellipse and the Sun is in one of the foci—that is near one end—a planet comes nearest to the Sun when at this end of the ellipse; and when the planet is at the other end of its orbit, it is farthest away. See Aphelion.
Period. A certain interval of time which is marked by a repeated occurrence. The period of the Earth round its axis is about 24 hours. Its period around the Sun is 365¼ days.
Periodic. Recurring regularly; as a periodic comet.
Perpendicular. [See Appendix B].
Phase. One of the peculiar aspects of a heavenly body; as the phases of the Moon, or the phases of Venus.