=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. =Pivot.= A pin, spindle or shaft on which a wheel, lever or device rotates or is rotated. =Plane.= A level surface, as a table top. =Point.= A sharp end. A starting place. =Pores.= Minute spaces which separate the molecules of a substance. =Position.= The place of a thing when compared to the places of other things. =Precede.= To go ahead of. That which is before. =Precession.= The act of going ahead or preceding. When a revolving body such as a spinning top or the Earth is acted upon by forces which tend to change the direction of its axis. This change in the direction of its axis is called _precessional motion_. The pole of the equator of the Earth makes a complete turn round the pole of the ecliptic in a little over 25,000 years and this change in the direction of the axis of the Earth causes the points of the _equinox_, that is the places where the _ecliptic_ and the _equator_ cross each other to move slowly from west to east and this is termed the precession of the equinoxes. =Prediction.= To foretell; to forecast. Predictions may be based on fancy or founded on fact. =Principal.= The chief one; the most important. =Principle.= The cause of a result. That on which a thing is based. =Process.= (1) The way of working. (2) The course of procedure. =Produce.= In astronomy the word _produce_ means the lengthening of a line. It is a mathematical term. =Protractor.= See Fig. 98. =Quarter circle.= See Appendix B. =Ray.= (1) A single line of light or heat. (2) The path of light and heat. =Relative position.= The position of a thing when compared with the position of something else. See Position. =Revolution.= The turning of a thing completely round on its axis. =Revolve.= (1) To turn completely round a circle. (2) To turn on an axis, as a top, or the Earth. =Rigid.= Firm, that which cannot be easily moved out of its place. =Ring.= See Appendix B.