Glossary P

Parapegma.

Planet.

Precession. More specifically known as axial precession, and historically called precession of the equinoxes, this refers to the slow but continuous change in the orientation of the Earth's rotational axis. To an observer on Earth it has the effect of moving the equinoxes steadily westwards along the ecliptic against the background of the fixed stars. Alternatively, it appears that the stars move steadily eastwards against the ecliptic, causing their ecliptic longitudes to increase. At the moment, the rotation is once per 25,772 years or 50.287'' per year (1°24' per century) but it is not constant. The phenomenon was discovered by Hipparchos.

Procyon.

Proper Motion.