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Astronomy & Telescopes Glossary - P
Parabolic mirror - A mirror ground to a shape of a paraboloid, which brings all incoming light rays to a focus.
Parsec - A unit of distance equal to 3.26 light-years, and the the distance at which a star would have a parallax of 1 second of arc.
Partial eclipse - When the moon partially blocks the sun or the Earth's shadow partially the Moon.
Penumbra - During an eclipse, the area of partial shadow.
Perihelion - A planet or comet's closest approach to the Sun. The Earth is at perihelion in January.
Photometry - The measurement of apparent magnitudes of stars.
Planetary alignement - When many of the planets in the solar system are in a line.
Planetary nebula - A type of nebular where a cloud of gas that has been discharged by a central star.
Planetoid - A large asteroid or other celestial body, also called a minor planet.
Polar axis - The telescope mount axis that is parallel to the Earth's axis. With a motorized drive, celestial objects can be tracked and remain in the eyepiece field of view throughout the night.
Pole Star - A star located almost due north or due south and used for navigation or telescope alignment. Polaris is currently the pole star of the Northern Hemisphere.
Prominence - A solar prominence is an cloud-like arc of gas that erupts from the surface of the Sun.