Astronomy & telescopes glossary - P

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.