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Astronomy & Telescopes Glossary - O
Objective - The primary or largest optics piece in a telescope."
Occultation - When a smaller astronomical body passes behind a larger one, totally obscuring its view. The Moon often occults a star or other planet.
Open cluster - A group of stars, usually of the same age, that are bound together gravitationally.
Oort Cloud - The cloud of rocks and dust that surrounds our solar systems. The Oort cloud is believed to contain long-period comets.
Optical tube assembly (OTA) - The housing and optical system of a telescope. It does not include the mount, eyepieces or other accessories.
Opposition - When the Earth is exactly between a planet and the Sun, that planet is said to be in opposition.
Outer planets - The planets that orbit far from the Sun, including Jupiter, Saturn, Uranus, Neptune and Pluto.