Glossary O

Occultation: The passage of one body in front of another, which causes the more distant object to be wholly or partially hidden.

Oort Cloud: A spherical distribution of trillions of icy planetesimals and cometary nuclei that surrounds the Solar System.

Open Cluster: A loose cluster of up to a few thousand stars that lies in or close to the plane of the Milky Way galaxy.

Opposition: The position of a planet when it is exactly on the opposite side of the Earth from the Sun.

Orbit: The path of a body that is moving within the gravitational field of another.