Astronomy Glossary

All | # A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
There are currently 5 names in this directory beginning with the letter O.
O

occultation
(a) Eclipse of a star by another celestial body.
(b) The cutoff of the light from a celestial body caused by its passage behind another object. (Strictly speaking, a Solar “eclipse” is a Solar occultation.)

open cluster
A gravitationally bound group of up to one thousand stars that formed together in the same molecular cloud.

opposition
The positioning of two celestial objects on opposite sides of the sky, from the perspective of an observer. This occurs, for example, when a planet makes its closest approach to the Earth, placing it in opposition to the Sun.

orbit
The gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Though the smaller body is often said to orbit the larger body itself, both bodies actually follow approximately elliptical orbits around a common center of mass positioned at a focal point of each ellipse. The word "orbit" can variously refer to the elliptical trajectory itself or the act of following this trajectory, and can refer to a stable, regularly repeating trajectory as well as a non-repeating trajectory.

outer space
The vast, nearly empty expanse that exists beyond the Earth and between all celestial bodies, characterized generally by extremely low densities of particles, extremely low temperatures, and minimal gravity. Most of the volume of the Universe is intergalactic space, and even galaxies and star systems consist almost entirely of empty space.