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objective
a telescope’s primary lens or mirror that gathers light and brings it to a focus
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obliquity
the angle between a planet’s equator and the plane of its orbit
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occultation
the passage of one object in front of a smaller one, temporarily obscuring all or part of the background object from view
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omega
1. the ratio of the density of the universe to the critical density
2. the 24th letter of the Greek alphabet
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open cluster
a system containing a few dozen to a few thousand stars that formed from the same stellar nursery
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opposition
the moment when a planet farther from the sun than Earth appears opposite the sun in the sky; it is the best time to observe a planet
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optical double
Two stars at different distances that lie along nearly the same line of sight and thus appear close together
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optics
the study of light and its properties; or lenses and mirrors
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orbit
the path an object follows around a more massive object or common center of mass; usually elliptical in shape
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orbital period
the length of time it takes one body to orbit another
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outgassing
the release of gas from a rocky body
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