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O

Obliquity
The angle between a body's equatorial plane and orbital plane.

Oblateness
A measure of flattening at the poles of a planet or other celestial body.

Occultation
An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon.

Oort Cloud
A theoretical shell of comets that is believed to exist at the outermost regions of our solar system. The Oort cloud was named after the Dutch astronomer who first proposed it.

Open Cluster
A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.

Opposition
The position of a planet when it is exactly opposite the Sun in the sky as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.

Orbit
The path of a celestial body as it moves through space.

 

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