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oceanus
A large, cicular plain.

old
A planetary surface that has been modified little since its formation typically featuring large numbers of impact craters (compare to young).

Oort Cloud
A theory, now widely accepted, that the Sun is surrounded by a distant cloud of comet-stuff, now called the Oort Cloud, bits of which are occasionally hurled into the solar system as comets.

opposition
A superior planet is said to be "in opposition" when it is directly on the opposite side of the Earth from the Sun. This is generally the closest it comes to the Earth and the time at which it is most easily visible. (e.g. when Mars, Earth and the Sun are located along a straight line, Mars is in opposition as seen from Earth.) Nice diagram.

opposition surge
An increase in brightness as an observer approaches the line between the sun and a target. The strength of the surge is an indicator of small-scale surface texture.

orbit
The path followed by an object in space as it goes around another object.

orbital period
The time required for an object to make a complete revolution along its orbit.

orbital resonance
A relationship in which the orbital period of one body is related to that of another by a simple integer fraction.

organic compounds
Complex chemical compounds that contain carbon.

ovoid
Shaped like an egg.

oxygen
A colorless and odorless gas that is needed by people and animals to live.