Astronomy dictionary – O

Browse through our astronomy dictionary to find definitions for some of the most common terms used in practical astronomy and space science.

Click on one of the letters below to search for a term.



An occultation is when a celestial body passes in front of another. So if the Moon passes in front of Saturn or a star it is known as a lunar occultation.

Ocular adjustment

Increasing or decreasing the distance between the two eyepieces (which can also be called the ocular lenses) in a pair of binoculars.

OIII filter

This is a special filter designed to let through a very specific wavelength of light emitted by oxygen in nebulae. It can help filter out light pollution and improves the view of objects such as planetary nebulae.

Oort Cloud

This is a vast cloud of small icy bodies that envelops the Solar System. It is thought to contain trillions of objects and resides 30 trillion kilometres from the Sun.

Open cluster

A group of stars that formed together in the same cloud of gas, and are gravitationally bound together. Open clusters can contain up to a few thousand stars.


The position of a superior planet when it is opposite the Sun in the sky, as viewed from Earth.

Optical double star

An optical double star is actually two stars that appear to be close to each other on the sky. In fact they are not at all related and appear close only because they lie close to each other along our line of sight.

Optical tube assembly

This is the housing within which a telescope’s mirror and optical surfaces are mounted.

Orographic clouds

These are clouds that form as air is forced to rise over high obstructions on the surface of a planet. This could be a mountain on Earth or a volcano on Mars, for example.


A dynamic model of the Solar System that shows the relative positions and motion of the planets and moons.