Description: The angular diameter of an object is its visible diameter from a specific location measured as an angle. The angular diameter is used in astronomy as one way to express the size of celestial objects on the sky. The angular diameter increases with increasing physical size of an object and decreases when an object is farther away. For example, the Moon and Sun both have angular diameters of about half a degree when viewed from Earth. The Moon is about 400 times smaller than the Sun but appears the same size (about half a degree across), as the Sun is about 400 times further away.

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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