Angular diameter

Angular diameter is the angle between sightlines from an observer to opposite edges of an object. It measures apparent width independently of the object's physical size.

Angular size: The apparent extent of an object measured as an angle at an observer. Angular diameter is the angular-size measure for an object's width.
Solar diameter: The apparent angular width of the Sun as seen from Earth, about half a degree. It is a familiar example of angular diameter that changes slightly over Earth's orbit.
Angular radius: The angle between an object's center direction and the direction to its edge. For a circular object, angular diameter is twice its angular radius.
Supermoon: A full Moon occurring near perigee, when the Moon is relatively close to Earth. Its angular diameter is larger than at a full Moon near apogee.
Small-angle formula: An approximation relating an object's physical size, distance, and angular size when the angle is small. It estimates angular diameter from physical width and distance.
Lunar diameter: The apparent angular width of the Moon as seen from Earth, close to half a degree. Its variation helps explain why total solar eclipses are possible.
Angular separation: The angle between the directions to two distinct objects in the sky. It measures spacing between objects rather than the width of one object.
Annular solar eclipse: A solar eclipse in which the Moon appears too small to cover the Sun completely. The unequal angular diameters leave a bright ring of the Sun visible.
Parallax: The apparent shift of an object's position when viewed from different locations. Distance measurements from parallax help determine an object's angular diameter in physical units.
Stellar radius: The distance from a star's center to its surface. Combining a star's angular diameter with its distance yields its physical radius.

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