Glossary term: Angular Diameter

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 objects in space. The angular diameter increases with increasing physical size of an object and decreases when an object is further 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 as the Sun is about 400 times further away.

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The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".