Glossary term: Phase

Description: Phase is the partial illumination of a moon or planet when viewed by an observer. When a three-dimensional body is illuminated by a distant light source only about half of that body will be illuminated; the rest remains dark. An observer viewing this may see part of the body illuminated and part in shadow. An example of this would be the crescent moon when viewed from Earth with only a small fraction of the Moon's illuminated surface visible to the observer. The phase of an object changes if that object moves relative to the observer. An example of this is the Moon's changing phase as it orbits Earth. As well as the phases of the Moon, phases are also seen in planets in the Solar System (particularly prominent for Mercury and Venus) and have been inferred in exoplanet systems.

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Term and definition status: This term and its definition is still awaiting approval

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".