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Glossary term: Opposition

Description: When two astronomical objects are lined up or nearly lined up with an observer, in opposite directions in the sky, they are said to be in opposition. It is not necessary for both objects to be actually visible for the observer. For instance, at a full moon, the Sun, observer, and Moon are lined up, so the visible part of the Moon's surface is fully lit up by the Sun – unless the alignment is perfect, in which case there is a lunar eclipse. When a planet, comet or asteroid is said to be "in opposition", this commonly refers to the Sun and observers on Earth. When a planet is in opposition, it looks particularly bright, appears to move in a direction opposite than usual ("retrograde motion" as Earth moves faster on its inside track), and is particularly close to Earth.

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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".