Glossary term: Fase
Description: En astronomía, el término «fase» se refiere al estado de iluminación parcial de una luna o un planeta con respecto a un observador concreto. Una fuente de luz lejana suele iluminar solo la mitad de la superficie de un cuerpo esférico; el resto permanece a oscuras. Del mismo modo, solo aproximadamente la mitad de la superficie de un cuerpo esférico se encuentra dentro del campo de visión de un observador lejano. La fase especifica qué fracción de la superficie que se encuentra dentro del campo de visión del observador está iluminada. Cambia a medida que varían las posiciones relativas del objeto, el observador y la fuente de luz.
Los ejemplos más conocidos son las fases de la Luna. Las posiciones relativas de la Luna, de la Tierra como ubicación del observador y del Sol como fuente de luz cambian a medida que la Luna orbita alrededor de la Tierra a lo largo de aproximadamente un mes. Por lo tanto, durante ese tiempo, un observador en la Tierra verá diferentes fases de la Luna. La fase en la que toda la superficie de la Luna que se encuentra en el campo de visión del observador está iluminada se denomina «Luna llena»; cuando no se ven regiones iluminadas de la superficie, tenemos «Luna nueva». Las dos fases en las que la superficie está iluminada a la mitad se denominan «primer cuarto» y «segundo cuarto», respectivamente. Cuando la iluminación es inferior a la mitad, se forma una «media luna».
Las fases también se observan en los planetas del Sistema Solar (son especialmente evidentes en Mercurio y Venus) y se han deducido para sistemas de exoplanetas. En el caso de la Luna, ni siquiera las regiones no iluminadas están completamente a oscuras: reflejan la luz que les llega desde la Tierra, en un fenómeno conocido como «resplandor terrestre», documentado por primera vez por Leonardo da Vinci.
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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher The translation of 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".
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