In orbital mechanics, the point in an orbit around a star when an object is farthest from that star (the opposite of periastron).
From Greek apo- (away/off) + astron (star), literally meaning 'away from the star.' Coined in 20th-century astronomy to parallel the term 'aphelion' (farthest from the sun) but for any star system.
Apastron is crucial for understanding exoplanet orbits—some planets have extremely elliptical orbits where they swing from roasting at apastron's closest point to freezing at apastron's farthest point, creating wild seasonal swings if they had atmospheres.
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