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periastron(pĕr'ēăs`trən): see apsisapsis
(pl. apsides), point in the orbit of a body where the body is neither approaching nor receding from another body about which it revolves. Any elliptical orbit has two apsides.
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periastron(pe-ree-ass -tron) The closest approach of the two components of a binary star. At greatest separation the components are at apastron.
the point in the orbit of a star nearest the primary star around which it is revolving. If the influence of other celestial bodies is negligible, motion conforms to Kepler’s laws of planetary motion. Perturbations, caused by the attraction of a third star or by the aspherical nature of the star’s shape, result in variations in the position of the periastron in space.