apsis, n. [L. apsis, absis, Gr. "apsi`s, "apsi^dos, a tying, fastening, the hoop of a wheel, the wheel, a bow, arch, vault, fr. "a`ptein to fasten.].
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One of the two points of an orbit, as of a planet or satellite, which are at the greatest and least distance from the central body, corresponding to the aphelion and perihelion of a planet, or to the apogee and perigee of the moon. The more distant is called the higher apsis; the other, the lower apsis; and the line joining them, the line of apsides. [1913 Webster]
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In a curve referred to polar coördinates, any point for which the radius vector is a maximum or minimum. [1913 Webster]
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Same as
Apse. [
1913 Webster]