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Either of two points on the orbit of a planet or satellite that are nearest to or furthest from the body around which it moves.
‘However, the speed of the apsides is very much slower than in the figure.’
‘For the moon, the line of apsides completes one revolution around the sky in a period of 8.85 years.’
‘Here, is the Sun, the planet, the geometric center of the orbit, the equant, and the line of apsides.’
‘Because of gravitational disturbances by the other planets, the shape and orientation of the orbit are not fixed, and the apsides slowly move with respect to a fixed frame of reference.’
‘The line of apsides is a line which passes through both periapsis, Cassini's closest approach to Saturn in an orbit, and apoapsis, the farthest spot in the orbit from Saturn.’
‘At time II the line of apsides has turned through 90 degrees and we get a symmetric eclipse, indistinguishable from a circular orbit.’
‘He noted that if the bob was drawn back and released then it followed an elliptical path, and moreover the major axis rotated in the direction of revolution exactly as did the apsides of the moon's orbit.’
‘In particular there was, according to Aryabhata I, a basic period of 4320000 years called a mahayuga and it was assumed that the sun, the moon, their apsis and node, and the planets reached perfect conjunctions after this period.’
‘The angle between Mars and the line of apsides is greater than 90 degrees in the unbisected vicarious hypothesis, and less than 90 degrees in the bisected version.’
‘When the line of apsides points towards the Sun (which will happen when they coincide with the New or Full Moon), the eccentricity reaches a maximum.’
Origin
Early 17th century (denoting the orbit of a planet): via Latin from Greek apsis, hapsis ‘arch, vault’, perhaps from haptein ‘fasten, join’. Compare with apse.