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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 round which it moves.
‘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.’
‘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 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.’
‘However, the speed of the apsides is very much slower than in the figure.’
‘At time II the line of apsides has turned through 90 degrees and we get a symmetric eclipse, indistinguishable from a circular orbit.’
‘Here, is the Sun, the planet, the geometric center of the orbit, the equant, and the line of apsides.’
‘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.’
‘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.’
‘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.’
‘For the moon, the line of apsides completes one revolution around the sky in a period of 8.85 years.’
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.