The line of apsides is in continual motion, generally direct, and performs a revolution in about 12 years.
On comparing observations made at different times it was found that the line of apsides was not fixed, but made a complete revolution in the heavens, in the order of the signs of the zodiac, in about nine years.
They are called the lower or nearer, and the higher or more distant apsides respectively.
The "line of apsides" is that which joins them, forming the major axis of the orbit.
Tisserand in 1895.1 It involved the action of no third mass, but depended solely upon the progression of the line of apsides in a moderately elliptical orbit due to the spheroidal shape of the globes traversing it.