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Change of line of apsides; period about 21,000 years.

The more distant is called the higher apsis; the other, the lower apsis; and the line joining them, the line of apsides.

The “line of apsides” is that which joins them, forming the major axis of the orbit.

That is to say, he assumed a revolving line of apsides in an elliptical orbit traversed by a pretty strongly compressed pair of globes.

The time of revolution of a primary or secondary planet in reference to its line of apsides; that is, from one perigee or apogee to another.