Now such a constellation as the following must sometimes exist: the earth is in perihelion; the line of nodes coincides with the line of apsides and both lie in the line joining earth and sun.
The line of nodes rotates in a period of 18.612 years and the line of apsides in a period of 8.84 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.
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.
