Note C — RHYTHMS AND GEOLOGIC TIME.A By G. K. GILBERT, UNITED STATES GEOLOGICAL SURVEY.
Reading edition
Strictly speaking, 21,000 years is the period of the precession of the equinoxes as referred to perihelion; but the perihelion is itself in motion. As referred to a fixed star the precession of the equinoxes has an average period of about 25,700 years.