Concept
Celestial pole
Overview
The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at Earth's North Pole and South Pole, respectively. As Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other celestial points appear to rotate around them, completing one circuit per day (strictly, per sidereal day).
The celestial poles are also the poles of the celestial equatorial coordinate system, meaning they have declinations of +90 degrees and −90 degrees (for the north and south celestial poles, respectively). Despite their apparently fixed positions, the celestial poles in the long term do not actually remain permanently fixed against the background of the stars. Because of a phenomenon known as the precession of the equinoxes, the poles trace out circles on the celestial sphere, with a period of about 25,700 years. The Earth's axis is also subject to other complex motions which cause the celestial poles to shift slightly over cycles of varying lengths (see nutation, polar motion and axial tilt). Finally, over very long periods the positions of the stars themselves change, because of the stars' proper motions. To take into account such movement, celestial pole definitions come with an epoch to specify the date of the rotation axis; J2000.0 is the current standard.
An analogous concept applies to other planets: a planet's celestial poles are the points in the sky where the projection of the planet's axis of rotation intersects the celestial sphere. These points vary because different planets' axes are oriented differently (the apparent positions of the stars also change slightly because of parallax effects).
Overview adapted from Wikipedia (CC BY-SA 4.0; see the article history for authors).
Possible passages
Found by search and not yet checked by a person; they may only mention the idea in passing.
The same principle, the attraction of the Sun, which thus accounts for the motions of the Nodes, connects, too, another very perplexing irregularity in the appearances of the Moon; the perpetual variation in the inclination of her orbit to that of the Earth. As the Moon revolves in an ellipse, which has the centre of the {381} Earth in one of its foci, the longer axis of its orbit is called the Line of its Apsides…
Adam Smith, The Essays of Adam Smith
States what Celestial pole is.
Those causes happening to be progressive in the particular way which is called periodical, the effect necessarily is so too; because the quantities to be added together returning in a regular order, the same sums must also regularly return.
John Stuart Mill, A System of Logic, Ratiocinative and Inductive
States what Celestial pole is.
The globe is divided into five parts, termed zones, and all that portion is subject to severe cold and perpetual frost, which is under the two extremities, about each of the poles, the nearer of which is called the north, and the opposite the south, pole. The middle of the earth, over which is the orbit of the sun, is parched and burned by the flame, and is consumed by being so near the heat.
the Elder Pliny, The Boys' and Girls' Pliny
States what Celestial pole is.
Related ideas
Place in the tradition
Part of Confucianism.