Caption: The celestial sphere and the horizon.

Features:

1. The diagram is not-to-scale.

   The Earth is a point compared to the celestial sphere.

   The observer on the Earth is a point compared to the quasi-infinite plane defined by the horizon.

2. The horizon plane is just a plane tangent to the Earth at the location of the observer.

   It is actually centered on the Earth since, as aforesaid, the Earth is point compared to the celestial sphere.

3. The horizon itself is the circle formed by the intersection of the horizon plane and the celestial sphere.

   The horizon is a great circle since it cuts the celestial sphere in half.

4. The observer ideally sees everything above the horizon and nothing below it.

5. In the Earth-at-rest perspective, the celestial sphere rotates westward once per sidereal day (23:56:4.0905 = 86164.0905 s) = 1 day - 4 m + 4.0905 s (on average).
   1. The sidereal day (23:56:4.0905 = 86164.0905 s) = 1 day - 4 m + 4.0905 s (on average) is the rotation period relative to the observable universe (i.e., bulk mass of the observable universe).

      The sidereal day is about 4 minutes shorter than the solar day.

   2. The solar day is defined as solar noon to solar noon and its mean value is currently ∼ 86400.002 s (see Wikipedia: Solar time: Introduction).

   3. The civil day = 86400 s = 24 hours exactly.

6. All natural astronomical objects are carried by the celestial sphere as their primary motion.

7. Due north and due south are given by the projections of the celestial axis onto the horizon plane. The projections are in the plane of the meridian.

   The plane perpendicular to the horizon plane through the point Earth or the point observer on the point Earth cuts the celestial sphere in a great circle which is the celestial equator.

8. Due east and due west are points 90° from due north on the horizon plane. They are also the points where the horizon intersects celestial equator. Due east/due west is clockwise/counterclockwise from due north looking down on the horizon plane from the zenith side of the celestial sphere.