Caption: The celestial sphere and the horizon.

Features:

1. The diagram is not-to-scale.

   The Earth is 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 entered 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.
   1. The sidereal day (86164.1 s on average) is the rotation period relative to the inertial frame of the fixed stars: i.e., it is the physical rotation period

      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 is currently about 86400.002 s.

   3. The civil day is defined as exactly 86400 s = 24 hours.

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

7. The north and south ends of the horizon define circumpolar circles which are small circles parallel to the celestial equator.

8. The circumpolar circle angular radius is the angle from the NCP/SCP of the north/south circumpolar circle.

   The circumpolar circle angular radius equals the observer's latitude which can be either north or south latitude. So the circumpolar circles are latitude or observer dependent.

   You can see this must be true by just rotating the celestial axis onto the horizon around an axis passing the intersection of horizon and celestial equator.

9. Astronomical objects within the circumpolar circles never cross the horizon: i.e., they never rise or set. These astronomical objects are circumpolar objects. Other astronomical objects must rise and set as they are carried by the rotating celestial sphere. Since the circumpolar circles are latitude dependent so are which astronomical objects are circumpolar objects.

10. For a Northern Hemisphere observer, the north circumpolar objects are always above the horizon, and so always in the sky though they are usually unobservable in the daytime.

    The south circumpolar objects are always below the horizon and are NEVER observable.

    This is why you NEVER see the Southern Cross in Las Vegas, Nevada---it is ∼ 30° from the SCP and the circumpolar circle angular radius for Las Vegas is ∼ 36°.

    Southern Hemisphere observers have a mirror image situation with the celestial equator defining the mirror.

11. At the North Pole/South Pole, all astronomical objects are circumpolar objects. At the equator, none are.

12. Zenith is the projection of the observer location (the dot on the Earth on the diagram) from the Earth center onto the celestial sphere.

    Nadir is the antipodal point on the celestial sphere to zenith.

13. The meridian is a great circle passing through the north celestial pole (NCP) south celestial pole (SCP), and zenith.

14. 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.

15. Due east and due west are 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.