Figure: An explication of the horizontal coordinates used in astronomy

Image 1 Caption: An illustration of horizontal coordinate system with labels explicating its features.

Further explication:

1. The horizontal coordinate system has its origin is wherever you, the observer, are on the surface of the Earth.

2. The Earth is an infinite plane from your perspective that cuts the celestial sphere in half by a great circle called the horizon.

   The plane itself is the horizon plane.

3. The cardinal directions north (N), south (S), east (E), west (W)) are on the horizon.

4. Zenith is the point on the sky directly above the observation point. It's on the vertical from the observer.

5. Nadir is the opposite point on the sky to the zenith and you usually can't see since you-know-what is in the way---the Earth!

6. The meridian is a great circle that passes through due north, Zenith, due south, and Nadir.

   "The meridian" is how we traditionally refer to what more elaborately can be called the celestial meridian.

7. In the horizontal coordinate system, you locate an astronomical object using altitude (the angle from the horizon along a great circle through the zenith) and azimuth (an angle along the horizon measured east from due north).

8. The horizontal coordinate system is a local coordinate system in space time. The altitude and azimuth for an astronomical object depend on your latitude and time of day.

   Nowadays the computer (i.e., your phone app) will the horizontal coordinates for astronomical objects.

9. Specifying altitude is pretty common in casual astronomy: e.g., the Moon is at altitude 60° in that direction (pointing with your hand).

   

10. Measurements of the altitude of astronomical objects transiting are historically very important in precise astrometry.

    This is because those altitudes can be easily and accurately converted into declinations of the equatorial coordinate system.

11. Image 2 Caption: Image 2 also illustrates horizontal coordinate system and explicitly shows the cardinal directions, north celestial pole (NCP), south celestial pole (SCP), and nadir.

12. The observer is in the Northern Hemisphere at mid-latitudes and/or near the Arctic Circle (66.6° N Lat.) judging from altitude of the north celestial pole (NCP) is from due north. Recall, the altitude of the NCP from due north is given by A_N_NCP = L, where L = latitude (L measured negative for south lat.) (Celestial sphere file: celestial_sphere_005_circumpolar.html: Image 2).

13. Image 2 also shows how the cardinal directions (north (N), south (S), east (E), west (W)) are derived from the north celestial pole (NCP). The NCP is projected down on the horizon plane within the celestial meridian plane to give north and the others follow from right angle (90°) rotations: i.e, 90° clockwise to east, 180° clockwise to south, 270 clockwise to west, and 360° clockwise back to north.

14. Image 2 moreover shows (with a bit of imagination) how the celestial equator is found. The plane perpendicular to the celestial axis through the observer is the celestial equator plane which cuts the celestial sphere by a great circle which is the celestial equator which, in fact, passes through east (E) and west (W).