2. Horizon The horizon is a plane through the observer’s position that is perpendicular to the direction of gravity at that point and that intercepts the celestial sphere in a great circle. The direction of gravity, commonly called the direction of the plumb line, does not necessarily pass through the earth’s center. The horizon plane is considered tangent to the surface of the earth at the observer’s position For most star observations, the distance from this plane to the center of the earth is too small to affect the computations.
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4. Visible Horizon A projection of the local horizon outward to intersect with the celestial sphere. Also known as the apparent horizon.
5. Astronomical horizon (or sensible horizon) The great circle formed by the intersection of the celestial sphere with a plane perpendicular to the line from an observer to the zenith; in other words, the great circle whose poles are the nadir and zenith.
6. Zenith In general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there.
7. The zenith is used in the following scientific contexts: Serves as the direction of reference for measuring the zenith angle, which is the angular distance between a direction of interest (e.g., a star) and the local zenith, relative to the point for which the zenith is defined. Defines one of the axes of the horizontal coordinate system in astronomy.
8. Nadir The nadir (from Arabic nadhir, "opposite") is the direction pointing directly below a particular location.
9. Rational or Celestial Horizon It is a plane passing through the center of the Earth and parallel to the sensible horizon.
10. Geoid The geoid is that equipotentialsurface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest (relative to the rotating Earth), and extended through the continents (such as with very narrow canals).
11. Geoidal Horizon Is an imaginary plane parallel to the sensible horizon, but through the point on the geoid (the sea level surface of the Earth) vertically below the observer.
12. Geometrical Horizon It is an imaginary plane parallel to the sensible horizon, but through the point on the geoid (the sea level surface of the Earth) vertically below the observer.
13. This system is based upon the celestial horizon as the primary great circle and a series of secondary vertical circles which are great circles through the zenith and nadir of the observer and hence perpendicular to his horizon (Figure A).
15. As shown in Figure B, altitude is angular distance above the horizon. It is measured along a vertical circle, from 0° at the horizon through 90° at the zenith.
