Figure 15-4.-Horizon system of coordinates.system is the observer’s horizon. Figure 15-4 illustratesthe horizon system. In this figure, O represents both theearth and the location of the observer.The horizon is a plane through the observer’sposition that is perpendicular to the direction of gravityat that point and that intercepts the celestial sphere in agreat circle. The direction of gravity, commonly calledthe direction of the plumb line, does not necessarily passthrough the earth’s center. The horizon plane isconsidered tangent to the surface of the earth at theobserver’s position For most star observations, thedistance from this plane to the center of the earth is toosmall to affect the computations. However, observationsof the sun, planets, and some of the nearer stars, whenused in the more precise computations, must account forthe displacement of the horizon plane. This is called thecorrection for parallax.The point where the plumb line, extended overhead,pierces the celestial sphere is known as the zenith. Thepoint opposite this and underneath is the nadir. Greatcircles drawn through the zenith and nadir (with theirplanes perpendicular to that of the horizon) are calledvertical circles. The angular distance of a celestial bodymeasured along a vertical circle from the horizon is thealtitude (h) of the body. The complement of the altitudeis the coaltitude, or zenith distance, and is measuredalong the vertical circle from the zenith to the body.The vertical circle through the poles, which alsopasses through the zenith, is called the observer’smeridian. The azimuth of an object is the anglemeasured clockwise in the plane of the horizon from theobserver’s meridian to the vertical circle passingthrough the object. The northern intersection of themeridian with the horizon is used as the zero azimuth15-6