Altitude (celestial)

Altitude is the angle at the observer between the visible horizon and a celestial body, measured along the vertical circle through the body, from 0 degrees at the horizon to 90 degrees at the zenith. The sextant reads this as the sextant altitude Hs. After index correction and dip it becomes apparent altitude, and after refraction, semidiameter, and parallax it becomes the observed altitude Ho used in the sight. Altitude and zenith distance are complementary: zenith distance equals 90 degrees minus altitude. In the intercept method Ho is compared with the computed altitude Hc.