The correct answer is C) intercept. The distance in miles between the circle of equal altitude for the observed altitude (Ho) and the circle of equal altitude for the computed altitude (Hc) is called the intercept. This is because the intercept represents the difference between the observed and computed altitudes, which is used to determine the position of the observer relative to the celestial body. The other options are incorrect because: A) Zenith angle is the angle between the observer's zenith and the celestial body. B) Zenith distance is the angular distance between the observer's zenith and the celestial body. D) Equation of time is the difference between the true solar time and the mean solar time.