The correct answer is D) Greenwich hour angle. In the celestial equator system of coordinates, the Greenwich hour angle (GHA) is equivalent to the longitude in the Earth system of coordinates. The GHA is the angle measured eastward along the celestial equator from the vernal equinox to the hour circle passing through the celestial body. This is analogous to longitude, which is the angle measured eastward along the Earth's equator from the prime meridian (Greenwich) to the meridian passing through the location on Earth. The other options are incorrect because they represent different celestial coordinate system parameters - azimuth angle is the horizontal angle, zenith distance is the vertical angle, and declination is the celestial latitude. None of these directly correspond to longitude in the Earth system.