The correct answer is D) the primary great circle of the Earth perpendicular to the axis. The equator is the primary great circle of the Earth that is perpendicular to the Earth's axis. It divides the Earth into the Northern and Southern hemispheres and is the reference line from which the latitude of a location is measured. The other answer choices are incorrect because: A) The equator is not the line from which a celestial body's altitude is measured. That would be the celestial horizon. B) The equator is not the line to which all celestial observations are reduced. That would be the celestial equator.