Question 1580Navigation General

The correct answer is B) directly below the observer. The nadir is the point on the celestial sphere that is directly opposite the zenith, which is the point directly overhead. Since the zenith is directly above the observer, the nadir must be the point directly below the observer, or "directly below the observer." The other answer choices are incorrect because: A) Greenwich is a location on Earth, not a point on the celestial sphere; C) 90° away from the zenith is the horizon, not the nadir; and D) the western horizon is a location on Earth, not a point on the celestial sphere.