Nadirs

In astronomy, a nadir is the point directly opposite the zenith on the celestial sphere. The zenith is the point directly overhead an observer, and the nadir is the point directly below them, perpendicular to the horizon. Imagine a line drawn from the center of the Earth, through your position on the surface, and extending upwards to the sky; that point in the sky is your zenith. The nadir is the point on the opposite end of that line.

The concept of the nadir is fundamental in spherical astronomy and navigation. It helps define the local coordinate system for an observer on Earth. While the zenith changes as an observer moves across the Earth's surface, the nadir is also specific to that observer's location at any given moment. It's essentially the "down" direction in space from an observer's perspective.

In practical terms, the nadir is important for instruments that need to be precisely oriented, such as telescopes and satellite tracking systems. It provides a reference point for understanding orientation in space relative to a local vertical. While less frequently discussed than the zenith, the nadir plays an equally crucial role in defining an observer's spatial relationship with the celestial sphere.