The nadir is the imaginary point directly beneath your feet when you look straight down, sitting at an altitude of exactly -90° on the celestial sphere. Think of it as the opposite of the zenith (the point directly overhead) – if you could drill a hole straight through Earth from where you stand, it would emerge at your nadir point on the opposite side of the planet.</p><p>This concept proves essential for astronomical observations and navigation. When astronomers describe object positions, they use the nadir as a reference point for their coordinate system. For example, if you're stargazing in New York, your nadir point sits somewhere in the Indian Ocean near Australia. Satellite operators regularly use nadir measurements – when a weather satellite points its cameras toward Earth's nadir, it captures images looking straight down at the surface below.</p><p>The term originates from the Arabic "nazir," meaning "opposite," which medieval Islamic astronomers used when developing celestial coordinate systems. Today, nadir measurements remain crucial for GPS technology, where satellites must know their precise nadir points to calculate accurate positions. Interestingly, while you can never actually see your nadir point (Earth's in the way!), understanding this invisible reference point helps astronomers map the entire sky and navigate our planet with remarkable precision.