The Three Conditions for Seeing the ISS — A Browser-Only Visibility Calculator with Spherical Trigonometry

When ISS passes overhead at the right moment, you can step outside and watch a bright point of light streak from west to east at ~7 km/s — apparent magnitude around −5, brighter than Venus, no telescope needed. The interesting part of building a "is it visible right now?" tool is that just plotting the dot on a map doesn't answer the question. Three independent conditions all have to be true: ISS above your horizon (geometry) ISS sunlit (not in Earth's shadow) Your sky dark enough (sun far enoug