▸ Orbital Mechanics — How Fly-Bys Work
Why Satellites Stay Up
Satellites aren't fighting gravity — they're falling around it. At ~7.7 km/s, they fall toward Earth just as fast as Earth's surface curves away. The ISS completes one orbit every ~92–93 minutes at roughly 400–420 km altitude.
Inclination & Visibility
The ISS orbits at 51.6° inclination, so its ground track covers everything between 51.6°N and 51.6°S — and because it is 400+ km up, it can still be seen from a few hundred km further north or south. Starlink uses several shells (about 43°, 53°, 70° and 97°) to cover the globe.
When Can You See It?
The satellite must be in sunlight while you're in darkness — typically 1–2 hours after sunset or before sunrise. The ISS can outshine every star and planet except Venus, and at its highest it crosses the sky at roughly 1° per second.
Two-Line Elements (TLE)
Each satellite's orbit is described by a Two-Line Element set — a standardized format encoding inclination, RAAN, eccentricity, mean motion, and more. This app downloads fresh TLEs from CelesTrak (NORAD data) and runs the standard SGP4 model — which includes Earth’s bulge and air drag — to predict where each satellite will be.
Elevation Angle
A pass is visible if the satellite rises above 10° elevation from your horizon. Higher passes (above 45°) are longer and brighter. The azimuth (compass direction) tells you exactly where to look in the sky.
Starlink Trains
Shortly after launch, Starlink satellites fly in a tight formation — a "train" of bright dots crossing the sky. SpaceX's mega-constellation — thousands of satellites, most around 480–560 km up — passes over most places many times a day, but single Starlinks are usually faint.
