Sun-synchronous orbit

A near-polar orbit that crosses the equator at the same local time every day.

Earth is not a sphere; its equatorial bulge tugs on satellite orbits and makes them precess. Choose the altitude and inclination correctly — typically 600–800 km and about 98°, slightly retrograde — and that precession exactly matches Earth's progress around the Sun, once a year. The result is an orbit fixed relative to the Sun rather than the stars, so the satellite passes over any given place at the same local solar time on every pass. That means consistent lighting and shadows, which is what makes images from different dates comparable. Nearly every Earth-observation satellite is in one, which is why so many launches head south from Vandenberg.