Iridium is a satellite constellation consisting of 66 satellites. It provides satellite phone services over the entire Earth surface. The first generation satellites have three door-sized reflective antennas. Because of their reflectivity, Iridium satellites will occasionally reflect sunlight to Earth’s surface. The Iridium flare will look like a bright flash over a few seconds.
Most flat-Earthers deny the existence of satellites. Iridium flares provide an easy opportunity to confirm the existence of satellites, at least until the end of 2018, when they are replaced by newer non-reflecting satellites.