THAON-LES-VOSGES (88) 24.03.2021

A witness reported repeated observations of a coloured light low in the nighttime sky between 24 and 30 March 2021, photographing and filming it on 29 March. Gendarmerie neighbourhood enquiries found no other witnesses. The account’s description and recurrence pointed to astronomy. Investigation identified Sirius, without needing to explore other hypotheses (see report). Its brightness relative to other stars troubled the witness; GEIPAN notes Sirius is the brightest visible star. The slight rightward movement matches azimuth drift from Earth’s rotation. He did not clearly notice the gradual decrease in elevation because the horizon was obstructed and he likely moved slightly around his viewing position. He did notice its unexplained disappearance. On 30 March it disappeared at 00:16 local time, when Sirius was 2.5° high, compatible with setting behind the local terrain rather than the true horizon. Random colour changes account for much of the perceived strangeness. GEIPAN describes the colours as covering much of the stellar spectrum and attributes them to diffraction; it considers atmospheric diffraction, turbulence, refraction, absorption and scattering sufficient to explain the appearance. Its non-point-like shape was not seen directly and comes from camera optical distortion, common or virtually systematic in such circumstances. GEIPAN finds the astronomical context leaves no doubt. Classification A: misidentification of Sirius as it set.