PLESSIS-TREVISE (LE) (94) 20.06.2017

Late on the morning of 20 June 2017, a witness at a female friend’s home heard a helicopter, went onto the balcony and looked for it. In daylight he then saw luminous points whose number increased from 1 to 3 and then 6. They appeared and moved in a clear sky, changing direction rapidly before disappearing suddenly. Later that day he saw a single point make similar movements. Returning to his friend’s home the next day around 12:10, he again saw a point in the same area of a cloudier sky but did not watch it for long. Only 1 account was collected. The investigation considered sky lanterns, balloons and birds (see the investigation report). Rapid curved movements were broadly compatible with these candidates. Sky lanterns were less plausible in broad daylight, when their recreational visual effect would be lost, and were rejected. Balloons might have been associated with the Fête de la Musique, but the light wind did not fit the UAPs’ movement well, and their paths varied. Circling birds offer features that could explain the sighting. GEIPAN notes that their visibility against a blue sky, without landmarks to establish distance, depends on several factors: - Illumination of a highly reflective part, such as white wings on a darker-bodied bird. Late on a 20 June morning the Sun is very high, so wings viewed from below would be lit indirectly by atmospheric scattering. Similarly, aircraft wings or fuselage can sometimes be the only visible parts. - Great distance may prevent anything but the most reflective parts from being resolved. - Circling in a glide exposes a larger reflective wing surface than flapping flight. The exact visibility threshold between these modes is difficult to establish because many relevant parameters are unknown. Under the bird hypothesis, GEIPAN considers the sighting only slightly unusual. Evidential strength is moderate: 1 witness, no UAP photograph and slight doubt about the date, since the witness mentioned clouds when there were none or very few. The descriptions are nevertheless fairly precise. GEIPAN finds the absence of other witnesses surprising for a daytime event over a densely urbanised region. In synthesis, the rapid curved movements could fit balloons or birds, with birds providing the better overall match to the account. The single account remains a limitation despite the daytime urban setting. GEIPAN regards this as a possible misidentification of sunlight reflected from birds and classifies it B: probable misidentification of sunlight reflections from birds in flight.