Private pilot Kenneth Arnold, searching for a crashed Marine C-46 near Mount Rainier, saw nine bright, thin objects in an echelon chain weaving past Rainier toward Mount Adams. He timed them at 1 min 42 s between the peaks, implying ~1,200+ mph (he rounded down to ~1,200), about three times any 1947 aircraft. It was the first nationally covered post-war sighting and gave the press 'flying saucer'/'flying disc' (headline coinage; Arnold used 'saucer-like', 'pie-pan' and 'skipping' motion). AAF intelligence (Hamilton Field, 12 Jul) found him credible, but the public AAF conclusion was 'a mirage'. The secret AAF/FBI best-cases review (from 9 Jul) concluded 'something real' was flying, leading to Twining's Sept 1947 memo and then Project Sign.
Nine objects; Rainier → Adams in 1:42; ~1,200+ mph
Arnold's report to AAF intelligence (12 Jul 1947) with sketches; radio interview 26 Jun
report
Corroboration: fire lookout at Diamond Gap saw 'flashes' over Rainier at 3:00 pm; L.G. Bernier (Richland) saw three objects ~30 min earlier; United Airlines crew saw 5–9 discs pacing them for 10–15 min (4 Jul)
AAF investigation; press
corroborating reports
'Flying saucer' coined by headline writers, not Arnold's first quotes
Strentz (1970); Bequette's articles
term origin
AAF investigators (Brown and Davidson) found him credible; Lt. Eustace called it 'silly season'
Declassified reports
split assessment
How it was handled
Method
What happened
Tag
PUBLIC VS. PRIVATE
Public AAF line 'a mirage'; the secret AAF/FBI review of best cases concluded something real was flying (Jul 1947)
two-track
ESCALATION IN SECRET
Twining memo → Project Sign; publicly acknowledged only later
internal escalation
Explanations offered (for balance)
Explanation
Proposer
Tag
P-80 jets / jets in formation
McChord officials; White Sands commander (nearest P-80s ~600 mi away at March Field)
jets
Mirages of snow peaks
Steuart Campbell
mirage
Meteors
Klass / Davidson
meteors
American white pelicans
James Easton
pelicans
Blowing snow → wave clouds → water drops on the window
Menzel (1953, 1963, 1971; three different explanations)