Van Morrison himself, musicians in the band, critics, journalists etc..called it Jazz. So who are we to argue?
I suppose how Jazzy and swinging it is, really depends on who's playing it. EDIT: ( I just noticed that 'Mangotango' beat me to it, above)...
"Moondance' started as a Jazz saxophone instrumental, and Van played that original sax solo he wrote for the song. He told Rolling Stone magazine:".
"I used to play this sax number over and over, anytime I picked up my horn."
"Pianist and organist Jeff Labes recalled the recording of the track to Uncut: "I remember 'Moondance' itself was a big question mark. It was jazzy, and didn't seem to belong to the pack. "
"Moondance" was recorded at the Mastertone Studio in New York City in August 1969, with Lewis Merenstein. as producer.
The song is played mostly acoustic, anchored by a walking bass line (played on electric bass by John Klingberg), with accompaniment by piano, guitar, saxophones, and flute with the instruments played with a soft jazz swing. It's a song about autumn, the composer's favorite season. Towards the end of the song, Morrison imitates a saxophone. The song also features a piano solo, played by Jeff Labes, which is immediately followed by an alto saxophone solo by Jack Schroer. The song ends with a trill on the Flute during the cadenza that fades out.
Music journalist Erik Hage wrote that the significance of the song "lies in its direct jazz approach", expanding that observation with "Astral Weeks had suggestions of jazz, but this song would take the genre head on. It would become Van Morrison's most successful and definitive jazz composition."
"Moondance' has always been a jazz song."