“If I ventured in the slipstream, between the viaducts of your dream.” That's a cracker of an opening line and it leads and beguiles you into the eponymous opening track of Astral Weeks, an LP record that was markedly different from pretty much anything that went before.

It still seems so different because it seems so little thought out, so big on spontaneity. Van's almost forgotten TB Sheets album preceded – yes, there is some kind of argument about which marked the watershed – but Astral Weeks was the real leap in the dark, the kind of artistic gamble that young men are perhaps inclined to make.

Van had clearly forsaken the gritty, smoky crackle and pop of Them, a band which had enjoyed, gasp, hit singles. Now he was searching in the great unknown of musical experimentation, with no guru, no method, no teacher. He was to make an album that would yield no singles and fail to chart, but in the aleatory log-book of Kerouac and the Beat sensibility, it's up there at number one.

The band which assembled at New York’s Century Sound studios on the evening of September 25, 1968 comprised a quartet of jazz musos who were probably as uncertain about what would result as Van himself. The unit comprised double bassist Richard Davis, guitarist Jay Berliner, percussionist Warren Smith jnr and Modern Jazz Quartet drummer Connie Kay. Further recordings were done in October, it was basically a three-day session, produced by Lewis Merenstein.

Astral Weeks itself is a wholly confident opening track, everybody purring along, softly, shifty, slidey, groovy.But you can sense that air of foraging in the dark in Beside You, restlessly treading modally, resisting the soulful entity it is warmed into by the band, who coax the music out of Van (or should that be the other way around?). Then Slim Slow Slider, all sinister cloak and dagger, ending in the rattling of the bow of the double bass, like a scattering of birds of ill-omen. A wild screech on the flute as Van sings the line, "I don't know just what to do." What a tensely dramatic way to end the song, a headlong car run off the road, into chaos.

The original album as presented here is 47 minutes and 10 seconds long, but with the extras, obviously, the total listening time increases somewhat. Check out the first take of Beside You, one of the CD extras. There is also a longer version of Slim Slow Slider, while the alternate Madame George has no strings, but more of that vibraphone that lent so much airy agility to the album.

Cypress Avenue was a set of black-and-white pictures, it was a door into the dark but star-lit summer night of the singer's Belfast adolescence. There was the raw excitement of Young Lovers Do whose brass swept you up into a strange fiesta inside a tropical rainstorm.You had young-blade sexual hunger preening itself on Ballerina, which was a song left over from Them daysEach track on Astral Weeks had its own aura, its own strange orbit, and the lyrics were street language made hymnal.

But can Astral Weeks mean as much to newcomers now, forty-seven years on - does it speak in the same way? Over to you, guys.

Paddy Kehoe