The numbers attached to ‘The Dark Side of the Moon’ are pretty impressive. More than 50 million copies sold worldwide. Fifteen straight years on Billboard’s album chart. Consistently ranked in the Top 10 of many best-albums-ever-made polls. But Pink Floyd’s achievements with their eighth LP go deeper than that. In a way, their 1973 epic changed the way people made and listened to albums. Forty years after its release, there’s still no better head trip -- legally at least -- available.