In April 1973, Pink Floyd's "Dark Side of the Moon" went to No. 1 on the Billboard Top 200 pop album chart. There it beamed in the Top 10 among LPs by Elvis Presley, Led Zeppelin, the Temptations, Diana Ross, Alice Cooper, Bread, Diana Ross and Focus.

That same year Richard Nixon accepted the resignations of his aides as Watergate unfolded. The Oakland A's beat the Mets in the World Series.Some things never change. Fifteen years later Nixon, has resurfaced on "Meet the Press," and "Dark Side of the Moon" remains on the chart, usually bobbing back and forth somewhere between 160 and 180.

"Dark Side of the Moon" marked Week 724 at No. 164. The landmark album hit Week 491 in late 1983 and knocked "Johnny Mathis' Greatest Hits" out of the top spot on the longevity list.

Johnny's Greatest Hits was on the charts from 1958 until July 1968. "My Fair Lady" is No. 3 with 480 weeks, "Oklahoma!" is No. 4 at 305 weeks, and Carole King's "Tapestry" holds fifth place with 302 weeks.

The devotion of Pink Floyd fans is legendary, though not easily explained.

Not as visible as the Grateful Dead's Dead Heads, Floyd fans apparently are a little more affluent and put their money where their ears are, buying an estimated 15,000 copies a week of the album in vinyl, cassette or compact disc.

Paul Grein, the Chart Beat columnist for Billboard, makes a living keeping track of such statistics. "Every so often I look to see if it's still bobbing there. I guess it's a sign we're growing old," said Grein, 33. "It is comforting, when you think about it . . . `Dark Side of the Moon' is almost into its fifth president."

What surprises Grein is that comparable LPs from the era have long since been eclipsed.

"What I can't figure out is why `Abbey Road,' `Led Zeppelin 4,' "Exile On Main Street,' `Who's Next,' `Tapestry,' `Bridge Over Troubled Water' and other classic rock albums had their run of a year or two and then fell off the chart. Where's `Sgt. Pepper'? Everybody loves the Beatles, but maybe the Pink Floyd cult is stronger."

Grein and others attribute the album's long life to several factors. There's the music, of course. Also the spectacular engineering, production and recording techniques that make it, even in 1988, a favorite with audiophiles and Floyd freaks alike. The CD revolution also injected fresh life into the record.

"When most people get a CD player, `Dark Side of the Moon' is one of the first CDs that they have to have," said Jim Leupold, a buyer for Listenup Disc Connection. "It's not our biggest seller week by week, but it is one of our biggest sellers in terms of longevity. It has a lot of neat things on it that come out on the CD version. A lot of pop albums just don't have as much substance."