Who would have thought it would be this hard to say goodbye to Johnny Carson?

True, 12 million viewers still tune in to "The Tonight Show Starring Johnny Carson" each weeknight on NBC, but many of us stopped watching -- or at least caring -- years ago, when the Carson show became less a program than a given.

But now that he's leaving, after a 30-year-stint as television's No. 1 late-night talk show host in which he stamped his indelible impression on the American psyche, the impact of Carson's departure is just beginning to sink in.

And it goes deeper than many of us who've come to take Carson for granted may even realize.

Sure, "The Tonight Show" will go on -- with Jay Leno, starting May 25.

There was a "Tonight Show" for eight years before Carson (with Steve Allen, Jack Paar and a series of guest hosts) and there will be a "Tonight Show" after Carson.

But "The Tonight Show Starring Johnny Carson" cannot be replaced.

Television, the nation, just won't be the same without him.

Carson's retirement, which he announced last year, is in fact the most significant event in the medium's history since Walter Cronkite stepped down from the anchor desk at CBS News in 1981. Life magazine, in a May cover story, declared Carson's "TV's greatest career."

For months, there have been tributes, goodbyes and retrospectives. Stars from Elizabeth Taylor to Ted Koppel have queued up on Carson's couch grateful to pay homage in person, to genuflect before one of television's last true royals -- and perhaps secure their special place in history with him. Arsenio Hall, the syndicated talk show host who grew up on Carson only to become his chief competitor, is taking this week off, putting his show in reruns, an obvious sign of respect either for Carson or for the ratings "Tonight" is bound to get this week.

Cable's Comedy Central will do even better than that. On the

big night, Friday, the service will go dark, leaving a video gone-fishing sign saying the people are out "watching Johnny Carson's last show and so should you."

It will, no doubt, be a strange sensation at 11:35 p.m. when Doc Severinsen strikes up the band and "Johnny's Theme" one last time, Carson's sidekick and announcer Ed McMahon shouts "Heeeerrre's Johnny!" and Carson, nervous tics in tow, delivers his final "Tonight Show" monologue and then takes a swing of his imaginary golf club and drives off into history.

There will be no guests -- just Johnny and Ed and Doc and their memories.

As usual, Carson's timing is impeccable, his exit a graceful, class act.

The show, at least for Carson, is over. The era, the sensibilities, the entertainment world that created him and that he influenced so greatly with his monologues and the alignment of Hollywood stars on his couch, has been largely left behind.

Johnny Carson represents what we were more than what we are.

When the talk show host, now 66, succeeded Paar as host of "The Tonight Show" on Oct. 1, 1962, network television was a monolithic force in American culture. The top-rated prime-time shows of the day were "The Beverly Hillbillies," "Candid Camera," "The Red Skelton Show" and "Bonanza."

Carson, a Midwesterner who avoided controversial humor and the mercurial style of his predecessor, appealed to the great, broad and mostly white middle of the country.

The complexion of America and the television industry, however, has since changed. Where once there were three choices -- the networks -- now, with cable, there are dozens. The audience and the industry has exploded and fragmented.

In the 1968-69 season, for instance, Carson's "Tonight Show" drew 34 percent of the available audience. Now, he's got less than half that, just 16 percent. Viewer loyalty, clearly, is a thing of the past. One night it's Hall, the next it's Dennis Miller or ABC News' "Nightline" or maybe a video rental.

America changed, but Carson didn't. That worked for a certain, older kind of audience, increasingly cast aside by the rest of television. But for others, instead of being a cultural touchstone, Carson merely appeared out of touch, unhip.

Always topical but rarely overtly political, Carson nevertheless seemed to prefer the ridiculous to the sublime. His stock comedy and characters -- "Carnac the Magnificent," arch conservative Floyd R. Turbo, Aunt Blabby and leering TV pitchman Art Fern, with his busty human prop Carol Wayne -- aimed low. Just Thursday night, for example, in his opening monologue, Carson dug deep into his pocket of sexist jokes, recalling his prom date with "Gina Statutoria."

What kept Carson on top, however, and ahead of pretenders to his TV throne -- Joey Bishop, Alan Thicke, Merv Griffin, Dick Cavett, Joan Rivers, Pat Sajak and the rest -- was a keen sense of balance. Carson could do it all. As an interviewer, he was first-rate. Kinder than David Letterman, less cerebral and more accessible than Cavett, Carson always came across as interested and, in contrast to Hall and Miller, prepared. He carried on the kind of intelligent, entertaining conversations anyone could follow.

As America's most famous host, he made his guests feel welcome, whether he was entertaining a president or the latest ingenue, or

turning his desk into a petting zoo. He was the gateway to Hollywood, to stardom, for comedians in particular -- dozens, including Roseanne Arnold, Garry Shandling and Jerry Seinfeld -- who knew they were on their way to the big time when their standup comedy got the approval sign from Carson on "Tonight."

Recently, Carson has been offering bits and pieces of his own pop-culture history with a variety of classic clips. The other night, it was a tongue-twisting send-up of the just-the-facts-ma'am "Dragnet" show featuring the series' star, Jack Webb, and Carson.

Many viewers have their own memories: the 1969 wedding of Tiny Tim and Miss Vicki, the highest rated show in Carson's history (21.4 million households watched), or the infamous misguided tomahawk toss from Ed Ames that struck its target below the belt and got one of TV's biggest laughs ever.