For years, we’ve wondered how Apple could possibly continue should its mercurial CEO, Steve Jobs, ever step down. And yet today, with Apple at the height of its market power, the unthinkable has happened.

Because of Jobs’ various health-related issues over the years and the lack of a real explanation, his sudden exit raises more questions than answers.

“I have always said if there ever came a day when I could no longer meet my duties and expectations as Apple’s CEO, I would be the first to let you know,” Jobs wrote in an open letter. “Unfortunately, that day has come.”

“I hereby resign as CEO of Apple,” the letter continues. “I would like to serve, if the Board sees fit, as Chairman of the Board, director, and Apple employee. As far as my successor goes, I strongly recommend that we execute our succession plan and name Tim Cook as CEO of Apple.”

Apple has done just that. In a separate press release, the Cupertino electronics giant announced that Tim Cook, who previously served as Apple’s Chief Operating Officer, was the new CEO. Likewise, Jobs was elected Chairman of the Board.

Cook is an excellent and even obvious choice to succeed Jobs. But the bigger concern, of course, is for Jobs himself. His increasingly wan appearance and a spate of health issues since a 2004 cancer scare has triggered worldwide concerns about his future. He’s taken two leaves of absence from Apple since then, most recently in January, and has always been very secretive about the reasons.

Steve Jobs’ contributions to the tech industry are, of course, innumerable and he will be remembered throughout history as a truly unique visionary, akin to such luminaries as Thomas Edison and Albert Einstein. Shorter term, he leaves Apple in incredibly good shape, dominating the PC, smartphone, and electronic-devices markets in ways that were unimaginable when he returned to the company in 1996.