Via MarketWatch, here’s what Apple shares are doing today. See if you can guess when news broke that Jobs was stepping down.

To an Apple fanboy like me, this is like finding out that Yoda’s resigned his position as a Jedi.

Jobs submitted a letter to the Apple board of directors announcing that he would leave as CEO, and the board has appointed long-time lieutenant Tim Cook to replace him. Jobs, however, will remain with Apple as the company’s chairman.

“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. Unfortunately, that day has come,” Jobs said in his letter.

He didn’t say why the day has come but he’s been on medical leave since January and has been dogged by serious illnesses for ages. Pancreatic cancer in 2004, a “hormone imbalance” in early 2009, then a liver transplant later that same year. He came back from all of it but this latest malady presumably was too much. I hope his resignation isn’t a hint of worse news to come.

He’s staying on as chairman of the board, but presumably that’s more a ceremonial role at this point. The new CEO is the guy who’s been running the company for the past seven months — Tim Cook, who joined Apple during its downturn in the 90s and helped turn it into the world’s most valuable corporation. Reuters ran a nifty short profile of him back in February emphasizing his “operations genius” and the contrast in personalities between him and the volcanic Jobs. He’s supremely well regarded and has been the heir apparent for ages so I’m not sure why the share price would nosedive this afternoon. A testament to Yoda’s cult of personality, I guess.

Here’s a quickie timeline from the NYT of Apple with and without Jobs.