That collective gasp just heard around the world? Steve Jobs has announced his resignation from Apple (AAPL).

The news was not unexpected, given Jobs' longstanding health issues, but it's still a shock.

The Apple chief sent an email to the world Wednesday afternoon saying he "can no longer meet his duties and expectations" as head of the world's largest company. He plans to serve as chairman, as long as the board gives its all but guaranteed approval.

Jobs didn't mention his health problems, but many people assume them to be the reason behind the resignation.

Investors didn't seem too panicked about Jobs' resignation in after-hours trading. Shares of Apple were down only about 5% to $357.49, which some took to mean that this resignation was already factored into the stock price.

Is Steve Jobs replaceable? Check out that discussion in the following video.

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"This is tragic news, not just for Apple and the Apple community but for Silicon Valley and the country," wrote Henry Blodget at The Business Insider.

Here's the full text of the email:

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.

I hereby resign as CEO of Apple. 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.

I believe Apple's brightest and most innovative days are ahead of it. And I look forward to watching and contributing to its success in a new role.

I have made some of the best friends of my life at Apple, and I thank you all for the many years of being able to work alongside you.

Many people in Silicon Valley took some time to credit Jobs for the industry-changing work he has done at the company. "Steve Jobs' run at Apple is likely the very best CEO execution we will see in our lifetime. Amazing," wrote venture capitalist Bill Gurley.

CNBC anchor Jim Cramer said he didn't think the news was a reason to sell Apple shares. "Very sad news about Steve Jobs" at Apple, Cramer wrote on Twitter. "He is America's greatest industrialist. Perhaps the greatest ever."

• Why Apple trusts Tim Cook

Business journalist Simon Caulkin was confident the company would survive. Apple has disrupted three major industries and defied convention to become one of the most valuable companies in the world, he writes. And Apple can keep doing it without Jobs. He adds this:

Unlike almost all conventional companies, Apple is thus not a standard corporate dictatorship. Although certainly casting apprehensive glances over their shoulder towards their demanding boss, Apple employees are mostly, and rightly, facing towards their customers. Apple, in short, is an ‘outside-in’ organisation rather than just a top-down one – and Jobs’ sometimes unattractive control-freakery serves to enforce this orientation. That, not its CEO's obsessive genius, is Apple’s greatest strength. In time it may be seen as Jobs’ most important and lasting legacy.

The Hedgeye newsletter admired Jobs' tenacity and dedication to his vision. "Jobs knew that democracy didn't work. He pursued a vision, people called him arrogant, and where are they now? It took time, but he was right," the newsletter editors wrote on Twitter.

The Wall Street Journal published the best Steve Jobs quotes over the years. I really like this one, given as part of a Stanford commencement speech in 2005:

"Your work is going to fill a large part of your life, and the only way to be truly satisfied is to do what you believe is great work. And the only way to do great work is to love what you do. If you haven’t found it yet, keep looking. Don’t settle. As with all matters of the heart, you’ll know when you find it."