It's even worse than that. As I understand it, the definition is not "a measurement of how often a user executes a search, and then goes to a webpage from the set of links" but rather "a measurement of how often a user executes a search, and then goes to a different website." That is, I believe Hitwise has no way of knowing whether the user clicks on a link from the search results vs. just navigating to a different page.

Bing also shows the answer, however: http://www.bing.com/search?q=circumference+of+the+earth&...

Perhaps Google does this better, but I would be surprised if it accounted for the large difference.

I see it in the second result from Google!

This is one of the rare queries where I'd try Wolfram Alpha first:

http://www.wolframalpha.com/input/?i=circumference+of+the+ea...