A few hours ago I posted about how the Google rankings for this blog had dipped suddenly lower — if you searched for Clive Thompson, you found Collision Detection only on the second page. Waaah!

But then some posters quickly noted that my blog was, in fact, appearing at different places on Google — it was sort of moving around, flipping from high on the first page of results to low on the second page. What was going on?

In a posting on my discussion boards, Bud pointed to one possible explanation: The Google Dance. A bunch of geeks at the Germany firm efactory.de use that name to describe the effect, which they outline in a really fascinating essay on their site:

The name “Google Dance” is often used to describe the index update of the Google search engine. Google’s index update occurs on average once per month. It can be identified by significant movement in search results and especially by Google’s cache of all indexed pages reflecting the status of Google’s last spidering. But the update does not proceed as a switch from one index to another at one point in time. In fact, it takes several days to complete the index update. During this period, the old and the new index alternate on www.google.com. At an early stage, the results from the new index occur sporadically. But later on, they appear more frequently. Google dances.

What I didn’t know is that when Google updates its indexes, before it releases them on the public it demos them on a couple of “test” domains available to a group of webmasters — www2.google.com and www3.google.com:

As soon as Google’s test community of forums members does not find any severe malfunctions caused by the new index, Google’s DNS records are ready to make www.google.com resolve the the data center that is updated first. This is the time when the Google Dance begins.

These efactory.de guys are insanely smart. They’ve also written the coolest exploration I’ve ever read of Google’s Pagerank technology — the main genius that makes Google rock.