What Is The Google Dance?

Whenever we are at trade shows, run seminars, or speak at symposiums we get asked the question "what is the Google dance?" We've heard a few different things referred to as "the Google Dance", but only one is really correct. It's the period when Google is rebuilding its rankings, and results fluctuate widely for a 3 to 5 day period.
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How Often Does The Google Dance Happen?

The name "Google Dance" was in the past used to describe the period that a major index update of the Google search engine are being implemented. These major Google index update occured on average every 36 days or 10 times per year. It was easiest be identified by significant changes in search results, and by an updating of Google's cache of all indexed pages. These changes would be evident from one minute to the next. But the update did not proceed as a switch from one index to another like the flip of a switch. In fact, it took several days to finish the complete update of the index.
Because Google, like every other search engine, depends on their customers knowing that they deliver authoritative reliable results 24 hours of the day, seven days a week, updates pose a serious issue. They can not shut down for maintenance and they cannot afford to go offline for even one minute. Hence, we had the Dance. Every search engine goes through it, some more or less often than Google. However, it is only because of Google's reach that we pay attention to its rebuild more than that of any other engine.
Since August 2003, the famous / infamous Google Dance is no more. Or rather it has become less dramatic. Google now performs updates every week, with most movement occuring on Mondays. These ongoing updates feature mostly minor algorithm and index updates.
So, during any month there will be minor changes in rankings. This is because Google's bot or spider is always running and finding new material. It also happens because the bot may have detected that a website no longer exists, and needs to be deleted from the index. During the Dance, the Googlebot will revisit every website, figure out how many sites link to it, and how many it links out to, and how valuable these links are.
Because Google is constantly crawling and updated selected pages, their search results will vary slightly over the course of the month. However, it is only during the Google Dance that these results can swing wildly. You also need to consider that Google has multiple data centers, sharing more than 10,000 servers. Somehow, the updates to the index that occur during the month, and outside of the Google Dance have to get transferred throughout. It's a constant process for Google, and every other search engine. These ongoing, incremental updates only affect parts of the index at any one time.