The Google dance explained

The results of a particular search on Google highly fluctuate when Google Shuffles the rankings of websites in its search engine results pages (SERPs). This concept is termed as dance. During the dance, the results for a particular search on Google vary minute to minute. The index update at Google is changed at an interval of about a month. Though the outcome of this change in the index is instantly reflected on the SERPs, the actual process takes several days to complete. It is not viable for Google to shut down operations or go offline till the updated index is transferred to all their servers. Hence the results for a particular search fluctuate or ‘dance’ till the index update is transferred to all Google servers.

It has been observed that Google has three main servers – www.google.com, www2.google.com, and www3.google.com. During an index update, the results displayed by these three servers vary to a large extent. This happens because one server reflects the updated index while the other is still showing the results of the old index.
An easy way of determining whether the dance is on is to make a search request on www.google.com and look at the blue bar on top of the page. It shows ‘pages 1-12 of about 564,000’. Then search for the same on www2.google.com and www3.google.com. If the number of total pages differs in each server, you know that the dance is on. You know the dance is over when the numbers do not vary and the order of the results is the same.
Google’s spider constantly visits websites to check their validity and to delete non existent websites. It is, therefore, common to see nominal changes in the results of a search over a month or so. But, if there is an alarming variation in the ranking of a website, or a major alteration in the results within a few days, it can be guessed that the Google dance is on.