IP Addresses and Domains of Google's Data Centers
The progression of a Google Dance could basically be watched by querying the IP addresses of Google's data centers. But queries on the IP addresses are normally redirected to www.google.com . However, Google has domains, which resolve to the single data centers' IP addresses.
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It's best to think of www.google.com, www2.google.com, www3.google.com and toolbarqueries.google.com as load balancing domains that can be pointed at datacentres.
www2.google.com and www3.google.com have been pointed at datacentres with the new index some time before www.google.com starts to show it. There's no reason why any of these four domains shouldn't swap between old and new indices during the update process, hence the term 'Google dance'.