A brief scan of the graphics on this site offers an idea of a “gyre” as a two-thousand year cycle.

http://www.yeatsvision.com/History.html#Cycles

For an idea of how many essays have been penned about Yeats’ symbolism and “The Second Coming,” this is a link to a 394-page book of essays that is quite in-depth.

http://www.clemson.edu/cedp/cudp/pubs/vision/vision_book.pdf

Lastly, look at the wikipedia entry to see an example of how wiki can be quite tepid. (Even Schmoop beats them in this particular case.)