Madison Square Park's newest art exhibit is a 30-foot tall lighted orb called "Bucky Ball". It pulsates with changing colors and is very pretty! It's in the SW corner of the park, the entrance nearest to the Flatiron Building, and will be up through February 1, 2013.
I just put some Bucky Ball photos up on Flickr...I updated this page with ~40 NYC autumn photos taken in recent weeks:
I highly recommend stopping by at dusk or after dark - it's a great installation!