pi/3.
http://wiki.answers.com/Q/What_is_a_60_degrees_ang...

1. Look at your ruler. One side is marked with inches, with each inch divided into a number of sections, either eighths or sixteenths. Each mark represents that fraction of an inch.
http://www.ehow.com/how_7966873_useconstructing6...

Cosine of 60 =
http://www.chacha.com/question/whatisthecosine...

It's called the inverse cosine of one half, or 'Bob' to its friends.
http://answers.yahoo.com/question/index?qid=200712...
