Rotation Angle
All points on a CD travel in circular arcs. The pits along a line from the center to the edge all move through the same angle Δ in a time Δt.
Source
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"OpenStax College, College Physics. November 10, 2012."
http://cnx.org/content/m42083/latest/?collection=col11406/1.7
OpenStax CNX
CC BY 3.0.