Unit Circle Manipulative

From the attached PDF: A unit circle, similar to the one above, is printed on a piece of clear plexiglass. A spindle is affixed at the center of the unit circle. A radius like the one shown below, perhaps printed in red to distinguish it from black lines on a piece of paper, is created on a piece of clear plastic, and is made in such a way that it fits on the spindle, forming a radius that can be turned 360 degrees on the plexiglass unit circle. Individuals can place the plexiglass over any geom