Unit Circle and Sine Curve

Change the value of angle a to discover the graphs of the sine and cosine.

What is angle a measuring?
Where do the values for the coordinates of the point on the sine graph come from? Cosine?

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When is the sine positive? Negative?
When is the cosine positive? Negative?
Are there any points on the graphs that seem especially significant?
What is the relationship between the unit circle and the trigonometric ratios?

Jenna Jerez, 26 October 2013, Created with GeoGebra