- Let's take a look at an animation of graphing the sine function using the unit circle. If an angle is in standard position, the point where the terminal side of the angle intersects the unit circle will give us, both, the cosine function value and the sine function value, where the x-coordinate is the cosine function value and the y-coordinate is the sine function value. 

So we're going to animate this point around the unit circle. And when we do this, we'll plot the y-coordinate or the sine function value on the coordinate plane for the angles from 0 to 2 pi radians. So notice that 0, the sine function value is equal to 0. And then as it approaches pi over 2 radians, the sine function value approaches positive 1. So it's increasing now. 

And then at pi over 2, it reaches a value of 1. And then from pi over 2 to pi, it decreases back to 0. And then from pi to 3 pi over 2, it continues to decrease to a function value of negative 1. And then from 3 pi over 2 to 2 pi, it increases back to a function value of 0. And that's one period of the sine function. As we continue to rotate around the unit circle, this graph would repeat itself. 

Let's take a look at that one more time. So the sine function value starts at 0, increases to 1, decreases to 0, and then decreases to negative 1, and then increases back to 0. All of these sine function values came from the y-coordinates of points on the unit circle.