So Sin is the y-axis on the unit circle. Why? It can't be the length of the opposite side since it can be negative values.

Sin is also defined as a function. Why? To me it seems that sin is a fixed number depending on the placement of the y-coordinate on the unit circle. So what is it that sin does to a number for it to be defined as a function? How does sin convert 30 into 0.5?

Thanks!