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In this lesson, you will learn what radian angle measure is and how it compares to degree measure. Every circle has 360 degrees, and every circle has 2 Pi radians. The reason is because if we consider a unit circle to have a radius of one, then the circumference of a unit circle is 2 Pi distance units. We can then use the distance around the unit circle to be a new measure of angle, where if we go all the way around the unit circle we travel 2 Pi radian angle units around the circle. This is a much more natural distance unit because it is fundamental to all circles and is not arbitrarily invented or defined. It is a fundamental feature of trigonometry, geometry, and calculus.