This lesson extends Lesson 4-2, which was about angles and stuff.  Here we talk about radian angle measures and why there used to be only 360 days in a year.

Angles, Degrees, and Radians, Part 1  

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Angles, Degrees, and Radians, Part 2

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Angles, Degrees, and Radians, Part 3

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Cosmology in Crisis?

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Geometry 4(A) Distinguish between undefined terms, definitions, postulates, conjectures, and theorems

Geometry 12(D) describe radian measure of an angle as the ratio of the length of an arc intercepted by a central angle and the radius of the circle