Kepler uses the construction below to first divide a line in the Divine Proportion. The areas of the square and rectangle below (on the left) are equal. In other words the larger part of the line of 2 is the mean proportional between the smaller part and the whole line. You may have to do some work on your own to prove this. The diameter is also made up of these parts. In the figure below, on the right, you see that the radius2 + decagon 2 = pentagon 2. You also see the side of the starpentagon and star decagon in this construction.