8/13 is an approximation of the Fibonacci or 'Golden' ratio, 1:1.6180339.... which, as I am sure you know, is a proportion that describes curves in nature, from the curvature of the roots of our teeth to the curvature of the spiral arms of the galaxies. A fourth-dimensional version of it also describes the curvature of space-time. The pentagram, which as you have pointed out above, is mapped out by the apparent movement of Venus from Earth, gives the exact Golden Ratio in the ratio of the distance between two adjacent points and the distance from one point to another. Yep, that can't be a coincidence!