The width of your thumb at arms length is a very natural unit of measure. As the article states, it is approximately 2 degrees of solid angle. If you ray trace your thumb at arms length back into your eyeballs, the image of your thumb will cover the fovea http://en.wikipedia.org/wiki/Fovea The fovea is the little dimple on your retina responsible for sharp vision. This happens to be the solid angle that is used to define the CIE color charts http://en.wikipedia.org/wiki/CIE_1931_color_space

If I am not misremembering, two degrees solid angle (your thumb at arms length) also covers the angular width of the sun and moon.

Now that's cool. This seems like one of those things they should teach recon soldiers.