[LAU] Where do the 60 degrees for stereo come from?

Hi there, in a discussion today someone asked me where those 60 degrees necessary for the production of phantom images come from and I couldn't deliver a satisfactory answer. Someone tried to explain to me that it has something to do with wavelengths or whatever but couldn't explain it in a way that anyone would understand. My best guess is that with a larger angle the head gets in the way and the ears have an easier time telling the signals apart. Also, I guess 60 degrees is a rough estimate and chosen because this leads to a nice Equilateral triangle. So, what's the real reason behind those 60 degrees? Regards, Philipp