This subject is discussed in the popular astronomy literature[1] but only a few rigorous studies[2][citation needed] have quantified the effect.
[6] Averted vision works because there are virtually no rods (cells which detect dim light in black and white) in the fovea: a small area in the center of the eye.
The fovea contains primarily cone cells, which serve as bright light and color detectors and are not as useful during the night.
Based on the early work of Osterberg (1935), and later confirmed by modern adaptive optics,[7] the density of the rod cells usually reaches a maximum around 20 degrees off the center of vision.
Some researchers have contested the claim that averted vision is due solely to rod cell density, because the peak sensitivity to stars is not at 20 degrees.