Intensity measurements of the two lines of the sodium nightglow, at 589.0 and 589.6 nm, show that their ratio is not constant. It is generally assumed that this ratio is fixed, with a value of I(589.0)/ I(589.6) = 2.0. In measurements made at a variety of sites, most often utilizing the echelle spectrographs at large telescopes, it is demonstrated that the ratio typically varies within the 1.3-1.8 range, with 1.6 being a common value. Because the nightglow emission is relatively strong, individual measurements are quite precise. Both the intensity ratio and the summed intensity fluctuate with a semi- annual oscillation. Laboratory spectra of the ratio show a similar variability of values, and the current hypothesis is that the ratio reflects the [O(3P)]/[O2] ratio of the environment, in the laboratory or in the mesosphere.