Like Santini, Colorado optometrist and amateur astronomer Keith Bowen attributes night myopia mostly to instrument myopia, a hardwired psychological and physiological reflex that causes us to focus on nearby things in the absence of strong visual stimuli.
Although the wavelength difference between the peak sensitivities of retinal rod and cone cells is only a modest 50 nanometers (roughly one-tenth the wavelength of yellow light), it can produce a half diopter's worth of night myopia when combined with the eye's substantial chromatic aberration, says Donald Miller, also at Indiana University.
Schwiegerling and Williams both give the lion's share of the credit (or blame) for night myopia to the eye's substantial spherical aberration, which becomes noticeable only when pupils are dilated.
Schwiegerling estimates that an amateur astronomer whose dark-adapted pupils span 6 millimeters typically will experience a half diopter of night myopia, while the rare stargazer whose pupil can open up as wide as 8 mm may experience a 1 1/2-diopter effect--though both numbers vary widely within the human population.
Finally, consider the possibility that each of your eyes experiences a different amount of night myopia.
Proceed as a nearsighted eyeglass wearer would, using flippers to calibrate your night myopia. If you wear contact lenses by day and experience a measurable amount of night myopia with them on, you may wish to augment your contacts with night-myopia glasses rather than getting a second set of contacts--particularly if you favor extended-wear contacts, or if you are likely to have trouble reading your star charts with souped-up vision.