The magnitude of the dimmest star visible to the unaided eye is calculated from conditions specified in the form below. This number depends on the location and phase of the Moon, the location of Sun, the humidity, your location and height as well as the zenith angle of the star of interest. Try various imput parameters to observe the effects of each variable. For more details see the reference sited above.
Intermediate results are presented in the tables at the bottom. Atmospheric extinction and brightness as well as air mass are given
FROM "ASTRONOMICAL COMPUTING"
SKY & TELESCOPE, MAY 1998 page 52
'To the Visual Limits'
by Bradley E. Schaefer
Corrected with the aide of
and David Fisher