Sky Brightness Nomogram
With this nomogram the brightness of the night sky can be easily converted from one scale to the other. The horizontal red line shows the natural sky brightness level for an unpolluted and clear starry sky. At this natural level the Milky Way can be seen in all it’s beauty and around 6000 stars with the naked eye at the hemisphere of an observer.
The comparison between the scales can be made by drawing a horizontal line and reading the various values at this horizontal line. The more light polluted the sky is, the higher the horizontal line will be drawn.
A white text against a white background is hard to read
A white text against a black ground reads much easier.
The same is this for seeing the stars
The left scale gives the factor for a light polluted sky to the natural sky. The natural sky brightness level is around 0.25 mcd/m2. So a factor 2 at this scale means that the night sky brightness is 2 times the natural level or 0.5 mcd/m2 and at a factor 10 the night sky brightness is 10 times the natural level (or 2.5 mcd/m2). The higher this factor the lesser dim stars can be seen and details in the Milky Way disappear or the Milky Way cannot be seen at all.
The second scale is given in the well-known astronomical scale of magnitude per arc second (mag/sec2) in the visual range (V-band). This gives the brightness of the sky of one square arc second. The natural level is around 21.6 mag/sec2 at a solar activity minimum.
The third scale in mcd/m2 (millicandela per square meter) is a scale often used by lighting enigineers and represents the brightness at the maximum response of the eye at 555 nm (green-yellow). As a comparison: the natural sky brightness level of about 0.25 mcd/m2 is about one million times less bright than a computer screen of about 300 cd/m2.
The fourth scale is the often used Bortle scale from 1 to 9. With scale 1 for a excellent dark sky up to scale 9 for the sky above an inner city.
The fifth scale (second from the right) shows the magnitude of the stars that can be seen by the naked eye. It is an approximate scale because it strongly depends on the experience, age, sharpness and pupil diameter of the eye. Younger people have larger pupils and sharper eye sight and will in general see fainter stars (higher magnitudes) then older people and people with less sharper eye sight. At an excellent dark sky visual magnitudes are around 6.6—6.8 for “average” observers, but values up to magnitude 8 are reported for experienced observers.
The most right scale gives the approximate number of stars that can be seen by an observer at the observers hemisphere.
Please note that the scales are approximate and should be used only for a first estimate. For more refined and exact values the formulas from literature should be used. Design by H. Spoelstra; email: firstname.lastname@example.org.