Bortle Dark Sky Scale

 
The Bortle Dark Sky Scale was developed by John Bortle "based on nearly 50 years of observing experience," to describe the amount of light pollution in a night sky. It was first published in a 2001 Sky & Telescope article

The reality behind the use of the scale is the enormous amount of artificial light pushed into the sky by human habitation, as documented on this light pollution globe. Nearly all points in the contiguous USA east of the 98° longitude and nearly all points in Euroope are wastefully bathed in photons produced by carbon consuming infrastructure.

To facilitate learning and using the scale, I've adapted Bortle's indicators of sky brightness as a table (below), including the color codes used in available light pollution maps.

For the amateur astronomer, the most robust and convenient relative measure of sky brightness is the naked eye or telescopic limiting magnitude. This is also a criterion that can be directly reported without recourse to the Bortle classification categories.

The five star charts below document stars down to visual magnitude 7.7 in sky areas 25° on a side that culminate near the zenith for continental United States observers (δ = 18° to 43°, with the exception of Lyra-Hercules and Equuleus-Deliphinus). Although these areas are not evenly spaced around the celestial sphere (to avoid the effect of Milky Way background brightness), at least one should be convenient to observe near the zenith at any time of the year.

To calculate the sky darkness using these charts, simply canvas the entire area of the chart and mark as many stars as you can recognize that are near your averted vision threshold. Do not mark stars that you can identify with direct vision or that are easy with averted vision; try to select stars near your threshold. Identify in this way at least 10 faint stars. Later, tally the number of stars that fall within each magnitude bin shown in the key at bottom left, which identifies the half magnitude steps corresponding to the Bortle categories. The prevailing sky brightness is the average magnitude of the two faintest bins marked:

SB = (t₁*m₁ + t₂*m₂) / (t₁+t₂)

where t is a tally and m is the fainter bracket magnitude that defines the magnitude interval bin. For example, at my home location I tallied 7 stars of magnitude 5.0–5.49 and 9 stars of magnitude 5.5–5.99, so:

SB = (7*5.5+9*6.0)/(7+9) = (38.5+54)/16 = 5.78 = Bortle 5 (suburban)

Your limit magnitude may differ from another observer's, but this difference in visual acuity will transfer to all other visual tasks. The Bortle scale inevitably combines differences in sky brightness and differences in individual detection capabilities.