How I think the new atlas of global light pollution should be 'read'

Master table of sky brightness values and associated colors - in the atlas - and statements (in the paper and by me):

mag./sq. arcsec lim. mag. Ácd/m^2 "ratio" color statement source
22 6.6 171 0 black natural sky brightness paper page 2
Cut "14" 188 8% dark blue -> blue "polluted" for IAU sets in / used in paper as light pollution threshold paper page 2 and esp. 4 plus table 2
202 16% blue -> light blue
230 32% light blue -> dark green
Cut "87" 260 52% dark green paper table
21.5 6.4 271 58% dark green
285 64% dark green -> green
21.3 326 91% green impressive summer MW West Havelland 2014, WHAT 2014
397 128% green -> yellow
21.0 6.1 430 151% yellow no more winter NW paper page 2
20.8 517 2x yellow still quite nice to have Eifel 2015 (dito), remote Rhodes 2015
619 2.6x yellow -> orange
20.5 5.8 681 3x orange
Cut "688" 860 4x orange all Milky Way gone / used in paper as MW visibility threshold paper pages 3 and 11 and table 2
20.2 898 4.2x orange useful but not great (MW still there) suburban Rhodes 2015
20.0 5.5 1080 5x orange -> red scotopic -> metopic vision paper page 2
1400 7.2x red like nautical -> astronomical twilight paper page 2
1960 10x red -> magenta
19.0 4.8 2713 15x magenta
Cut "3000" 3170 17.5x magenta no more dark adaptation paper page 11 and table
3740 20x magenta -> pink
18.0 4.0 6814 39x pink
7300 41x pink -> white
17.0 3.1 17117 99x white


Col. 1: astronomical magnitude per square arc degree, as measured e.g. with an SQM
Col. 2: naked eye limiting magnitude, calculated with Schaefer's formula, only for rough orientation!
Col. 3: microcandela per square meter, calculated with the formula from here and here (with 171 for 22, not 174 as in the paper)
Col. 4: ratio of the excess(!) brightness to the base (here: 171) for a perfect sky
Col. 5: color of that brightness level in the atlas maps, "->" meaning the exact switch between colors
Col. 6: claims about the meaning of the respective brightness level.
Col. 7: source of that claim - either the 2016 paper or my 2015 and 2014 impressions in two Dark Sky Places in Germany and on Rhodes.

"Cut" in Col. 2 refers to the excess sky brightness cuts used in the paper in fig. 11-14 and table 2.

Conclusion: the criteria for both the onset of light pollution (relevant to an ordinary citizen enjoying a nice night sky) and the total loss of the Milky Way are too tough - based on my experience (and folding it back into the table now) significant degradation of the night sky does not set in until about 400 Ácd/m^2 are reached, i.e. an artificial brightness of ~230 or a ratio of 133%. And a seriously degraded sky does not occur until perhaps 850 Ácd/m^2 are crossed or a factor 4 in excess.

Thus my recommendation for "reading" the figures 11-14 and table 2 would be use the "87" cut as criterion for very good and - for layman's purposes - unpolluted skies and the "688" cut not as a loss-of-Milky-Way threshold which it isn't yet but as a criterion for significantly degraded but still inspiring skies. (I've seen such skies inspire people!)And colorwise everything up to and including green is fine, yellow is so-so, and only from orange onwards things begin to turn ugly. The statistics suddenly don't look that bleak anymore ...

Daniel Fischer - cosmos4u (at) web.de - on 13 June 2016