[SQM] GaN maps; mag not PER arcsec

Jan Hollan jhollan at amper.ped.muni.cz
Sun Feb 27 12:05:38 UTC 2011

Just 2 remarks, hope they help.

> also collect SQM measurements around the world.
> http://www.globeatnight.org/
> http://www.globeatnight.org/observe_magnitude.html
> http://www.globeatnight.org/webapp/

For maps:
      the full (large) set of maps prepared for Globe at Night and 
explained as needed is at


  -- including all-sky maps, in subdirectories for appropriate dates. 
Actually, dates are not so important, the local stellar time is. So a 
Feb23 map for 21 h is OK a week later at approx. 20:30 and another week 
later at 20 h.

Luminance units:

it is a bad habit to write mag/arcsec or to say mag per arcsec. It is like 
saying db/car. Any metrologist, good physics teacher or a caring 
technician would wave a hand and think that such people speaking nonsense 
language cannot be taken seriously, as they don't understand science at 

If there are hundred cars around, producing noise amounting to 70 db, it 
would be absurd to claim that they have "a polluting power" of 0.7 db/car. 
"21 mag/arcsec" is no less absurd. The slash is a mathematical operation 
and cannot be employed here. If it could, then
               21 mag / arcsec = 42 mag / 2 arcsec
  -- the nonsense is apparent, isn't it.

The only way is to say something like that 1 arcsec has 21 mag. Then 2 
arsec have 20.25 mag, 10 arcsec have 18.5 mag, 100 arsec have 16 mag. Or, 
using my programme lum (available online as well, as
                http://amper.ped.muni.cz/jenik/astro/lum.php ):

jhollan at amper:~$ lum s 21

The equivalent of Luminance expressed by a star of faintness of
21.00 mag  defocused to "one square second"
       means a Luminance of some
                             4.30E-4 cd/m^2

and alternative equivalent stars might be faint

21.00 mag  when defocused to one "square second",
12.11 mag  when defocused to one "square minute",
  3.22 mag  when defocused to one "square degree".
  8.88 mag   defocused to a circle of diameter of 5 angular minutes
             (on the verge of being a "point" for human nightime vision)

Arcsec is so tiny that it has no bearing for human perception of the sky. 
And it's boring to carry those large numbers in mind, starting somewhere 
from 16 in cities. Tenth of a square degree would be a more reasonable 
piece of the sky -- actually, it would hint how faint stars may be 
well seen. (3.22 + 2.5 = 5.7) mag ones, for the above luminance of 
the sky of 0.43 mcd/m2. Numbers from 0 to 7 are easier to remember 
and have some idea how bright or dark the sky really is.

best regards,
  Jenik Hollan

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