Re: [magnitude6] exact answer on shielding and skyglow

[Jan Hollan <jhollan@amper....muni.cz>]

Hi Fabio,

I supposed the ``brave new idea'' is not new at all... It's so obvious.
I'm very happy you have a measurement like that.

I'd say there is no problem at all. We don't need to demonstrate the exact
amount of the ``snow signal'', your example (where the contribution of
snow is hardly perceivable), is completely satisfactory. It shows, beyond
any doubt, that the vast majority of skyglow is not produced by the lit
terrain. This is what we need to prove, and you have proven it.
Congratulations!

From your measurement, even confidence intervals for the relative
influence of non-terrain light could be given.

The only thing which is needed, is showing the dependence of sky luminance
at Observatory of San Benedetto Po on the extinction coefficient. It is
surely different than in my example: with turbid air, most light from
distant settlements disperses up or down before it reaches the air above
the observatory. Of course, you can model it for the site with
Pierantonio, as you have done earlier. But the most persuasive way would
be simply to show the observed dependence... Then there would be an
expected sky luminance (for case of no snow) and the observed one (with
snow). If there would be no difference, it would be possible to say:
  ``All sky luminance over the observatory is caused by direct light, or
at least ...per cent of it (90?), on a 0.95 confidence level.''

It is not so very important if the contribution of direct light is 0.9,
0.8 or 0.7. It's enough to prove, for the start, that it is the majority
contribution. That we could have much better sky when it would be avoided.
So, please publish the measurement you have!

I suspect that even James Benya and another people preparing the IDA's
Model Outdoor Lighting Code simply don't know that. They believe lighting
engineers, who (almost) all say that the influence of direct light from
current luminaires (on increased skyglow, which they mostly consider to be
just another name for light pollution, neglecting all the other influences
of light-at-night) is minor, or even negligible.  Otherwise they would not
allow such a nonsense as ``partial shielding'' (it's really there, e.g.
for bulbs below 100 W in towns, look at the JB's presentation, I checked
today!) for lamps which are still very strong.

jenik

PS. in my country, not many streets are cleaned (strange word, snow is
no pollutant, unlike light) to dark asphalt soon after the snow storm...
the snow sky signal could be larger.

> will raise to a 19.8 one (a 20% increase, to a 7.2 times the natural one),
> the rise will be from the 50% of the flux falling on snow covered regions
> off the streets. That means that the 1.2 more luminance (in unit of natural
> sky luminance: NSL) is due by the snow. The contribute from the lighted
> surfaces with no snow will be 1.2/(4-1)=0.4 NSL. So, of the total 6 NSL, 5.6
> will be from direct lights, 0.4 from reflected. This will be valid only for
> that specific site.
>
> I made such a measure from the Observatory of San Benedetto Po on December
> 15, 2001. The site is in the very urbanized Padana valley, but it is far
> from mayor cities. I found a 20.08 mag/arcsec^2 sky. The average value of
> that site was 20.03 +- 0.06. The contribute from the snow is covered by the
> errors. Unfortunately the extinction coefficient was 0.65 mag/airmass, when
> usually it was between 0.25 and 0.4.
>
> Your idea needs to be studied in depth.
>
> Fabio