Earth from above (continuing ``for your amusement'')

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

> Hi Jenik,
>
> > At a 250 km distance, it would amount to (25e6 / 6e10 =) 4e-4 lx, or
> > almost -6 mag (0 mag corresponds to 2.56e-6 lx). This may be surely
>
>   Many thanks for doing that calculation in a unit that an astronomer can
> understand!
>
>   But would it stand out against the rest of the light pollution?  i.e.
> what is the background magnitude of light pollution?
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Dr Darren Baskill
> http://www.star.le.ac.uk/~dbl

Hi Das, that's another good question!

The brightest ``dot'' (in the published data, it has some 3 km size) in
the 1996/97 winter as available from the DMSP had a radiance of some
 3 kW/(km2*sr), inside Prague.

Assuming 200 lm/W for the DMSP-recorded radiation, this dot amounts to
  3 (kW/(km2*sr)) 200 (lm/W) = 600 klm/(km2*sr) = 0.6 cd/m2. It
corresponds to the luminances demanded for main roads in technical
standards (the road is never 0.25 km wide, but there is, unfortunately,
straight light from the bad luminaires as well).

From 250 km distance, a ``point'' has some 0.25 km size only (one
milliradian, 3 angular minutes), or 1/16 km2, so its brightness is some 38
kcd. In 250 km distance, it produces just 6e-7 lm/m2, or appears like a
1.5 mag star. The whole Moon-sized (angularly) area of 3 km size
would have just 1e-5 lm/m2 or -1.5 mag.

As I wrote previously, a hypothetical 1 kW source with a 5 degree beam
would produce 1e-4 lm/m2 (-4 mag), hundred times more than the strongest
points of Prague. This can be called glare indeed!

Most of the urbanised landscape has luminances hundred times less than the
most luminous 3-km dot in Prague. In another words, the average luminance
of the land in central Bohemia around Prague would be just 6 mcd/m2, like
the best London zenith night sky. But the sources (villages) are resolved,
and most of the landscape around them is illuminated just from the
polluted sky. Natural sky would illuminate the landscape just with some 1
millilux. Even an urban sky, like in Brno, gives several centilux only.

With an albedo of 15 per cent, the luminance of the landscape
(illuminance*albedo/3.14) may be just several time larger than that of the
natural zenith sky (in unlit parks in centres of cities), about 1 mcd/m2.
Any rural landscape is much darker, about 0.1 mcd/m2, twice darker than
the best zenith sky. It corresponds to the fact, that the DMSP data
register many areas as zero-signal ones (with a threshold of some 0.15
mcd/m2).

Of course this is a common knowledge that earth is mostly darker than the
sky above it. This holds if you walk on it, and partly even if you look
from the International Space Station (just because sky is darker over the
ISS... but not completely dark -- there is Milky Way and zodiac light).
From the ISS, cities, towns and villages are definitely less glaring than
fixed stars in the sky! Accidental presence of a Venus-like glaring point
changes and spoils the scene completely. And the beamers can be still much
stronger... (DMSP maps have been produced in such a way that the beamers
are not contained there).

The story that ``view of Northern Europe at night is being blinded by the
brilliant beams illuminating the Boston Stump in Lincolnshire'' is
completely plausible.

The glory of Boston Stump beamers (not of the church) can reach the Moon
(it may be a single star-like object on Earth visible from the Moon with a
pair of binoculars, being still too faint to be seen with naked eyes).
Don't you have an URL of the article?

Looking on the night landscape from hills has an advantage, that the
strongest beamers point always higher to the sky. We see mostly just the
ordinary bad lights which are not aimed specifically at us. The real
damage caused by stationary skybeamers illuminating the buildings from
below can be seen just from planes and spaceships. Or indirectly, from the
dead birds for which the confusion by them was fatal.

It's a pity that IDA never supported explicitly a Lombardy-like
legislation forbidding such neglecting architectural illumination
(including my proposal in http://www.astro.cz/darksky/eu_law).

Illuminating buildings further from below is strongly discouraged but not
completely forbidden in those three Italian provinces. The ultimate demand
is, that no light is allowed to miss the buildings and proceed to the
heavens, with a safety margin of 1 m, so that obeying the demand could be
verified by a single look at the illuminated structure. (I gave up this
margin in my extremely liberal proposal.)

Obeying such demands is a win-win strategy. Much more pleasant view of the
monuments at night for much less running costs. Of course, no artificial
illumination of buildings may be still better, in most cases. This is the
sustainable development.

cheers,
 jenik