Outdoor Lighting and Crime - Amper

A decade of satellite measurements of optical energy radiated from Earth’s populated areas
(Cinzano, Falchi and Elvidge 2001a) appears to indicate a generally exponential growth of
skyglow. Worldwide, only a few areas such as central India and some cities in Russia have
shown a reduction in upward light emissions in recent years (NASA 2000, Sutton 2002).
Satellite data for 1993 to 2000 has been analysed to determine that the total skyglow over the
UK increased in this time by 24% (CPRE 2003).
2.1.3 Skyglow, outdoor lighting and population
With a moonless clear sky and well into the dark hours, the absolute values of both natural
and artificial skyglow are dependent on geographical position and direction of observation.
For the present purpose, the exponential growth in the artificial component added to the value
of the natural component is a key issue. The luminance of the natural component varies
according to the phase of solar activity. Garstang (1989a) used 53.7 nanoLambert (nL) (171
µcd/m 2 , or 0.171 mcd/m 2 ) as the minimum, occurring at solar activity minimum, and 55 nL
for the same quantity in Garstang (2000). Cinzano (2000d) allowed a 1 stellar magnitude 11
increase over the minimum natural skyglow at solar maximum, which would give a value of
0.43 mcd/m 2 and a logarithmic mean of 0.27 mcd/m 2 . The value used by Cinzano, Falchi and
Elvidge (2001b) as typical was close to the latter. The amount of artificial skyglow in remote
rural areas can still be much smaller than the minimum natural value. In the total skyglow
over large cities, however, the natural component, even at its maximum, generally became
negligible decades ago.
Walker’s law (eg Garstang 2000; Mizon 2002, p 65) is an empirical relationship for predicting
the artificial component of sky luminance at 45º above a city with known characteristics at a
known distance. A simplified version of the law is based on the assumption that the amount
of outdoor lighting per member of the population is a constant. Astronomers such as Pike
(1976) once hoped it to be so, but time has shown that this factor has continued to increase.
Pike’s pioneering work on modelling skyglow growth in southern Ontario during the 1970s
provided indications even then that the growth was exponential. His predictions about the
Milky Way being blotted out in southern Ontario by 2000 have unfortunately proved correct.
Garstang (1991) calculated the effect of airborne dust on skyglow. Desert dust and volcanic
dust have closely comparable effects. Dust below about 10 km altitude reduces skyglow and
higher dust increases it. For the present purpose, the effect on skyglow is unimportant. The
same conclusion applies to Los Angeles smog (Garstang 2000), so the effects of dust and
smog generally can be ignored in this document.
Garstang (2000) calculated the growth of skyglow over Mount Wilson Observatory in
California from 1910 to 1990 using a set amount of light flux emission (1000 lumen) per
person from 124 cities in the Los Angeles basin. As in earlier papers, Garstang chose to
exclude effects of changes in lighting technology, not because they were unimportant but to
give a simpler basis for comparison of results. Within a large margin, the results were
11 Historically, bright stars were of first magnitude and those near the limit of unaided vision,
magnitude 6. The more modern logarithmic adaptation of this system has the five magnitude
difference set to a factor of 100 in apparent intensity, so that each stellar magnitude represents
a change in intensity of the fifth root of 100, about 2.512.
8