Editing Light pollution (section)

=== Issues to measuring light pollution ===
Measuring the effect of sky glow on a global scale is a complex procedure.<ref>{{Cite journal |last1=Matsumoto |first1=T. |last2=Tsumura |first2=K. |last3=Matsuoka |first3=Y. |last4=Pyo |first4=J. |date=2018-08-09 |title=Zodiacal Light Beyond Earth Orbit Observed with Pioneer 10 |journal=The Astronomical Journal |volume=156 |issue=3 |pages=86 |doi=10.3847/1538-3881/aad0f0 |doi-access=free |arxiv=1808.03759 |bibcode=2018AJ....156...86M |issn=0004-6256}}</ref> The natural atmosphere is not completely dark, even in the absence of terrestrial sources of light and illumination from the Moon. This is caused by two main sources: ''airglow'' and ''scattered light''.

At high altitudes, primarily above the [[mesosphere]], there is enough UV radiation from the sun at very short wavelengths to cause [[ionization]]. When the ions collide with electrically neutral particles they recombine and emit photons in the process, causing [[airglow]]. The degree of ionization is sufficiently large to allow a constant emission of radiation even during the night when the upper atmosphere is in the Earth's shadow. Lower in the atmosphere all the solar photons with energies above the ionization potential of N<sub>2</sub> and O<sub>2</sub> have already been absorbed by the higher layers and thus no appreciable ionization occurs.

Apart from emitting light, the sky also scatters incoming light, primarily from distant stars and the [[Milky Way]], but also the [[zodiacal light]], sunlight that is reflected and backscattered from interplanetary dust particles. <ref>{{Cite journal |last1=Carleton |first1=Timothy |last2=Windhorst |first2=Rogier A. |last3=O’Brien |first3=Rosalia |last4=Cohen |first4=Seth H. |last5=Carter |first5=Delondrae |last6=Jansen |first6=Rolf |last7=Tompkins |first7=Scott |last8=Arendt |first8=Richard G. |last9=Caddy |first9=Sarah |last10=Grogin |first10=Norman |last11=Kenyon |first11=Scott J. |last12=Koekemoer |first12=Anton |last13=MacKenty |first13=John |last14=Casertano |first14=Stefano |last15=Davies |first15=Luke J. M. |date=2022-10-04 |title=SKYSURF: Constraints on Zodiacal Light and Extragalactic Background Light through Panchromatic HST All-sky Surface-brightness Measurements: II. First Limits on Diffuse Light at 1.25, 1.4, and 1.6 μm |journal=The Astronomical Journal |volume=164 |issue=5 |pages=170 |doi=10.3847/1538-3881/ac8d02 |doi-access=free |arxiv=2205.06347 |bibcode=2022AJ....164..170C |issn=0004-6256}}</ref>

The amount of airglow and zodiacal light is quite varied (depending, amongst other things on sunspot activity and the [[Solar cycle]]) but given optimal conditions, the darkest possible sky has a brightness of about 22 magnitude/square arc second. If a full moon is present, the [[sky brightness]] increases to about 18 magnitude/sq. arcsecond depending on local atmospheric transparency, 40 times brighter than the darkest sky. In densely populated areas a sky brightness of 17 magnitude/sq. an arcsecond is not uncommon, or as much as 100 times brighter than is natural.