Editing Light pollution (section)

==Measurement==

=== 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.

=== Satellite imagery measuring ===
To precisely measure how bright the sky gets, night time satellite imagery of the earth is used as raw input for the number and intensity of light sources. These are put into a physical model<ref name="Cinzano-2001">{{cite journal
 |url         = http://debora.pd.astro.it/cinzano/download/0108052.pdf
 |title       = The first world atlas of the artificial night sky brightness
 |journal     = Mon. Not. R. Astron. Soc.
 |volume      = 328
 |date        = 2001
 |pages       = 689–707
 |doi         = 10.1046/j.1365-8711.2001.04882.x
 |issue       = 3
 |arxiv       = astro-ph/0108052
 |bibcode     = 2001MNRAS.328..689C
 |archive-url  = https://web.archive.org/web/20060819051651/http://debora.pd.astro.it/cinzano/download/0108052.pdf
 |archive-date = 2006-08-19
|last1        = Cinzano
 |first1      = P.
 |last2       = Falchi
 |first2      = F.
 |last3       = Elvidge
 |first3      = C. D.
 |last4       = Baugh
 |first4      = K. E.
 |doi-access = free
 |s2cid = 15365532
 }}</ref> of scattering due to air molecules and aerosoles to calculate cumulative sky brightness. Maps that show the enhanced sky brightness have been prepared for the entire world.<ref>{{in lang|it}} [http://www.lightpollution.it/worldatlas/pages/fig1.htm The World Atlas of the Artificial Night Sky Brightness] {{Webarchive|url=https://web.archive.org/web/20100923212042/http://www.lightpollution.it/worldatlas/pages/fig1.htm |date=2010-09-23 }}. Lightpollution.it. Retrieved 2011-12-03.</ref>

=== Ground-based monitoring ===
In addition to satellite-based observations, ground-based networks of photometers have become essential for monitoring light pollution over time. One of the most widely used instruments is the [[Sky Quality Meter]] (SQM), a compact device that measures night sky brightness (NSB) in magnitudes per square arcsecond. SQMs are deployed by both professional observatories and citizen scientists worldwide, providing high temporal resolution data that complements remote sensing approaches.

Long-term SQM datasets from urban, intermediate, and rural sites have revealed measurable increases in light pollution. A 2023 study analyzing over a decade of data from 26 sites across Europe - including cities such as Stockholm, Berlin, and Vienna - found average annual increases in NSB of 1.7% in rural areas, 1.8% in urban areas, and 3.7% in intermediate areas. These trends were corrected for sensor aging using twilight calibration methods and adjusted for atmospheric factors such as albedo, vegetation cover, and aerosols through an empirical regression model.<ref>Puschnig, J., Wallner, S., Schwope, A., Näslund, M. (2023). Long-term trends of light pollution assessed from SQM measurements and an empirical atmospheric model. ''Monthly Notices of the Royal Astronomical Society'', 518(3), 4449–4465. https://doi.org/10.1093/mnras/stac3003</ref>

Ground-based studies have also shown that high levels of artificial light at night can suppress the natural circalunar pattern in sky brightness. In urban areas where the NSB exceeds 16.5 mag/arcsec², the variation associated with the moon cycle becomes nearly undetectable, potentially affecting species that rely on moonlight for behavior or navigation.<ref>Puschnig, J., Wallner, S., & Posch, T. (2020). Circalunar variations of the night sky brightness – an FFT perspective on the impact of light pollution. ''MNRAS'', 492(2), 2622–2637. https://doi.org/10.1093/mnras/stz3514</ref>

National SQM networks have been established in several countries. In Austria, the provincial government of Upper Austria operates a dense SQM network to support both astronomical and environmental research.<ref>{{Cite journal | last1 = Posch
 | first1 = T. | last2 = Binder | first2 = F. | last3 = Puschnig | first3 = J. | title = Quantitative assessment of light pollution in Upper Austria | journal = Journal of Quantitative Spectroscopy and Radiative Transfer | volume = 211
 | pages = 144–153 | date = 2018 | doi = 10.1016/j.jqsrt.2018.03.010 | arxiv = 1803.09811 | url = https://doi.org/10.1016/j.jqsrt.2018.03.010 }}</ref> In Spain, coordinated efforts by researchers including Bará and colleagues have helped quantify the relative contributions of streetlights, traffic, and residential lighting to NSB.<ref>{{Cite journal | last1 = Bará | first1 = S.
 | last2 = Lima | first2 = R. C. | last3 = Zamorano | first3 = J. | title = Monitoring Long-Term Trends in the Anthropogenic Night Sky Brightness  | journal = Sustainability | volume = 11 | issue = 11 | pages = 3070 | year = 2019 | doi = 10.3390/su11113070 | doi-access = free
 | bibcode = 2019Sust...11.3070B
 | hdl = 10316/107395 | hdl-access = free }}</ref> In Italy, SQM data have been used to monitor urban and protected areas.<ref>{{Cite journal | last1 = Bertolo | first1 = A. | last2 = Binotto | first2 = R.
 | last3 = Ortolani | first3 = S. | last4 = Sapienza | first4 = S. | title = Measurements of Night Sky Brightness in the Veneto Region of Italy: Sky Quality Meter Network Results and Differential Photometry by Digital Single Lens Reflex
 | journal = Journal of Imaging | volume = 5 | issue = 5 | pages = 56 | year = 2019 | doi = 10.3390/jimaging5050056
 | doi-access = free | pmid = 34460494 | pmc = 8320935 }}</ref> The Netherlands also maintains a national monitoring program using SQMs to track long-term trends.<ref>{{Cite web | last1 = Schmidt | first1 = T. S. | last2 = Spoelstra | first2 = H. | title = Darkness monitoring in the Netherlands 2009-2019 | date = 2020
 | url = https://zenodo.org/record/4293366 | doi = 10.5281/zenodo.4293366 | access-date = 8 May 2025}}</ref>

These ground-based networks provide continuous data under varied weather conditions and offer a crucial complement to satellite observations, especially for evaluating local lighting policies and environmental impacts.

=== Bortle scale ===
The [[Bortle scale]] is a nine-level measuring system used to track how much light pollution there is in the sky. A Bortle scale of four or less is required to see the [[Milky Way]] whilst one is "pristine", the darkest possible.<ref>{{Cite web|url=https://www.forbes.com/sites/startswithabang/2016/06/14/the-milky-way-invisible-to-most-of-us-but-accessible-to-all/|title=The Milky Way: Invisible To Most Of Us, But Accessible To All|last=Siegel|first=Ethan|date=June 14, 2016|website=[[Forbes]]|access-date=November 16, 2019|archive-date=March 11, 2021|archive-url=https://web.archive.org/web/20210311005605/https://www.forbes.com/sites/startswithabang/2016/06/14/the-milky-way-invisible-to-most-of-us-but-accessible-to-all/|url-status=live}}</ref>