You do not have permission to edit this page, for the following reasons:

This IP address has been blocked from editing Wikipedia.
This does not affect your ability to read Wikipedia pages.
Most people who see this message have done nothing wrong. Some kinds of blocks restrict editing from specific service providers or telecom companies in response to recent abuse or vandalism, and can sometimes affect other users who are unrelated to that abuse. Review the information below for assistance if you do not believe that you have done anything wrong.

The IP address or range 3.95.0.0/16 has been blocked (disabled) by ‪Blablubbs‬ for the following reason(s):

The IP address that you are currently using has been blocked because it is believed to be a web host provider or colocation provider. To prevent abuse, web hosts and colocation providers may be blocked from editing Wikipedia.
You will not be able to edit Wikipedia using a web host or colocation provider because it hides your IP address, much like a proxy or VPN.
We recommend that you attempt to use another connection to edit. For example, if you use a proxy or VPN to connect to the internet, turn it off when editing Wikipedia. If you edit using a mobile connection, try using a Wi-Fi connection, and vice versa. If you are using a corporate internet connection, switch to a different Wi-Fi network. If you have a Wikipedia account, please log in.
If you do not have any other way to edit Wikipedia, you will need to request an IP block exemption.

How to appeal if you are confident that your connection does not use a colocation provider's IP address:
If you are confident that you are not using a web host, you may appeal this block by adding the following text on your talk page: ((unblock|reason=Caught by a colocation web host block but this host or IP is not a web host. My IP address is _______. Place any further information here. ~~~~)). You must fill in the blank with your IP address for this block to be investigated. Your IP address can be determined here. Alternatively, if you wish to keep your IP address private you can use the unblock ticket request system. There are several reasons you might be editing using the IP address of a web host or colocation provider (such as if you are using VPN software or a business network); please use this method of appeal only if you think your IP address is in fact not a web host or colocation provider.

Administrators: The IP block exemption user right should only be applied to allow users to edit using web host in exceptional circumstances, and requests should usually be directed to the functionaries team via email. If you intend to give the IPBE user right, a CheckUser needs to take a look at the account. This can be requested most easily at SPI Quick Checkuser Requests. Unblocking an IP or IP range with this template is highly discouraged without at least contacting the blocking administrator.

This block will expire on 08:49, 2 November 2024.

Even when blocked, you will usually still be able to edit your user talk page, as well as email administrators and other editors.

For information on how to proceed, please read the FAQ for blocked users and the guideline on block appeals. The guide to appealing blocks may also be helpful.

Other useful links: Blocking policy · Help:I have been blocked
Your IP address is in a range that has been blocked on all Wikimedia Foundation wikis. The block was made by ‪Jon Kolbert‬. The reason given is Open proxy/Webhost: See the help page if you are affected .
- Start of block: 05:35, 21 February 2024
- Expiry of block: 05:35, 21 February 2029
Your current IP address is 3.95.186.134. The blocked range is 3.95.0.0/16. Please include all above details in any queries you make. If you believe you were blocked by mistake, you can find additional information and instructions in the No open proxies global policy. Otherwise, to discuss the block please post a request for review on Meta-Wiki. You could also send an email to the stewards VRT queue at stewards@wikimedia.org including all above details.

You can view and copy the source of this page:

((short description|Visual perception of the sky and how it scatters and diffuses light.))
((Hatnote|This article is primarily about the brightness of the [[night sky|'''night''' sky]]. For daytime situations, see [[diffuse sky radiation]] and [[Rayleigh scattering]].))
((Refimprove|date=January 2017))
[[File:Orion1 big.jpg|thumb|300px|right|Airglow made visible from aboard the [[International Space Station|ISS]]]]

'''Sky brightness''' refers to the [[visual perception]] of the [[sky]] and how it [[light scattering|scatters]] and [[diffuse sky radiation|diffuses]] light.  The fact that the sky is not completely dark at [[night]] is easily visible.  If light sources (e.g. the [[moonlight|Moon]] and [[light pollution]]) were removed from the [[night sky]], only direct [[starlight]] would be visible. 

The sky's brightness varies greatly over the day, and the primary cause differs as well. During [[daytime]], when the [[Sun]] is above the horizon, the [[Rayleigh scattering|direct scattering]] of [[sunlight]] is the overwhelmingly dominant source of light. During [[twilight]] (the duration after [[sunset]] or before [[sunrise]] until or since, respectively, the full darkness of night), the situation is more complicated, and a further differentiation is required.

Twilight (both [[dusk]] and [[dawn]]) is divided into three 6° segments that mark the Sun's position below the horizon. At [[civil twilight]], the center of the Sun's disk appears to be between 1/4° and 6° below the horizon. At [[nautical twilight]], the Sun's [[horizontal coordinate system|altitude]] is between –6° and –12°. At [[astronomical twilight]], the Sun is between –12° and –18°. When the Sun's depth is more than 18°, the sky generally attains its maximum darkness.

Sources of the night sky's intrinsic brightness include [[airglow]], indirect scattering of [[sunlight]], scattering of [[starlight]], and [[skyglow|light pollution]].<ref>((cite web |author=F. Patat |url=http://www.eso.org/~fpatat/science/skybright/ |title=The Brightness of the Night Sky |publisher=[[European Southern Observatory|ESO]] |accessdate=2015-11-27))</ref>

==Airglow==
((Main|Airglow))
When physicist [[Anders Ångström]] examined the spectrum of the [[aurora borealis]], he discovered that even on nights when the aurora was absent, its characteristic green line was still present. It was not until the 1920s that scientists were beginning to identify and understand the [[emission line]]s in aurorae and of the sky itself, and what was causing them. The green line Angstrom observed is in fact an emission line with a wavelength of 557.7&nbsp;nm, caused by the recombination of oxygen in the upper atmosphere.

Airglow is the collective name of the various processes in the upper atmosphere that result in the emission of photons, with the driving force being primarily [[ultraviolet|UV radiation]] from the Sun. Several emission lines are dominant: a green line from oxygen at 557.7&nbsp;nm, a yellow doublet from sodium at 589.0 and 589.6&nbsp;nm, and red lines from oxygen at 630.0 and 636.4&nbsp;nm.

The sodium emissions come from a thin [[sodium layer]] approximately 10&nbsp;km thick at an altitude of 90–100&nbsp;km, above the [[mesopause]] and in the D-layer of the [[ionosphere]]. The red oxygen lines originate at altitudes of about 300&nbsp;km, in the F-layer. The green oxygen emissions are more spatially distributed. How sodium gets to mesospheric heights is not yet well understood, but it is believed to be a combination of upward transport of [[sea salt]] and [[meteor]]itic dust.

In daytime, sodium and red oxygen emissions are dominant and roughly 1,000 times as bright as nighttime emissions because in daytime, the upper atmosphere is fully exposed to solar UV radiation. The effect is however not noticeable to the human eye, since the glare of [[Rayleigh scattering|directly scattered]] sunlight outshines and obscures it.

==Indirect scattering of sunlight==
[[Image:Illuminated-arimass.png|250px|thumb|Amount of air still illuminated after sunset, at the horizon. Normalized so that zenith is 1 airmass]]
Indirectly scattered sunlight comes from two directions. From the atmosphere itself, and from outer space. In the first case, the Sun has just set but still illuminates the upper atmosphere directly. Because the amount of scattered sunlight is proportional to the number of scatterers (i.e. air molecules) in the line of sight, the intensity of this light decreases rapidly as the Sun drops further below the horizon and illuminates less of the atmosphere.

When the Sun's altitude is < -6° 99% of the atmosphere in [[zenith]] is in the Earth's shadow and second order scattering takes over. At the horizon, however, 35% of the atmosphere along the line of sight is still directly illuminated, and continues to be until the sun reaches -12°. From -12° to -18° only the uppermost parts of the atmosphere along the horizon, directly above the spot where the Sun is, is still illuminated. After that, all direct illumination ceases and astronomical darkness sets in.

A second source sunlight is the [[zodiacal light]], which is caused by reflection and scattering of sunlight on interplanetary dust. Zodiacal light varies quite a lot in intensity depending on the position of the Earth, location of the observer, time of year, and composition and distribution of the reflecting dust.

==Scattered light from extraterrestrial sources==
Not only sunlight is scattered by the molecules in the air. Starlight and the diffuse light of the [[Milky Way]] are also scattered by the air, and it is found that stars up to [[Apparent magnitude|V magnitude]] 16 contribute to the diffuse scattered starlight.

Other sources such as galaxies and nebulae don't contribute significantly.

The total brightness of all the stars was first measured by Burns in 1899, with a calculated result that the total brightness reaching earth was equivalent to that of 2,000 first-magnitude stars <ref>Burns, G. J., "The total amount of starlight and the brightness of the sky," ''The Observatory, Vol. 33'', p. 123-129, March 1910; available at [http://adsabs.harvard.edu/full/1910Obs....33..123B SAO/NASA Astrophysics Data System] (retrieved 27 Nov. 2015)</ref> with subsequent measurements by others.<ref>Yntema, L., "On the Brightness of the Sky and Total Amount of Starlight," ''Publications of the Kapteyn Astronomical Laboratory Groningen, vol. 22'', pp.1-55 (1909); available at [http://adsabs.harvard.edu/full/1909PGro...22....1Y SAO/NASA Astrophysics Data System] (retrieved 27 Nov. 2015)</ref>

==Light pollution==
((Main|Light pollution))
Light pollution is an ever-increasing source of sky brightness in [[Urban area|urbanized areas]]. In densely populated areas that do not have stringent light pollution control, the entire night sky is regularly 5 to 50 times brighter than it would be if all lights were switched off, and very often the influence of light pollution is far greater than natural sources (including moonlight). With [[urbanization]] and light pollution, one third of humanity, and the majority of those in developed countries, cannot see the [[Milky Way]].<ref>((Cite web|url=https://www.theguardian.com/science/2016/jun/10/milky-way-no-longer-visible-to-one-third-of-humanity-light-pollution|title=Milky Way no longer visible to one third of humanity, light pollution atlas shows|last=Davis|first=Nicola|date=2016-06-10|website=the Guardian|access-date=2016-07-11))</ref>

==Twilight==
When the Sun has just set, the brightness of the sky decreases rapidly, thereby enabling the viewing of the airglow that is caused from such high altitudes that they are still fully sunlit until the Sun drops more than about 12° below the horizon. During this time, yellow emissions from the sodium layer and red emissions from the 630&nbsp;nm oxygen lines are dominant, and contribute to the purplish color sometimes seen during civil and nautical twilight.

After the Sun has also set for these altitudes at the end of nautical twilight, the intensity of light emanating from earlier mentioned lines decreases, until the oxygen-green remains as the dominant source.

When astronomical darkness has set in, the green 557.7&nbsp;nm oxygen line is dominant, and atmospheric scattering of starlight occurs.

Differential refraction causes different parts of the spectrum to dominate, producing a [[Golden hour (photography)|golden hour]] and a [[blue hour]].

==Relative contributions==
The following table gives the relative and absolute contributions to night sky brightness at zenith on a perfectly dark night at middle latitudes without moonlight and in the absence of any [[light pollution]].

{| class="wikitable" style="text-align:center;border-collapse:collapse;" cellpadding="2"
|+ '''Night sky brightness'''
|-
! style="background:#efefef;" | Cause
! style="background:#efefef;" | Surface brightness [S<sub>10</sub>]
! style="background:#efefef;" | Percentage
|- 
| [[Airglow]]
| 145
| 65
|-
| [[Zodiacal light]]
| 60
| 27
|-
| Scattered [[starlight]]
| ~15
| 7
|}

(The S<sub>10</sub> unit is defined as the surface brightness of a star whose V-magnitude is 10 and whose light is smeared over one square degree, or 27.78 mag arcsec<sup>−2</sup>.)

The total sky brightness in zenith is therefore ~220 S<sub>10</sub> or 21.9 mag/arcsec² in the V-band. Note that the contributions from Airglow and Zodiacal light vary with the time of year, the solar cycle, and the observer's latitude roughly as follows:

:<math> {\rm Airglow}/{\rm S}_{10} = 145+108(S-0.8)</math>
where ''S'' is the solar 10.7&nbsp;cm flux in MJy, and various sinusoidally between 0.8 and 2.0 with the 11-year solar cycle, yielding an upper contribution of ~270 S<sub>10</sub> at solar maximum.

The intensity of zodiacal light depends on the [[Ecliptic coordinate system|ecliptic latitude and longitude]] of the point in the sky being observed relative to that of the Sun. At ecliptic longitudes differing from the Sun's by > 90 degrees, the relation is 
:<math> {\rm ZodiacalLight}/{\rm S}_{10} = 140 - 90\sin(|\beta|)</math> 
where ''β'' is the ecliptic latitude and is smaller than 60°, when larger than 60 degrees the contribution is that given in the table. Along the ecliptic plane there are enhancements in the zodiacal light where it is much brighter near the Sun and with a secondary maximum opposite the Sun at 180 degrees longitude (the [[gegenschein]]).

In extreme cases natural zenith sky brightness can be as high as ~21.0 mag/arcsec², roughly twice as bright as nominal conditions.

==See also==
*[[Night sky]]
*[[Airglow]]
*[[Zodiacal light]]
*[[Light pollution]]
*[[Skyglow]]

==References==
((reflist))

[[Category:Observational astronomy]]
[[Category:Atmospheric optical phenomena]]
[[Category:Space plasmas]]
[[Category:Earth phenomena]]
[[Category:Light pollution]]
[[Category:Sky]]

Pages transcluded onto the current version of this page (help):

Sky brightness (edit)
Template:Ambox (view source) (template editor protected)
Template:Cite web (view source) (protected)
Template:Find sources mainspace (view source) (template editor protected)
Template:Hatnote (view source) (template editor protected)
Template:Main (view source) (template editor protected)
Template:Main other (view source) (protected)
Template:More citations needed (view source) (template editor protected)
Template:Pagetype (view source) (protected)
Template:Refimprove (view source) (template editor protected)
Template:Reflist (view source) (protected)
Template:Reflist/styles.css (view source) (protected)
Template:SDcat (view source) (protected)
Template:Short description (view source) (protected)
Template:Short description/lowercasecheck (view source) (protected)
Module:Arguments (view source) (protected)
Module:Category handler (view source) (protected)
Module:Category handler/blacklist (view source) (protected)
Module:Category handler/config (view source) (protected)
Module:Category handler/data (view source) (protected)
Module:Category handler/shared (view source) (protected)
Module:Check for unknown parameters (view source) (protected)
Module:Citation/CS1 (view source) (protected)
Module:Citation/CS1/COinS (view source) (protected)
Module:Citation/CS1/Configuration (view source) (protected)
Module:Citation/CS1/Date validation (view source) (protected)
Module:Citation/CS1/Identifiers (view source) (protected)
Module:Citation/CS1/Utilities (view source) (protected)
Module:Citation/CS1/Whitelist (view source) (protected)
Module:Citation/CS1/styles.css (view source) (protected)
Module:Disambiguation/templates (view source) (protected)
Module:Find sources (view source) (template editor protected)
Module:Find sources/config (view source) (template editor protected)
Module:Find sources/links (view source) (template editor protected)
Module:Find sources/templates/Find sources mainspace (view source) (template editor protected)
Module:Format link (view source) (template editor protected)
Module:Hatnote (view source) (template editor protected)
Module:Hatnote/styles.css (view source) (template editor protected)
Module:Hatnote list (view source) (template editor protected)
Module:Labelled list hatnote (view source) (template editor protected)
Module:Message box (view source) (protected)
Module:Message box/ambox.css (view source) (protected)
Module:Message box/configuration (view source) (protected)
Module:Namespace detect/config (view source) (protected)
Module:Namespace detect/data (view source) (protected)
Module:Pagetype (view source) (protected)
Module:Pagetype/config (view source) (protected)
Module:Pagetype/disambiguation (view source) (protected)
Module:Pagetype/setindex (view source) (protected)
Module:Pagetype/softredirect (view source) (protected)
Module:SDcat (view source) (protected)
Module:String (view source) (protected)
Module:Unsubst (view source) (protected)
Module:Wikitext Parsing (view source) (protected)
Module:Yesno (view source) (protected)

Return to Sky brightness.

Retrieved from "https://en.wikipedia.org/wiki/Sky_brightness"