Editing Solar constant (section)

==Calculation==

[[Solar irradiance]] is measured by satellites above [[Earth's atmosphere]],<ref name=TSI20>{{cite web|url=http://acrim.com/TSI%20Monitoring.htm|title=Satellite observations of total solar irradiance|website=acrim.com|access-date=2010-10-02|archive-date=2011-07-16|archive-url=https://web.archive.org/web/20110716140550/http://www.acrim.com/TSI%20Monitoring.htm|url-status=dead}}</ref> and is then adjusted using the [[inverse square law]] to infer the magnitude of solar irradiance at one [[Astronomical Unit]] (au) to evaluate the solar constant.<ref>{{Cite web | url=http://www.ngdc.noaa.gov/stp/SOLAR/ftpsolarirradiance.html | archive-url=https://archive.today/20031204001103/http://www.ngdc.noaa.gov/stp/SOLAR/ftpsolarirradiance.html | url-status=dead | archive-date=December 4, 2003 | title=NOAA Status Alert}}</ref> The approximate average value cited,<ref name=KoppLean11/> 1.3608 ± 0.0005 &nbsp;kW/m<sup>2</sup>, which is 81.65 kJ/m<sup>2</sup> per minute, is equivalent to approximately 1.951 calories per minute per square centimeter, or 1.951 [[Langley (unit)|langleys]] per minute.

Solar output is nearly, but not quite, constant. Variations in [[total solar irradiance]] (TSI) were small and difficult to detect accurately with technology available before the satellite era (±2% in 1954). Total solar output is now measured as varying (over the last three 11-year [[sunspot]] cycles) by approximately 0.1%;<ref name=Willson91>{{cite journal |last=Willson |first=Richard C.|author2=H.S. Hudson|date=1991 |title=The Sun's luminosity over a complete solar cycle |journal=Nature |volume=351 |issue=6321 |pages=42–4 |doi=10.1038/351042a0 |bibcode=1991Natur.351...42W|s2cid=4273483}}<!-- {{harvnb|Willson|1991}} --></ref> see [[solar variation]] for details.

===For extrasolar planets===
<math>L=4\pi R_{\rm star}^2\sigma T_{\rm star}^4</math> <br>
<math>L=4\pi f \  d ^2</math> <br>
Therefore:  <math>4\pi f\  d ^2 = 4\pi R_{\rm star}^2\sigma T_{\rm star}^4</math> <br>
<math display="block">   f=  \left (  \frac{R_{\rm star}^2\sigma T_{\rm star}^4}{ d ^2}  \right )  </math> <br>
Where <math>\sigma</math> is the Stefan-Boltzmann constant (<math>\approx 5.67e^{-8} W m^{-2} K^{-4}</math>) and f is the irradiance of the star at the extrasolar planet at distance d.