Editing Solar constant (section)

{{short description|Intensity of sunlight or solar constant}}

[[File:Solar irradiance spectrum 1992.gif|thumb|Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against [[wavenumber]].]]

The '''solar constant''' ('''''G<sub>SC</sub>''''') measures the amount of energy received by a given area one astronomical unit away from the Sun. More specifically, it is a [[flux density]] measuring mean [[Sun|solar]] [[electromagnetic radiation]] ([[total solar irradiance]]) per unit area. It is measured on a surface perpendicular to the rays, one [[astronomical unit|astronomical unit (au)]] from the Sun (roughly the distance from the Sun to the Earth).

The solar constant includes radiation over the entire [[electromagnetic spectrum]]. It is measured by satellite as being 1.361 [[Kilo-|kilo]][[watt]]s per square meter (kW/m<sup>2</sup>) at [[solar minimum]] (the time in the 11-year [[solar cycle]] when the number of [[sunspot]]s is minimal) and approximately 0.1% greater (roughly 1.362&nbsp;kW/m<sup>2</sup>) at [[solar maximum]].<ref name=KoppLean11>{{cite journal |last1= Kopp |first1= G. |last2= Lean |first2= J. L. |author2-link=Judith Lean |doi= 10.1029/2010GL045777 |title= A new, lower value of total solar irradiance: Evidence and climate significance |journal= Geophysical Research Letters |volume= 38 |issue= 1 |pages= n/a |year= 2011 |bibcode= 2011GeoRL..38.1706K|doi-access= free }}</ref>

The solar "constant" is not a [[physical constant]] in the modern [[CODATA]] scientific sense; that is, it is not like the [[Planck constant]] or the [[speed of light]] which are absolutely constant in physics. The solar constant is an average of a varying value. In the past 400 years it has varied less than 0.2 percent.<ref name=sorce>http://lasp.colorado.edu/home/sorce/data/tsi-data/ Total Solar Irradiance Data, [[SORCE]]</ref> Billions of years ago, it was [[Faint young Sun paradox|significantly lower]].

This constant is used in the calculation of [[radiation pressure]], which aids in the calculation of a force on a [[solar sail]].