Editing Solar constant (section)

==Past variations in solar irradiance==
Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies with the 11-year sunspot [[solar cycle]].
When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years.
Such reconstructions show that solar irradiance varies with distinct periodicities. These cycles are: 11 years (Schwabe), 88 years (Gleisberg cycle), 208 years (DeVries cycle) and 1,000 years (Eddy cycle).<ref>{{cite journal |last1= Wang |display-authors= etal |year= 2005 |title= Modeling the Sun's Magnetic Field and Irradiance since 1713|journal= The Astrophysical Journal |volume= 625 |issue= 1| pages= 522–538 |doi= 10.1086/429689 |bibcode=2005ApJ...625..522W|doi-access= free }}</ref><ref>Steinhilber et al. (2009), Geophysical Research Letters, Volume 36, L19704, {{doi|10.1051/0004-6361/200811446}}</ref><ref>{{cite journal |last1= Vieira |display-authors= etal |year= 2011 |title= Evolution of the solar irradiance during the Holocene|journal= Astronomy & Astrophysics |volume= 531 |page= A6 |doi= 10.1051/0004-6361/201015843 |bibcode=2011A&A...531A...6V|arxiv= 1103.4958|s2cid= 119190565 }}</ref><ref>{{cite journal |last1= Steinhilber |display-authors= etal |year= 2012 |title= 9,400 years of cosmic radiation and solar activity from ice cores and tree rings| journal= Proceedings of the National Academy of Sciences |volume= 109| issue= 16|pages= 5967–5971| doi= 10.1073/pnas.1118965109 |pmid=22474348 |pmc=3341045|bibcode= 2012PNAS..109.5967S|doi-access= free }}</ref><ref>{{cite journal |last1= Vieira |first1= L. E. A. |last2= Norton |first2= A. |last3= Kretzschmar |first3= M. |last4= Schmidt |first4= G. A. |last5= Cheung |first5= M. C. M. |year= 2012 |title= How the inclination of Earth's orbit affects incoming solar irradiance |url= https://hal-insu.archives-ouvertes.fr/insu-01179873/file/grl29502.pdf|journal= Geophys. Res. Lett. |volume= 39 |issue= 16|page= L16104 |doi= 10.1029/2012GL052950 |bibcode=2012GeoRL..3916104V|doi-access= free }}</ref>

Over billions of years, the Sun is gradually expanding, and emitting more energy from the resultant larger surface area. The unsolved question of how to account for the clear geological evidence of liquid water on the Earth billions of years ago, at a time when the sun's luminosity was only 70% of its current value, is known as the [[faint young Sun paradox]].