Editing Sunlight (section)

==Variations in solar irradiance==

===Seasonal and orbital variation===
{{Further|Insolation|Sunshine duration}}
On Earth, the solar radiation varies with the angle of the Sun above the [[horizon]], with longer sunlight duration at high latitudes during summer, varying to no sunlight at all in winter near the pertinent pole. When the direct radiation is not blocked by clouds, it is experienced as ''sunshine''. The warming of the ground (and other objects) depends on the [[absorption (electromagnetic radiation)|absorption of the electromagnetic radiation]] in the form of [[heat]].

The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. Earth's [[orbit]] and [[obliquity]] change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an [[orbital eccentricity]] of 5% (currently 1.67%). As the orbital eccentricity changes, the average distance from the Sun (the [[semimajor axis]] does not significantly vary, and so the total [[insolation]] over a year remains almost constant due to [[Kepler's second law]],
:<math>\tfrac{2A}{r^2}dt = d\theta,</math>

where <math>A</math> is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant.

:<math>\int_{0}^{T} \tfrac{2A}{r^2}dt = \int_{0}^{2\pi} d\theta = \mathrm{constant}.</math>

If we assume the solar radiation power&nbsp;{{mvar|P}} as a constant over time and the [[solar irradiation]] given by the [[inverse-square law]], we obtain also the average insolation as a constant. However, the [[season]]al and latitudinal distribution and intensity of solar radiation received at Earth's surface does vary.<ref>{{cite web |url=http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html |title=Graph of variation of seasonal and latitudinal distribution of solar radiation |publisher=Museum.state.il.us |date=2007-08-30 |access-date=2012-02-12 |url-status=live |archive-url=https://web.archive.org/web/20120112043906/http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html |archive-date=2012-01-12 }}</ref> The [[effect of Sun angle on climate]] results in the change in solar energy in summer and winter. For example, at [[latitude]]s of 65&nbsp;degrees, this can vary by more than 25% as a result of Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent [[ice age]]s (see: [[Milankovitch cycles]]).

===Solar intensity variation===
{{Further|Solar variation}}
Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies on many time scales, including the 11-year sunspot solar cycle.<ref name="acrim"/> When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400&nbsp;years or cosmogenic radionuclides for going back&nbsp;10,000 years.
Such reconstructions have been done.<ref>{{cite journal | title = Modeling the Sun's Magnetic Field and Irradiance since 1713 | last1 = Wang | display-authors = etal   | date = 2005 | journal = The Astrophysical Journal | volume = 625 | issue = 1| pages = 522–538 | doi = 10.1086/429689 | bibcode=2005ApJ...625..522W| doi-access = free }}</ref><ref>{{cite journal | title = Total solar irradiance since 1996: is there a long-term variation unrelated to solar surface magnetic phenomena? | last1 = Steinhilber | display-authors = etal   | date = 2009 | url =https://www.dora.lib4ri.ch/eawag/islandora/object/eawag%3A6539/datastream/PDF/view | journal = Geophysical Research Letters | volume = 36 | page = L19704 | doi = 10.1051/0004-6361/200811446 | bibcode=2010A&A...523A..39S| doi-access = free }}</ref><ref>{{cite journal | title = Evolution of the solar irradiance during the Holocene | last1 = Vieira | display-authors = etal   | date = 2011 | 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 | title = 9,400 years of cosmic radiation and solar activity from ice cores and tree rings | last1 = Steinhilber | display-authors = etal   | date = 2012 | journal = Proceedings of the National Academy of Sciences | volume = 109| issue = 16| pages = 5967–5971| doi = 10.1073/pnas.1118965109 |bibcode = 2012PNAS..109.5967S | pmid=22474348 | pmc=3341045| url = http://epic.awi.de/30297/1/PNAS-2012-Steinhilber-1118965109.pdf| doi-access = free }}</ref> These studies show that in addition to the solar irradiance variation with the solar cycle (the (Schwabe) cycle), the solar activity varies with longer cycles, such as the proposed 88 year (Gleisberg cycle), 208 year (DeVries cycle) and 1,000 year (Eddy cycle).