Editing Solar constant (section)

===Solar irradiance===
{{Main|Solar irradiance}}
{{See also|Earth's energy budget}}
The actual direct solar irradiance at the top of the atmosphere fluctuates by about 6.9% during a year (from 1.412&nbsp;kW/m<sup>2</sup> in early January to 1.321&nbsp;kW/m<sup>2</sup> in early July) due to the Earth's varying distance from the Sun, and typically by much less than 0.1% from day to day. Thus, for the whole [[Earth]] (which has a [[cross section (geometry)|cross section]] of 127,400,000&nbsp;km<sup>2</sup>), the power is 1.730×10<sup>17</sup>&nbsp;[[watt|W]] (or 173,000 [[terawatt]]s),<ref>{{cite book|isbn=978-0-470-94341-0|author=Archer, D.|date=2012|title=Global Warming: Understanding the Forecast|publisher=John Wiley & Sons }}</ref> plus or minus 3.5% (half the approximately 6.9% annual range). The solar constant does not remain constant over long periods of time (see [[Solar variation]]), but over a year the solar constant varies much less than the solar irradiance measured at the top of the atmosphere. This is because the solar constant is evaluated at a fixed distance of 1 [[Astronomical Unit]] (au) while the solar irradiance will be affected by the [[Orbital eccentricity|eccentricity]] of the Earth's orbit. Its distance to the Sun varies annually between 147.1·10<sup>6</sup> km at [[perihelion]] and 152.1·10<sup>6</sup>&nbsp;km at [[aphelion]]. In addition, several long term (tens to hundreds of millennia) cycles of subtle variation in the Earth's orbit ([[Milankovich cycles]]) affect the solar irradiance and insolation (but not the solar constant).

The Earth receives a total amount of radiation determined by its cross section (π·R<sub>E</sub><sup>2</sup>), but as it rotates this energy is distributed across the entire [[surface area]] (4·π·R<sub>E</sub><sup>2</sup>). Hence the average incoming solar radiation, taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar radiation, is one-fourth the solar constant (approximately 340&nbsp;W/m<sup>2</sup>). The amount reaching the Earth's surface (as [[insolation]]) is further reduced by atmospheric attenuation, which varies. At any given moment, the amount of solar radiation received at a location on the Earth's surface depends on the state of the atmosphere, the location's [[latitude]], and the time of day.