Solar radiation

Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum. When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause a change in the skin color of humans.

Solar radiation is commonly measured with a pyranometer or pyrheliometer.

Solar constant
The solar constant is the amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth's atmosphere, in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1366 watts per square meter, though it fluctuates by about 6.9% during a year - from 1412 W/m2 in early January to 1321 W/m2 in early July, due to the earth's varying distance from the sun, and by a few parts per thousand from day to day. Thus, for the whole Earth, with a cross section of 127,400,000 km², the power is 1.740×1017 W, plus or minus 3.5%. The solar constant is not quite constant over long time periods either; see solar variation. The value 1366 W/m2 is equivalent to 1.96 calories per minute per square centimeter, which can also be expressed as 1.96 langleys (or Ly) per minute.

The Earth receives a total amount of radiation determined by its cross section (π R2), but as the planet rotates this energy is distributed across the entire surface area (4 π R2). Hence, the average incoming solar radiation (called sometimes the solar irradiance), taking into account the half of the planet not receiving any solar radiation at all, is one fourth the solar constant or ~342 W/m². At any given location and time, the amount received at the surface depends on the state of the atmosphere and the latitude.

The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light. (See electromagnetic spectrum for more details) It is linked to the apparent magnitude of the Sun, &minus;26.8, in that the solar constant and the magnitude of the sun are two methods of describing the apparent brightness of the Sun, though the magnitude only measures the visual output of the Sun.

In 1884 Samuel Pierpont Langley attempted to estimate the solar constant from Mount Whitney in California, and (by taking readings at different times of day) attempted to remove atmospheric absorption effects. However he obtained the incorrect value of 2903 W/m2, perhaps due to mathematical errors. Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1322 and 1465 W/m2. Abbott proved that one of Langley's corrections was erroneously applied, and his results varied between 1.89 and 2.22 calories (1318 to 1548 W/m2), and the variation appeared to be solar, not terrestrial.

The angular diameter of Earth seen from the sun is ca. 1/11,000 radian, so the solid angle of Earth seen from the sun is ca. 1/140,000,000 steradian. Thus, the sun emits about 2 billion times the amount of radiation that is caught by Earth, or about 3.86&times;1026 watts.

Climate effect of solar radiation
On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright yellow light (sunlight in the strict sense) and heat. The heat on the body, on objects, etc., that is directly produced by the radiation should be distinguished from the increase in air temperature.

The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The 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%). The total insolation remains almost constant but the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies. For example, at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more than 25% as a result of the 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 ages (see: Milankovitch cycles).