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Solar constant
The solar constant is the amount of the Sun's incoming electromagnetic radiation (Solar 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 (W/m²),[2] though this 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 (which has a cross section of 127,400,000 km²), the power is 1.740×1017 W, plus or minus 3.5%. The Solar constant does not remain constant over long periods of time (see Solar variation). 1366 W/m² is equivalent to 1.96 calories per minute per square centimeter, or 1.96 langleys (Ly) per minute.
The Earth receives a total amount of radiation determined by its cross section (π·RE²), but as it rotates this energy is distributed across the entire surface area (4·π·RE²). Hence the average incoming solar radiation (sometimes called the solar irradiance), 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 342 W/m²). 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 and the location's latitude.
The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light (see Electromagnetic spectrum). It is linked to the apparent magnitude of the Sun, −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. By taking readings at different times of day, he attempted to remove effects due to atmospheric absorption. However, the value he obtained, 2903 W/m², was still too great. 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. His results varied between 1.89 and 2.22 calories (1318 to 1548 W/m²), a variation that appeared to be due to the Sun and not the Earth's atmosphere.[3]
The angular diameter of the Earth as seen from the Sun is approximately 1/11,000 radians, meaning the solid angle of the Earth as seen from the sun is approximately 1/140,000,000 steradians. Thus the Sun emits about two billion times the amount of radiation that is caught by Earth, in other words about 3.86×1026 watts.[4]
Climate effect of solar radiation
- Further information: Solar dimming and Insolation
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, combining the perception of bright white light (sunlight in the strict sense) and warming. The warming on the body and surfaces of other objects is 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.[5] 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).
Notes
- ^ Climate Change 2001: The Scientific Basis
- ^ "Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present". Retrieved on 2005-10-05.
- ^ This article incorporates text from the Encyclopædia Britannica Eleventh Edition article "Sun", a publication now in the public domain.
- ^ The Sun at nine planets.org
- ^ Graph of variation of seasonal and latitudinal distribution of solar radiation
See also
- Insolation: a measure of solar radiation energy incident on a surface
- Solar heating: Predicting solar heating effects
- Solar neutrino problem: solar neutrino measurement problem
- Solar variation: variations in solar activity
- Solar wind: particles flowing from the Sun
- Coronal mass ejection: large ejection of electrons and protons
- Polar aurora: usually electrons hitting Earth's atmosphere
- Solar flare: eruption creates increase of solar wind particles
- Solar proton event: protons hitting Earth's atmosphere
- Pyranometer: solar radiation sensor
External links
- Solar radiation - Encyclopedia of Earth
- Total solar irradiance data archive 1978-2007 at the website of the National Geophysical Data Center
- A Comparison of Methods for Providing Solar Radiation Data to Crop Models and Decision Support Systems, Rivington et al.
- Evaluation of three model estimations of solar radiation at 24 UK stations, Rivington et al.
- High resolution spectrum of solar radiation from Observatoire de Paris
- Measuring Solar Radiation : A lesson plan from the National Science Digital Library.
- Websurf astronomical information: Online tools for calculating Rising and setting times of Sun, Moon or planet, Azimuth of Sun, Moon or planet at rising and setting, Altitude and azimuth of Sun, Moon or planet for a given date or range of dates, and more.
- Daylength - Formulas to calculate the daylength depending from latitude and day of year.
- An Excel workbook with a solar position and solar radiation time-series calculator; by Greg Pelletier
- DOE information about the ASTM standard solar spectrum for PV evaluation.
- ASTM Standard for solar spectrum at ground level in the US (latitude ~ 37 degrees).
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