Solar constant

The solar constant (G_SC) measures the amount of energy received by a given area one astronomical unit away from the Sun. More specifically, it is a

The solar constant includes radiation over the entire electromagnetic spectrum. It is measured by satellite as being 1.361 kilowatts per square meter (kW/m²) at solar minimum (the time in the 11-year solar cycle when the number of sunspots is minimal) and approximately 0.1% greater (roughly 1.362 kW/m²) at solar maximum.^[1]

The solar "constant" is not a

This constant is used in the calculation of radiation pressure, which aids in the calculation of a force on a solar sail.

Calculation

In 1838, Claude Pouillet made the first estimate of the solar constant. Using a very simple pyrheliometer he developed, he obtained a value of 1.228 kW/m²,^[6] close to the current estimate.

In 1875, Jules Violle resumed the work of Pouillet and offered a somewhat larger estimate of 1.7 kW/m² based, in part, on a measurement that he made from Mont Blanc in France.

In 1884,

Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1.322 and 1.465 kW/m². Abbot showed that one of Langley's corrections was erroneously applied. Abbot's results varied between 1.89 and 2.22 calories (1.318 to 1.548  kW/m²), a variation that appeared to be due to the Sun and not the Earth's atmosphere.^[7]

In 1954 the solar constant was evaluated as 2.00 cal/min/cm² ± 2%.^[8] Current results are about 2.5 percent lower.

Relationship to other measurements

Solar irradiance

The actual direct solar irradiance at the top of the atmosphere fluctuates by about 6.9% during a year (from 1.412 kW/m² in early January to 1.321 kW/m² 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

The Earth receives a total amount of radiation determined by its cross section (π·R_E²), but as it rotates this energy is distributed across the entire

The angular diameter of the Earth as seen from the Sun is approximately 1/11,700 radians (about 18 arcseconds), meaning the solid angle of the Earth as seen from the Sun is approximately 1/175,000,000 of a steradian. Thus the Sun emits about 2.2 billion times the amount of radiation that is caught by Earth, in other words about 3.846×10²⁶ watts.

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).^{[10][11][12][13][14]}

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.

Variations due to atmospheric conditions

At most about 75% of the solar energy actually reaches the earth's surface,^[15] as even with a cloudless sky it is partially reflected and absorbed by the atmosphere. Even light cirrus clouds reduce this to 50%, stronger cirrus clouds to 40%. Thus the solar energy arriving at the surface with the sun directly overhead can vary from 550 W/m² with cirrus clouds to 1025 W/m² with a clear sky.

See also

References