The solar constant is a flux density measuring mean solar electromagnetic radiation per unit area. It is measured on a surface perpendicular to the rays, one astronomical unit from the Sun. The solar constant includes all types of solar radiation other than neutrinos, not just the visible light. It is measured by satellite as being 1.361 kilowatts per square meter at solar minimum and approximately 0.1% greater at solar maximum. The solar "constant" is not a physical constant in the modern CODATA scientific sense; that is, it is not like the Planck constant or the speed of light which are absolutely constant in physics. The solar constant is an average of a varying value. In the past 400 years it has varied less than 0.2 percent. Billions of years ago, it was significantly lower. This constant is used in the calculation of radiation pressure, which aids in the calculation of a force on a solar sail.
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, 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, Samuel Pierpont Langley attempted to estimate the solar constant from Mount Whitney in California. By taking readings at different times of day, he tried to correct for effects due to atmospheric absorption. However, the final value he proposed, 2.903 kW/m², was much too large. 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, a variation that appeared to be due to the Sun and not the Earth's atmosphere. In 1954 the solar constant was evaluated as 2.00 cal/min/cm2 ± 2%. 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 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, the power is 1.730×1017W, plus or minus 3.5%. The solar constant does not remain constant over long periods of time, 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 while the solar irradiance will be affected by the eccentricity of the Earth's orbit. Its distance to the Sun varies annually between 147.1·106 km at perihelion and 152.1·106 km at aphelion. In addition, several long term cycles of subtle variation the Earth's orbit affect the solar irradiance and isolation. The Earth receives a total amount of radiation determined by its cross section, but as it rotates this energy is distributed across the entire surface area. 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. The amount reaching the Earth's surface 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.
Apparent magnitude
The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light. It is positively correlated with the apparent magnitude of the Sun which is −26.8. The solar constant and the magnitude of the Sun are two methods of describing the apparent brightness of the Sun, though the magnitude is based on the Sun's visual output only.
The Sun's total radiation
The angular diameter of the Earth as seen from the Sun is approximately 1/11,700 radians, 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 sunspotsolar 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, 88 years, 208 years and 1,000 years. 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, 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.
