Solar constant

The solar constant (GSC) measures the amount of energy received by a given area one astronomical unit away from the Sun.

Variations in total solar irradiance (TSI) were small and difficult to detect accurately with technology available before the satellite era (±2% in 1954).

) and f is the irradiance of the star at the extrasolar planet at distance d. 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, 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.

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'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.

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 Earth (which has a cross section of 127,400,000 km2), the power is 1.730×1017 W (or 173,000 terawatts),[9] plus or minus 3.5% (half the approximately 6.9% annual range).

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·RE2), but as it rotates this energy is distributed across the entire surface area (4·π·RE2).

The amount reaching the Earth's surface (as insolation) is further reduced by atmospheric attenuation, which varies.

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.

[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.

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.

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.