Solar constant

The solar constant is the solar power received per unit area at Earth’s mean distance from the Sun, measured outside the atmosphere on a surface perpendicular to sunlight.

Solar irradiance: Solar irradiance is the power of sunlight received per unit area. The solar constant is a standardized value of solar irradiance at a specified distance and orientation.
Claude Pouillet: Claude Pouillet was a French physicist who pioneered measurements of solar radiation in the nineteenth century. His actinometric work produced an influential early estimate of solar heating at Earth.
Solar energy: Solar energy is radiation from the Sun that can be converted into heat or electricity. The solar constant provides a reference for estimating sunlight available to solar technologies in space.
Solar flux density: Solar flux density is the rate of solar energy crossing a unit area, often specified by wavelength and location. It is a broader measurement concept than the solar constant’s fixed reference conditions.
Inverse-square law: The inverse-square law states that a point source’s intensity decreases as the square of distance increases. It explains how solar irradiance changes with distance from the Sun.
John Herschel: John Herschel was a British astronomer and polymath who studied solar radiation and climate. His South African observations helped quantify sunlight reaching Earth’s surface.
Planetary energy balance: Planetary energy balance compares incoming absorbed radiation with energy emitted back to space. The solar constant sets the incoming solar power used in global energy-budget calculations.
Solar spectral irradiance: Solar spectral irradiance is solar power per unit area per unit wavelength interval. Unlike the spectrally integrated solar constant, it resolves how energy is distributed by wavelength.
Solar luminosity: Solar luminosity is the total energy the Sun emits as radiation per unit time. Combined with Earth–Sun distance, luminosity determines the solar constant.
Samuel Pierpont Langley: Samuel Pierpont Langley was an American astronomer who measured solar radiation with a bolometer. His measurements helped establish the historical effort to determine solar input.

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