The solar constant measures the power of sunlight on a perpendicular plane above Earth’s atmosphere, but changes on Earth due to atmospheric conditions. It is important for solar power generation, meteorology, and space exploration. Solar radiation includes all electromagnetic radiation from the Sun, but insolation on Earth’s surface is reduced to 250 watts per square meter. The solar constant also affects satellite and space probe development, solar cycle searches, and can impact Earth’s climate. Manned space exploration must account for the solar constant, which varies on different planets and asteroids.
The solar constant is a measure of the power of one square meter of sunlight directly impacting a perpendicular plane of space above the Earth’s atmosphere, and is considered to be a uniform value of 1.370 watts per square meter. This changes dramatically, however, on the surface of the Earth, as sunlight has to pass through different layers of the atmosphere depending on latitude and sea level, as well as atmospheric conditions. Thus, the solar constant is largely a reference number used on which to base actual received sunlight values, and is instrumental in areas such as the placement of solar panels for photovoltaic or solar power generation, and in meteorological and agricultural calculations. As a pure value above the limits of the atmosphere, the solar constant also varies by 3% depending on where the Earth is in its orbit of the Sun, since the orbit is slightly elliptical.
While solar radiation values for the solar constant usually focus on visible light, the values are a calculation of all received solar electromagnetic radiation. This includes infrared light, X-rays and radio waves emitted by the Sun, although high-frequency waves such as X-rays make up less than 1% of the total energy emitted. Where sunlight has reached the earth’s surface, this radiation is referred to as insolation and has an optimum level of around 1,000 watts per square metre. Practical values due to higher latitudes, varying elevations, overcast skies and other causes of indirect light reduce this value to 250 watts per square meter, reducing the effective level of solar energy Earth receives in space by a factor of more than five a once it reaches the surface.
The solar constant is an important value to know in the field of satellite and space probe development. This is because these systems are often equipped with solar panels for power generation and can be damaged by some solar radiation if not properly shielded. Solar cycle searches for the Sun, which involve calculating solar storms and sunspot activity, also depend on the solar constant and its level of flux density, or the relative amount of solar energy transmitted per square meter. The Sun itself is known to have a slight variability in its radiation levels over 11-year cycles of ±0.2%. This, together with a 10% increase in the solar constant every 10,000,000,000 years, can have dramatic impacts on the Earth’s climate over time in regional areas such as the sea or on a global basis.
Manned space exploration to places like Earth’s Moon or the planet Mars also has to account for the solar constant for these regions. Solar energy is largely similar to Earth’s pure value when on the surface of the Moon, due to the same relative distance from the Sun and the fact that the Moon has no atmosphere. Mars, however, will have a different solar constant due to the fact that it is at any one time at least 30,000,000 miles (48,280,320 kilometers) farther from the Sun than the Earth and because it has its atmosphere weak. In space or on barren planets and asteroids, the solar constant is the primary indicator of how much energy is available to transform rocks into useful materials such as oxygen and hydrogen, or to generate electrical energy to sustain man-made environmental systems and communications equipment.
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