The solar constant is a measure of the power of a square meter of sunlight directly impacting on a perpendicular plane of space above the atmosphere of the Earth, and is considered to be a uniform value of 1,370 watts per square meter. This changes dramatically, however, at the surface of the Earth, as sunlight has to pass through varying layers of atmosphere depending on latitude and sea level as well as atmospheric conditions. Therefore, the solar constant is largely a reference number used off of which to base actual sunlight values received, and is instrumental in such areas as the placement of solar arrays for photovoltaic or solar furnace power generation, and in weather and agricultural calculations. As a pure value above the limits of the atmosphere, the solar constant also varies by 3% depending on the point at which 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 solar electromagnetic radiation received. This includes infrared light, X-rays, and radio waves that are transmitted by the Sun, though high frequency waves like 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 optimal level of around 1,000 watts per square meter. Practical values due to higher latitudes, varying elevations, overcast skies, and other causes for indirect light drop this value to 250 watts per square meter, reducing the actual solar energy level that the Earth receives in space by a factor of more than five 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 due to the fact that these systems often have solar panels for generating power, and that they can be damaged by some solar radiation if not properly shielded. Research into solar cycles for the Sun, involving the calculation of solar storms and sunspot activity, are also dependent on the solar constant and its level of flux density or the relative amount of solar power transmitted per square meter. The Sun itself is known to have a slight variability to its radiation levels over 11-year cycles of ±0.2%. This along with a 10% increase in the solar constant every 10,000,000,000 years can have dramatic impacts on Earth's climate in regional areas such as the sea or on a global basis over time.
Manned space exploration to locations such as the Earth's Moon or the planet Mars also have to take into account the solar constant for these regions. Solar energy is largely similar to the pure value for Earth when on the Moon's surface, 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 it being 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 own weak atmosphere. In space or on barren planets and asteroids, the solar constant is the primary indicator of how much energy is available for processing rocks into useful materials such as oxygen and hydrogen, or for generating electrical power to sustain artificial environmental systems and communications equipment.