The solar constant which is denoted by the symbol GSC is a flux density which is the measuring mean solar electromagnetic radiation. It is the solar irradiance per unit area. It is said to be measured on a surface perpendicular to the rays that is one astronomical unit denoted by AU from the Sun which is roughly the distance from the Sun to the planet.
The solar constant includes all types of solar radiation and not just visible light. It is said to be measured by satellite as being 1.361 kilowatts per square meter which is written as kW/m2 at solar minimum that is the time in the 11-year solar cycle when the number of sunspots is minimal) and approximately 0.1% greater roughly 1.362 kW/m2 at solar maximum.
Solar Constant Value
The time per unit of area on a theoretical surface that is perpendicular to the rays of the Sun and at Earth’s mean distance from the Suns said to be the most accurate measurement that is measured from satellites where atmospheric effects are absent. The value of the constant is approximately said to be 1.366 kilowatts per square metre.
The “solar constant” is fairly constant increasing by only 0.2 percent at the peak of each 11-year solar cycle. The sunspots usually block out the light and reduce the emission by a few tenths of a percent but the bright spot, also known as the plagues that are associated with the solar activity are more extensive and longer lived. Moreover, as the Sun burns up its hydrogen present, the solar constant increases by about 10 percent every billion years.
The solar constant is not a physical constant in the modern CODATA scientific sense; unlike the Planck constant or the speed of light which are absolutely constant in Physics. The solar constant is said to be an average of a varying value. In the past 400 years, it has varied even less than 0.2 percent. That is we can say the billions of years ago or so, it was significantly lower.
This constant is said to be used in the calculation of radiation pressure which helps in the calculation of a force on a solar sail.
Solar Constant Variation
The luminosity of the Sun is said to be approximately about 3.86 x 1026 watts. This is the total power that is said to be radiated out into space by the Sun. Most of this radiation is in the visible as well as the infrared part of the electromagnetic spectrum. With less than 1% emitted in the radio, UV and X-ray spectral bands. The energy of the sun is radiated uniformly in all directions.
As the Sun is about 150 million kilometres from the Earth and because the Earth is about 6300 km in radius, only 0.000000045% of this power is intercepted by our planet. This still amounts to a massive 1.75 x 1017 watts. For the purposes of the energy of the solar capture we normally talk about the amount of power in sunlight which is generally passing through a single square metre face-on to the Sun. The power of the Sun at the planet Earth as per square metre is known as the solar constant and is approximately 1370 watts per square metre is denoted by W/m2.
The constant that is the solar constant actually varies by +/-3% because of the planet Earth's slightly elliptical orbit around the star Sun. The distance between Sun-Earth is smaller when the Earth is at perihelion that is the first week in January and larger when the Earth is at aphelion that is the first week in July. The solar constant is also referred to as the power per unit area received at the average Earth-Sun distance of one Astronomical Unit denoted by AU which is 149.59787066 million kilometres.
There is also another variation which is smaller, due to a variation in the total luminosity of the Sun itself. This variation has been measured by radiometers aboard several satellites since the late 1970s.