Why Understanding the Solar Constant Matters in Physics
The measure of the solar electromagnetic radiation in a meter squared at Earth's distance from the sun is called a solar constant. To quantify the rate at the unit surface of a solar panel in which the energy is received upon the solar constant is used. In this case, the solar constant is absorbed at a given point and provides a total measurement of the sun's radiant energy. They are used in several atmospheric and geological sciences. Though it is called a constant, the solar constant is just nearly constant. Once every eleven years, the relative constant varies by 0.2% in a cycle that peaks. In 1838, Claude Pouillet made the first attempt to estimate the solar constant at 1.228 kW/m2. At a solar minimum of 1.361 kW/m2 and a solar maximum of 1.362 kW/m2, the constant is rated.
To measure the solar constant and not just the visible light, the entire spectrum of electromagnetic radiation is included in it. From the satellites, the solar constant is taken at the best direct measurements. To calculate a solar constant, the Stefan-Boltzman constant is used. In this case, the constant refers to the power per unit area emitted by a black body as a function of its thermodynamic temperature.
The Dimensional formula for solar constant
The solar constant is the incident ray of solar energy per unit area per second on the earth surface.
Solar constant = Energy / (Unit area x Unit time)
= ML²T⁻² / (L²T)
= MT⁻³
What is Solar Constant
The solar constant which is denoted by the symbol GSC is a flux density which is the measuring mean of 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 is 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 per cent 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, is more extensive and longer-lived. Moreover, as the Sun burns up its hydrogen presence, 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 per cent. That is we can say that 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 kilometers 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 that 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.
FAQs on Solar Constant in Physics: Definition, Formula & Significance
1. What is the solar constant and what is its standard value?
The solar constant is defined as the mean solar electromagnetic radiation (total energy from the Sun) received per unit area of a surface, held perpendicular to the incoming rays, at Earth's average distance from the Sun (one Astronomical Unit). Its accepted value is approximately 1366 watts per square metre (W/m²) or 1.366 kilowatts per square metre (kW/m²).
2. What is the formula for the solar constant and its dimensional formula?
Conceptually, the solar constant represents power per unit area. While there isn't a single calculation formula for it, its dimensional formula can be derived from its definition (Energy / (Area × Time)).
The dimensions are:
- Energy = [ML²T⁻²]
- Area = [L²]
- Time = [T]
Therefore, the dimensional formula for the solar constant is [ML²T⁻²] / ([L²][T]) = [MT⁻³].
3. Why is the “solar constant” not actually a true physical constant?
The term "constant" is a slight misnomer because its value varies slightly due to two main factors:
- Earth's Elliptical Orbit: The distance between the Earth and Sun changes throughout the year. The value is about 3.3% higher at perihelion (closest approach in January) and lower at aphelion (farthest point in July).
- Solar Cycle: The Sun's own energy output fluctuates by about 0.1% over its 11-year sunspot cycle.
Therefore, the solar constant is an average value, not a fundamental, unchanging constant of nature like the speed of light.
4. How does the solar radiation reaching Earth's surface (insolation) differ from the solar constant?
The solar constant is a theoretical value measured outside Earth's atmosphere. Insolation refers to the solar radiation that actually reaches the Earth's surface. Insolation is always less than the solar constant because a portion of the Sun's energy is lost as it passes through the atmosphere due to:
- Reflection by clouds and ice particles.
- Scattering by air molecules and aerosols.
- Absorption by gases like ozone, water vapour, and carbon dioxide.
5. What is the importance of the solar constant in physics and real-world applications?
The solar constant is a crucial parameter in several fields. Its importance includes:
- Climate Science: It is a fundamental input for Earth's climate models and for understanding the planet's overall energy budget.
- Astrophysics: It allows scientists to calculate the Sun's total energy output (luminosity) and its effective surface temperature using the Stefan-Boltzmann law.
- Engineering: It is used to design and determine the efficiency of solar panels, spacecraft, and satellites that rely on solar power.
6. How is the solar constant related to the Sun's temperature through the Stefan-Boltzmann law?
The Stefan-Boltzmann law states that the total power radiated by a black body is proportional to the fourth power of its surface temperature (L = σAT⁴). The Sun's total power output (luminosity, L) is radiated outwards in all directions. The solar constant (S) is this total luminosity spread over the surface of a giant sphere with a radius (r) equal to the Earth-Sun distance. Therefore, S = L / (4πr²). By measuring the solar constant S, we can work backwards to calculate the Sun's total luminosity and, consequently, its effective surface temperature.
7. Why is the solar constant defined at the average Earth-Sun distance of one Astronomical Unit (AU)?
The solar constant is defined at one Astronomical Unit (1 AU) to create a standardized, consistent reference value. The intensity of solar radiation follows the inverse square law, meaning it decreases as the square of the distance from the Sun increases. By fixing the measurement distance to the average Earth-Sun distance, scientists have a stable baseline that is directly relevant to our planet and can be used for reliable comparisons and calculations in climatology and astrophysics.
