The solar luminosity, , is a unit of luminosity (power emitted in the form of photons) conventionally used by astronomers to give the luminosities of stars. Luminosity has different meanings in several different fields of science In Physics, power (symbol P) is the rate at which work is performed or energy is transmitted or the amount of energy required or expended for In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth It is equal to the current accepted luminosity of the Sun, which is 3. The Sun (Sol is the Star at the center of the Solar System. 839 × 10²⁶ W, or 3. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. 839 × 10³³erg/s. An erg is the unit of Energy and Mechanical work in the centimetre-gram-second (CGS system of units symbol "erg" ^[1] Note that the Sun is a weak variable star and its luminosity therefore fluctuates. For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson Solar variations are changes in the amount of Solar radiation emitted by the Sun.

Calculating with this constant

You can calculate how much solar power hits the Earth by comparing a cross sectional area of the Earth and the total surface area of a sphere with a radius equal to the distance of the earth from the sun.

• The Earth's radius is 3963 miles (6,378 km). The Earth 's shape like that of all major Planets approximates a Sphere.
• The Earth's cross sectional area = π×radius² = 49. 3 million square miles (128,000,000 km²).
• The Sun's average distance is about 93 million miles (150,000,000 km).
• The surface area of a sphere = 4×π×radius² = 1. "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe 09×10¹⁷ square miles (2. 82×10¹⁷ km²).
• Power reaching the Earth = P(total) × Area(earth)/Area(sphere) = 1. 77×10¹⁷ W.
• The power hitting a square meter of area on Earth: (square meter = 1/1609² square miles)
  • Power over square meter = P(total)(1/1609²)/area(sphere) = 1387 W (the solar constant)
• Estimates have been made that humans use about 12×10¹² W.
• How much land area would be needed to power that?
  • The best solar cells can produce about 33% efficiency.
  • Area needed = 12×10¹²/(1387×0. 33) = 26×10⁹ m² = 10122 square miles ~100×100 mile square. (More is needed since the sun is not always straight over head, and because some fraction of the radiation does not reach the surface due to clouds and atmospheric scattering. )

References

• I. -J. Sackmann, A. I. Boothroyd (2003). "Our Sun. V. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars". The Astrophysical Journal 583 (2): 1024-1039.  

• Carroll, Bradley W. ; Dale A. Ostlie (2007). An Introduction to Modern Astrophysics. Pearson Addison Wesley, Appendix A. ISBN 0-8053-0402-9.  

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