All Topics  
Solar luminosity

 

 
   Email Print
   Bookmark   Link
 
 

 

Solar luminosity
 
 
  Topics Solar luminosity Topic Home

The solar luminosity, , is a unit of luminosityLuminosity

Luminosity has different meanings in several different fields of science....
|power]] emitted in the form of photonPhoton

In modern physics, the photon is the elementary particle responsible for electromagnetic phenomena....
s) conventionally used by astronomerAstronomer

An astronomer or astrophysicist is a person whose area of interest is astronomy or astrophysics....
s to give the luminosities of starStar

A star is a massive, compact body of plasma in outer space that is held together by its own gravity and, unlike a planet, is...
s.
It is equal to the current accepted luminosity of the SunSun

|+ The Sun   |+|-| colspan="2" align="center" | |-...
, which is 3.839 × 1026 WWatt

The watt is the SI derived unit of power, equal to one joule per second....
, or 3.839 × 1033ergErg

An erg is the unit of energy and mechanical work in the centimetre-gram-second system of units, symbol "erg"....
/s.
Note that the Sun is a weak variable starVariable star

Most stars are of almost constant luminosity....
 and its luminosity therefore fluctuates.

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 cross sectional area = p×radius2 = 49.3 million square miles (128,000,000 kmē).
  • The Sun's average distance is about 93 million miles (150,000,000 km).
  • The surface area of a sphereSphere

    A sphere is a perfectly symmetrical geometrical object....
     = 4×p×radius2 = 1.09×1017 square miles (2.82×1017 kmē).
  • 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 constantSolar constant

      The solar constant is the amount of incoming solar radiation per unit area, measured on the outer surface of Earth's atmosph...
      )
  • 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.)