The solar constant is the amount of radiant energy received per second per unit area by a perfect black body on the Earth with its surface perpendicular to the direction of radiation from the sun in the absence of atmosphere.

**Solar constant**

*The solar constant is the amount of radiant energy
received per second per unit area by a perfect black body on the Earth with its
surface perpendicular to the direction of radiation from the sun in the absence
of atmosphere. *It is denoted by* S *and
its value is 1.388 ? 10^{3}* W m*^{-2}.* *Surface temperature of the Sun can be
calculated from solar constant.

*Surface temperature of the Sun*

The Sun is a
perfect black body of radius *r* and
surface temperature *T*. According to
Stefan?s law, the energy radiated by the Sun per second* *per unit area is equal to σ*T*^{4}.

Where σ is Stefan?s Constant.

Hence, the total energy radiated
per second by the Sun will be given by

E = surface area of the Sun ? σT^{4}

Let us imagine a sphere with Sun
at the centre and the distance between the Sun and Earth R as radius
(Fig.). The heat energy from the Sun
will necessarily pass through this surface of the sphere.

If S is the solar constant, the
amount of heat energy that falls on this sphere per unit time is E = 4πR^{2}S ????.(2)

By definition, equations (1)
& (2) are equal.

4πr^{2}σT^{4}. =
4πR^{2}S

T^{4} = R^{2}S/r^{2}σ

T = (R/r)^{1/2}(S/ σ)^{1/4}

Knowing the values of *R, r, S* and σ the surface temperature of the Sun can be
calculated.

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