Solar constant is the amount of energy received from the sun per second per unit area by a perfect black body on the earth, the area being placed normal to the direction of radiation.

Let **S **be the solar constant **r **be the distance of the earth from the sun and **R **be the radius of the sun.

Total energy radiated by the sun per second is

Here **T **is the temperature of the sun and is the **Stefan’s constant.**

Energy received per second per unit are on the surface of the earth is

According to the definition of the solar constant, this is the solar constant.

Hence by knowing the value of **S, **the temperature of the sun can be estimated.