1. The problem statement, all variables and given/known data The flux from the Sun above the Earth's atmosphere is about 1370 watts/m^2. This quantity is called the solar constant S and equals pi*f(sun). Use the angular radius of the sun as seen from the Eart to find pi*F , the surface flux of the sun. 2. Relevant equations possible equations that may be relevant: Flux=sigma*T^4, T being the temeperature dependence and sigma=5.669*10^-8/m^2 *K^4; Luminosity = flux*area, a 3. The attempt at a solution I have to find the angular radius of the sun in order to find the surface flux of the sun. To find the angular radius of the sun, I would probably have to used the fact that theta=lambda/Diameter of the sun; theta=lambda/2*radius of the sun. L=4*pi*R^2 *sigma*T^4. There are still 3 unknown variables: lambda, and the Temperature. Perhaps I should approach a solution to this problem in a different fashion: I know the radius of the sun is 6.96*10^5 km and luminosity of the sun is 3.90*10^26 W and the flux from the sun over the earth's atmosphere is 1370 watts/m^2. L/A=flux => (3.90*10^26 W)/(4pi(6.96*10^8 m)^2) = 64067276.69 W/m^2. But that calculation gives me the total flux of the sun, it doesn't give me the surface flux of the sun. how do you find the surface flux of the sun?