According to the Stefan-Boltzmann law, the total power radiated by the sun acting as a black body is $P = \sigma A T^4$. The absolute temperature is $T = (t + 273) \text{ K}$ and the surface area of the sun is $A = 4\pi r^2$. Thus, $P = \sigma (4\pi r^2)(t+273)^4$. This radiated power spreads uniformly over all directions. At a distance $R$ from the centre of the sun, this power is distributed over a spherical surface area of $4\pi R^2$. The power received per unit area (intensity) at the earth is given by: $I = \frac{P}{4\pi R^2} = \frac{\sigma (4\pi r^2)(t+273)^4}{4\pi R^2} = \frac{r^2\sigma(t+273)^4}{R^2}$.