I was thinking how to solve this problem. $1\,\mathrm{AU}$ is roughly the distance from the Earth to the Sun, $1.4960 \times 10^{11}\,\mathrm{m}$.

The radius of Earth is approximately $6.4 \times 10^{6}\,\mathrm{m}$, and the radius of the Sun is approximately $6.96 \times 10^{8}\,\mathrm{m}$.

How could we estimate the percent of radiation which the Earth receives, ignoring astrophysical "noise" like dust?

The radiation emitted by the Sun roughly follows the Stefan-Boltzmann law and the radiation emitted is roughly $\propto T^4$, and the surface area of the Earth is roughly $\propto r^2$.

Would you simply take the ratio between the Sun's surface area divided by the Earth's surface area?


closed as off-topic by Rob Jeffries, Sebastian Riese, CuriousOne, Kyle Kanos, rob Sep 21 '15 at 2:27

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – Rob Jeffries, Sebastian Riese, CuriousOne, Kyle Kanos, rob
If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Consider the solid angle subtended by the sun with the earth. $\endgroup$ – SchrodingersCat Sep 20 '15 at 16:21
  • $\begingroup$ If you mean total radiation at the Earth's surface, would our atmosphere be more of an aspect you could consider than any dust between here and the sun? I am not sure how accurate you want to be, but you could of course check it on Wikipedia. $\endgroup$ – user81619 Sep 20 '15 at 16:22
  • 1
    $\begingroup$ The Sun radiates isotopically (roughly). So you work out what fraction of a $4\pi$ solid angle is occupied by the Earth as seen from the Sun. To first order it is the projected surface area divided by the area of a sphere at the Earth's orbit. $\endgroup$ – Rob Jeffries Sep 20 '15 at 16:41

Sun and Earth.

All the Sun's power $P$ passes uniformly through a sphere with radius of 1 AU. Calculate the total surface area of this sphere and call it $S$.

The Earth's disc also has a surface area that can be calculated from its radius. Call this surface $S_E=\pi R_E^2$.

The fraction of the Sun's power received by the Earth is thus:



Not the answer you're looking for? Browse other questions tagged or ask your own question.