How much energy radiated by the sun reaches Earth?

That number is the ratio of (the area of the half of the Earth facing the sun) to
(the area of a sphere whose radius is 93 million miles).

Radius of earth . . . 4,000 miles

Area of Earth . . . 4 (pi) R² = 64 x 10⁶ pi square miles.

Area of the side facing the sun . . . 32 x 10⁶ pi square miles.

Radius of the sphere at Earth's orbit . . . 93 x 10⁶ miles

Area through which the suns radiation is pouring = 4 (pi) R² = 3.46 x 10¹⁶ square miles

The ratio is . . . (32 x 10⁶) / (3.46 x 10¹⁶) = 9.25 x 10^-10 = roughly 1 billionth of it.

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