The amount of solar energy reaching Earth relative to the Sun's total energy is best described as:

• A. Large
• B. Huge
• C. Small
• D. Moderate

Answer: (C)

Energy from the Sun spreads outward in all directions, so by the time it reaches Earth, it’s flowing over a huge sphere with a radius equal to the Earth–Sun distance. The Earth presents only a small circular cross-section to that light. The fraction of the Sun’s total energy that Earth intercepts is the Earth's cross-sectional area divided by the surface area of that sphere, which works out to (R^2)/(4r^2). Using Earth's radius about 6,400 km and the Sun–Earth distance about 150 million km, this fraction is roughly 5×10^-10—a tiny portion of the Sun’s total energy. So the amount of solar energy reaching Earth relative to the Sun’s total energy is best described as small. The other options would imply Earth takes a much larger share, which isn’t consistent with this geometric reasoning, while the tiny fraction is precisely what the math shows even though the absolute energy Earth receives is still huge in real terms.