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SESK: Stuff Everyone Should Know · sesk.info/astro-inverse-square-law

If you move twice as far from a star, how much of its light reaches you?

One quarter, by the inverse square law.

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  1. One half
  2. One quarter ✔
  3. One eighth

Why: Light spreads over a sphere whose area grows with distance squared. Twice as far means 2 x 2 = 4 times less light; ten times as far means 100 times less.

Huh, didn't know that: This is the key link between how bright a star looks and how far away it is.

The dad joke
The inverse square law: light needs more space whenever it goes the distance.
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References

Answer: One quarter. Light spreads over a sphere whose area grows with distance squared. Twice as far means 2 x 2 = 4 times less light; ten times as far means 100 times less.

  1. OpenStax Astronomy 2e, 5.1 The Behavior of Light openstax.org/books/astronomy-2e/pages/5-1-the-behavior-of-light
    If we stand twice as far from the source, our eyes will intercept two-squared (2 × 2), or four times less light.
    Checked 2026-10-10.

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