When a star emits light, it emits an equal amount in all directions. You can imagine this as lots of little "light arrows" flying out of the star. Imagine these coming from the Sun. At Earth, we can measure how much light we detect each second; imagine we do this by holding up a "bucket" and seeing how many "arrows" fall into it. If we were to go to Jupiter, which is about 5 times farther from the Sun as Earth, and collect light in the same "bucket":

    A ) we would count the same amount of light as we do from Earth
    B) we would count 5 times the amount of light, because Jupiter is 5 times farther
    C) we would count 1/5th the light, because the total light from the Sun is spread out over a circle (Jupiter's orbit) that has 5 times the circumference of Earth's orbit
    D) we would count 1/25th the light, because the total light from the Sun is spread out over a sphere (the size of Jupiter's orbit) that has 25 times the area of a sphere the size of Earth's orbit, because the area of a sphere goes as the radius squared (specifically, 4 π R²)

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