What does a light-year measure?
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Answer: C
Correct answer: Distance light travels in one year. Despite the word year, a light-year measures length, not time, about 9.5 trillion kilometers or 5.9 trillion miles. Astronomers use it because star distances in ordinary units become unmanageably large; the nearest star beyond the Sun is roughly 4.2 light-years away. The trap is the word year, which suggests time, but it only tells you how long light needs to cover that distance.
- A: A light-year does not measure energy.
- B: An orbital year is a time interval, not a light-year distance.
- C (correct): Light-year is a length unit defined from light travel.
- D: Rotational speed is not what light-year denotes.