Despite the name, a light-year measures distance: how far light travels in one year.
A very long way
Light moves about 300,000 km every second, so a light-year is roughly 9.5 trillion kilometres.
Looking back in time
Light from a star 100 light-years away left it a century ago — you see it as it was.
Why astronomers use it
Kilometres become unreadable at these scales; light-years keep the numbers human.
Filed underScience
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