We can define distance as the amount of space between two points. We might use units like inches, centimeters, feet, yards, miles, or kilometers on a daily basis for measuring those distances. But in astronomy, measuring the distances between two points gets quite vast quite quickly. It’s not feasible to use miles or kilometers when the distances to astronomical objects are so very far away.
On Earth, it might be a few hundred kilometers from New York City to Washington, DC, and a few thousand kilometers from California to Maine. But out in the universe, the distance to the next nearest big galaxy, the Andromeda Galaxy, is about 25 quintillion km: 25,000,000,000,000,000,000 km (that’s a lot of zeroes to have to write).
Instead, distances in astronomy are measured in units of light-years, where one light-year is the distance that light travels in a year — about 10 trillion kilometers.
1 light-second ≈ 300,000 km
1 light-minute ≈ 2 million km
1 light-year ≈ 10 trillion km
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