What exactly is a light year?

Every time i watch a show on space they always use “light year” as a measurement of time. but what exactly is a light year? How is a light year measured and how long is it compared to a Gregorian calendar year (365 days)?

This is an extremely common point of confusion! A light year is the distance light travels in one year (365 days). It often gets misused as a unit of time, likely because ‘year’ is right there in the name. It will always take light 1 year to travel a distance of 1 light year.

In a vacuum, light always travels at 300,000 kilometers per second (or 670 million miles per hour). With about 8765 hours in a year, this makes the distance light travels in a year 5.87 x 10^12 miles (nearly 6 trillion miles), or 9.46 x 10^12 km (9 trillion kilometers). Once you start counting in trillions, using kilometers and miles becomes really impractical, so using the distance light travels in a year is a good way to measure very large distances. As a point of comparison, it takes 8 minutes for light from the sun (93 million miles away) to reach us, and our galaxy is a little more than 100,000 light years across.

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