Physical Cosmology

How long is a light year?
Answered by HowStuffWorks
  • HowStuffWorks

    HowStuffWorks

  1. A light year is not a length of time but a measure of distance. Objects in space are really far apart: The sun is about 89,280,000 miles (144,000,000 kilometers) away from the Earth, and the next closest star after that is approximately 24,000,000,000,000 miles (38,000,000,000,000 km) off. To make these huge numbers easier to write and work with, astronomers use a larger distance of measure: the light year, or the distance light travels in one year. After a star creates light, it travels at a rate of 186,000 miles (300,000 kilometers) per second. Multiply that by 60 seconds, 60 minutes, 24 hours and then 365 days, and over the course of an entire year, light travels 5,865,696,000,000 miles (9,460,800,000,000 kilometers). Light years also are useful to astronomers because they don't just give a distance, but can help determine age as well. Because a light year is the distance light can travel in a year, knowing how many light years away something is also tells you how many years ago the light you see was created.

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