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The Question

(Submitted February 11, 1998)

I am 8 years old. Could you explain what a light year is and give an example that I can understand or relate to.

The Answer

A light year is the distance that light travels in one year. The speed of light is 186,287.5 miles per second. You can find out the number of seconds in a year by multiplying the number of seconds in a minute (60) by the number of minutes in an hour (60). Then multiply that by the number of hours in a day (24), and multiply that by the number of days in a year (approximately 365.25).

  So we've got 60 x 60 = 3600 seconds in an hour
               3600 x 24 = 86400 seconds in a day
               86400 x 365.25 = 31,557,600 seconds in a year.

So a light year is about (I've rounded off a bit) 5,878,786,100,000 miles. That's almost 6 trillion miles. The distance from the earth to the Sun is 93 million miles. The distance to the nearest star is 4.3 light years, and the distance to the Andromeda galaxy is 2 million light years.

The universe is a big place !

Jim Lochner
for Ask an Astrophysicist

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