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What is a light year?

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A light year is a measure of distance, or the distance a particle of light will travel through a pure vacuum in one Earth year.

This distance is approximately 5.8785 x 1012 [5.87 trillion] miles, or 9.4605 x 1012 [9.46 trillion] kilometers.

In a vacuum, light travels at almost 300,000 kilometers per second (more than 186,000 miles every second) so in a minute it will travel 60 times this amount (as there are 60 seconds in a minute). In 1 hour (60 mins) it will travel 60 times this amount. In 1 day (24 hrs) it will travel 24 times this amount. In 1 year (365 days) it will travel 365 times this amount. This amount is equivalent to almost 9.4605284 × 1012 kilometers or six million, million miles.

The nearest star (Proxima Centauri) is 4 1/4 light years away or approx. 25 million, million miles. The galaxy is 100,000 light years across The known universe is over 13 billion light years across. If a star is 10,000 light years away, we are seeing it today as it was 10,000 years ago - as the light that started out then is only just reaching us now. Similarly, if there were aliens looking at us through a powerful telescope on a planet 4 light years away, then they would just today be seeing what happened here 4 years ago.

The measurement of a light year is useful in finding distances to very distant objects, and as we're assuming that there is a pure vacuum between here and there, we can then determine how long ago, in Earth years, that light originated from its source. It is entirely possible that some of the stars we see from Earth are simply no longer there! A light year is a unit of distance. It is the distance that light can travel in one year. In one year, light can travel about 10 trillion km. One light-year is equal to 9,500,000,000,000 kilometers.
A light year is the distance covered by light in one year travelling at 186,000 miles per second.
Basically, a light year is the distance light would travel in a year. (Though I've often wondered what calender is used to define the length of the year, and has an adjustment been made for a leap year?)
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