|What is a light year?
|Question Date: 2003-03-20|
A light year is a measure of distance. It is the
distance that light travels in one year.
Astronomers use this measure, because it is easier
to compare numbers like 1 and 100 light years than
numbers like 5 billion miles and 674 trillion
miles. If you want to know how many miles there
are in a light year, you can look up the speed of
light in miles per second and multiply by the
number of seconds in a minute and then by the
number of minutes in an hour and then by the
number of hours in a day and lastly by the number
of days in a year.
A light year is the distance that light travels in
a vacuum in one year. The speed of light is
300,000,000 meters/second, and there are 365 x 24
x 60 x 60 = 31,536,000 seconds in a year. Since
distance = speed x time, that gives 9.46 x 10^15
meters = 1 light year, or 9.46 x 10^12 km, or 5.88
x 10^15 miles.
A light year is about 6 trillion miles- the
distance light travels in a year. To give an idea
of the immense cosmic distances, consider these
measurements: Earth is about 1/8 of a light-second
around. A light beam could circle the world 8
times per second.
As you know, light travels very fast (186,000
miles per second). So in one second, a beam of
light travels 186,000 miles. So say we want to
think about how far away the sun is from Earth -
well, it takes light about 8 seconds for light to
get from the sun to earth so that gives 1,488,000
miles.Well, as you can imagine, these numbers get
unwieldy fairly fast considering that light from
the closest star takes 4 years to reach Earth. In
order to simplify thinking about these vast
distances, astronomers use a unit of distance
called a "light year". Simply put, it is the
distance light will travel in a year (in a perfect
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