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Question of the Day
Question
I was wondering if you might be able to explain
what a "Light Year" is. I am in the 6th grade and it doesn't
make much sense to me.
Answer
A light year is a way of measuring distance. That doesn't make much sense
because "light year" contains the word "year", which is normally a unit of time. Even so,
light years measure distance.
You are used to measuring distances in either inches/feet/miles or centimeters/meters/kilometers, depending
on where you live. You know how long a foot or a meter is - you are comfortable with these units
because you use them every day. Same thing with miles and kilometers. These are nice, human increments of distance.
When astronomers use their telescopes to look at stars, things are different. The distances are gigantic.
For example, the closest star to earth (besides our sun) is something like 24,000,000,000,000 miles
(38,000,000,000,000 kilometers) away. That's the closest star. There are stars that are billions of times
further away than that. When you start talking about distances that are that far away, a mile or kilometer
just isn't a practical unit to use because the numbers get too big. No one wants to write or talk about numbers
that have 20 digits in them!
So for really long distances, people use a unit called a light year to measure distance.
Light travels at 186,000 miles per second (300,000 kilometers per second). Therefore a light second is
186,000 miles (300,000 kilometers). A light year is the distance light could travel in a year, or:
Using a light year as a distance measurement has another advantage - it helps you determine age.
Let's say that a star is 1 million light years away. The light from that
star has traveled at the speed of light to reach us. Therefore it has taken the star's light 1 million years to
get here, and the light we are seeing was created 1 million years ago. So the star we are seeing is really how
the star looked a million years ago, not how it looks today. In the same way, our sun is 8 or so light minutes
away. If the sun were to suddenly explode right now, we wouldn't know about it for 8 minutes because
that is how long it would take for the light of the explosion to get here.
Fun fact: A light nanosecond - the distance light can travel in a billionth of a second - is
about a foot (about 30 cm). Radar uses this fact to measure how far away something like
an airplane is. A radar antenna sends out a short radio pulse and then waits for it to echo off an airplane
or other target. While it's waiting it counts the number of nanoseconds that pass. Radio waves travel at the speed of light, so
the number of nanoseconds
divided by 2 tells the radar unit how far away the object is!
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186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year =
5,865,696,000,000 miles/year
A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers). That's a long way!
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