The Light Year - a cosmology unit of distance

A light year is a way of measuring distance.

That doesn't make much sense at first sight because "light year" contains the word "year", which is normally a unit of time. But, light years measure distance..... the distance light can travel in a year!

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 (24 million, million miles away!) or 38,000,000,000,000 kilometers away.

..... and that's the closest star.

If you were to travel in a spaceship at a million miles per hour, it would take you 24 million hours to travel to that nearest star..... or a million days.... so it would take more than 2700 years for the journey! Wow, just think of that.

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.

So as it can travel 186,000 miles in one second, in one minute it would travel 186,000 miles x 60 miles

and in one hour it would travel 186,000 miles x 60 x 60 miles.

As there are 24 hours in a day it would travel 186,000 miles x 60 x 60 x 24 miles each day, and in a year (as there are 365 days in normal years but 366 days in a leap year - averaging 365.25 days in an average year) light travels:

186,000 x 60 x 60 x 24 x 365.25 miles

= 5.9 million, million miles

in metres that would be 300,000,000 x 60 x 60 x 24 x 365.25 m

= 9.5 thousand, million, million metres

or 9.5 million, million kilometres
 
  A light year is 9,500,000,000,000 km or 5,900,000,000,000 miles

  That's a long, long way!

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 travelled 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!

Physics 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!