The SI unit of distance is the meter. One meter is about 39 inches, just three inches bigger than a yard. So, if you think of a yard stick, that's almost the length of a meter.
The historical definition of a meter is quite interesting, and you can read about it. However, today, the length of a meter is based on the speed of light.
The speed of light traveling through a vacuum was measured to be 299,792,458 m/s, which we simply call c and round to 3x108 m/s. Let's say that a super-duper spaceship (that doesn't even exist) is traveling toward you at 1/2 the speed of light and turns on its headlights. Before Einstein, scientists thought that a person in the spaceship would measure the speed of light to be c, and you, the observer, would measure the speed of light to be 1.5c--that's 50% larger simply because the spaceship is moving toward you and you have to account for its speed as well as the speed of light.
However, Albert Einstein in 1905 proposed a revolutionary idea that turned out to be correct. He suggested that the speed of light is a constant of nature that does not depend on the motion of the observer. Thus, the person in the super-duper spaceship and you, on Earth, would BOTH measure the speed of light coming from the spaceship to be c.
Today, we define the speed of light to be 299,792,458 m/s. Since we've defined a second and we've defined the speed of light, we can calculate the distance light travels in 1/299,792,458 s. That distance is what we now call a meter.
The distance that light travels in one second is called a light-second which equals 299,792,458 meters. The light-second is therefore a unit of distance, not time.
Distances between objects in our galaxy and universe are so large, that we more often use the unit light-year. One light-year is 9.46 x 1015 meters.
The simulation shown below shows a pulse of light (called a two-dimensional wavefront) leaving a distant star (a red giant) and traveling toward Earth. Click the Run button to view the motion of the light pulse. Note that the diameters of the star and Earth are GREATLY exaggerated for the purpose of visualization. The unit for time in the simulation is years.
How much time elapses for light to travel from the star to Earth?
Now, we are going to create a lightyearstick (as opposed to a meterstick) which is a stick of length 1 light-year. Click the "Show Lightyear" button. After the light beam travels for a time of 1 year, the program will lay down a lightyearstick to show how far it has traveled.
What is the distance of the star from Earth in units of light-years? Note: just count the lightyearsticks! Also, note that the distance traveled in a time interval of 1 year is a light-year.
Sun is much closer to Earth than the star shown in the simulation. It takes about 8 minutes for light to travel from Sun to Earth. So, what is the distance from Sun to Earth? Well, we say that it's 8 light-minutes.
If Sun were to burn out, how long would it take for us to notice with our eyes that Sun burned out?