The Universe is immense. So immense, in fact, that astronomers had to come up with a new unit of distance to measure it with. Miles and kilometers were just too small. The unit that astronomers use to measure distances in the Universe is the light-year.
One light-year is defined as the distance traveled by light in one year in a vacuum. The speed of light in a vacuum is constant; it never speeds up and it never slows down. This means that the distance traveled by light in a vacuum in any given period of time is always the same and it can therefore be used as a measure of distance.
Light travels at 299 792 458 meters per second in a vacuum. This means that in space (which is essentially a vacuum) light travels 9,460,528,400,000,000 meters in one year.
In other words, one light-year is the same as 9,460,528,400,000,000 meters, or 9,460,528,400,000 kilometers. But whether you use meters or kilometers, it’s still a really big number and light-years are a much more convenient way to measure such large distances.
To give you another idea of how far a light-year is, here are some common astronomical distances given in light years:
– the average diameter of the Earth is 0.000000001 light-years
– the average distance from the Earth to the Moon is 0.00000004 light-years
– the average distance from the Earth to the Sun is 0.00002 light-years
– the average distance from Neptune to the Sun is 0.0005 light-years
– the average distance from Pluto to the Sun is 0.0006 light-years
– the average distance from the Sun to Proxima Centauri (the closest star to the Sun) is 4.2 light-years
– the average distance from the Earth to the North Star (which is also known as Polaris or Ursae Minoris) is 430 light-years
– the distance to the furthest known galaxy is 13,000,000,000 light-years.
Because light takes a finite time to reach us, when we look out into space we are actually looking back in time.
For example, the distance from the Earth to the Sun is 0.00002 light-years, or around 7 light-minutes. This means that when we look at the Sun, we are actually seeing the Sun as it was 7 minutes ago.
The closest star to our solar system, Proxima Centauri, is 4.2 light-years away, so when we look at it, we see it as it was 4.2 years ago. Looking at the North Star, we are seeing it as it was 430 years ago.
So in summary, light-years are a convenient unit with which to measure astronomical distances, such as the distance between stars. It is much easier to deal with 4 or 400 light-years, than it is to be writing down billions of meters!