Light Years and Astronomical Units
Measuring the dimensions of your science class is not a difficult task. All that is required is to obtain a tape measure and stretch it across the length and width of the floor. Finding the perimeter of your school’s athletic field, though offering a bit more of a challenge, is nonetheless within the realm of possibility.  A different method must be used, however, to find the distances between stars and galaxies.
It is obviously impossible to take a tape measure and pull it across the voids separating celestial bodies, so astronomers must use a bit of fancy math instead. At times, that means relying on geometry to measure how much the  position of stars vary over a given period of time. For objects that are farther away, astronomers must get even more technical and use computers to analyze the light stars emit.
In either case, one thing we know for sure is that the distances are going to be “astronomical.” For example, the distance to the nearest star, our own Sun, is approximately 93 million miles. The next closest star is Alpha Centauri, which is an amazing 25,670 billion miles away, and the distance to the center of our galaxy is an almost unimaginable 153,400 trillion miles from Earth. Beyond that, the distances become so vast as to almost be incomprehensible.
As you can imagine, measuring these distances using miles or kilometers does not make a lot of sense. It would be similar to using millimeters to measure the distance across an entire continent. Clearly a much larger unit of measure is required in such situations, which is why astrophysicists came up with an alternative method to measure huge distances.  Instead of miles, they use what are referred to as light years.
Though the term “light year” might sound as if it is referring to a unit of time, it is indeed specifying a precise distance, or to be more explicit, the distance light traverses in one year.  Given light’s rapid, rate of movement, it turns out that a light year is close to six trillion miles.
However, there are times when it is impractical to measure distances in miles or kilometers due to their enormity, but also impractical to measure them in light years, as is the case when planetary astronomers describe distances within our solar system. In such instances, they use astronomical units, or AU, the average distance between the Earth and the Sun.  The number of astronomical units separating each pair of planets advances at a steady rate as their distances from the sun increase.