Much of the agonizing that mankind has gone through has been directed at better understanding his place in time and space. Starting from an ethnocentric point of view that considered every tribe or civilization or religion at the center of things, we have slowly, and often painfully, come to understand that the universe in which we live is vastly different from anything that we conceived in the past and that our earth, in all its beauty and majesty is but a small part of a grand cosmos. 

One of the most awesome and incomprehensible facets of astronomy is the distances involved. Here on the earth, we measure larger distances in miles. But miles become small units when we begin to explore even the solar system. The Earth has a diameter of some 8000 miles and a circumference of some 25,000 miles. The moon is ten times as far away as the earth's circumference. But even a quarter of a million miles pales when compared to our distance from the sun, about 93 million miles, or the distance between Pluto and the sun, over 3.6 billion miles from the sun on average. So, to reduce these numbers to sizes that made sense when navigating the solar system, astronomers evolved another more convenient unit. The astronomical unit (AU) is defined as the mean earth sun distance (93 million miles). Using AUs, the earth would lie 1 AU from the sun, Pluto nearly 40 AUs. 

As we look beyond the Solar system, the vast scale of the universe renders the use of AUs cumbersome. To measure the universe, we use how far light travels in a given amount of time. Because light travels at 186,000 miles per second, at the end of a second it will have gone 186,000 miles. We call this a light second. A light year (LY) would we how far light has gone in a year, (some 6 trillion miles). We commonly use light years for interstellar (between stars) and intergalactic (between galaxies) distances. Astronomers also use the parsec, which is approximately three and a quarter light years (3.26 LY).