text: parsecs & lightyears? hello. what are the advantages of using the parsec over the lightyear? this is urgent. a parsec is a parallax-arcsecond, and equal to 3.26 light-years.  it's really *not* a much larger distance than a light-year, but it's a useful one for the following reasons:  if we look at a close star in january and then in june, the earth will be on opposite sides of the sun at each of those times.  although the earth's distance to the sun is only a very small fraction of our distance from the star, it will cause stars to slightly shift in the sky from this "binocular vision" perspective effect.  this is what's known as parallax.  stars which are closer will shift more, and we can use the amount of parallax shift to tell how far away it is.  a star which shifts 1 arcsecond (1/3600th of a degree) will be exactly 1 parsec away.  if it shifts only 1/2 of an arc-second, it's 2 parsecs away, 1/3 of an arc-second, 3 parsecs away, etc.  so, that's one advantage to using it - it makes your parallax to distance conversion really easy.  another advantage is that parallax is tied to the astronomical unit (au) which is the distance between the earth and the sun.  because of how the parallax geometry works out, it makes it such that the number of arc-seconds in a radian is exactly equal to the number of au in a parsec: 206265.  this makes converting between solar system distances and interstellar distances very easy.