**par·sec**(pär'sĕk') n. A unit of astronomical length based on the distance from Earth at which stellar parallax is one second of arc and equal to 3.258 light-years, 3.086×10^13 kilometers, or 1.918 × 10^13 miles.

**par·al·lax**(păr'э-lăks') n. An apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight. --par”al·lac“tic (-lăk'tĭk) adj.

Another word I was sort of familiar with. I had heard it used in science fiction in regards to distances in interstellar flight. I never paid much attention to it, but I figured it was a long ways. As it turns out, it is not that much better than "light year" in describing distance. It came into use as shorthand having to do with a second of an arch in viewing the heavens. It has no relation to the distance "covered" in the arc itself, but rather is figured out using equations based on the distance from the sun to the earth.

You see, we figure out how far "near by" stars are by looking at them six months apart when compared to "stationary" stars or galaxies much farther away. In doing this, a triangulation is performed by which the distance is figured by how much of an arc a star apparently moves. The point in space forming a focal point formed by a right triangle with a one second point opposite the vertical line from the earth to the sun. Here is the illustration taken from the Wikipedia article

The triangle is not to scale! There are 60 "minutes" in a degree, and of course 60 seconds to a degree. The figure above represents an approximate distance based on the value of pi (an irrational number), but basically a parsec is over 200,000 times the distance to the sun (93,000,000 miles)! Long story short, the imaginary point in space is a long way away. However, the closest star apart from Sol is a bit further away, appearing to move a bit over a "second" in the arc of the sky.

It has been figured that a "light nano-second" is about 1 foot, so a "nano-parsec" would be around 40 inches (3.25 * 12), or around a meter. Either way, of course, brings it down to a distance we can visualize.

As used in science fiction, though, the parcec is not a good term to use as a universal measurement. It is based on the earth's relationship to it's star. Any other planets would be about the same distance, so an average would have to be worked out. The same thing goes for the "light year," since years would be of differing lengths. That leads to other discussions about earth's special place in the universe, which probably does not float well in sci-fi circles.

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