Few people truly appreciate the scale of interstellar space. Space is so big that astronomers had to invent new units of measure: the astronomical unit, the light-year, the parsec. People hear these words, they can do calculations with them, but rarely do these terms evoke a feeling as concrete “mile” or “kilometer.” I stumbled on a way to correct this.

There are ≈63,240 AU in a light-year. In a fortunate coincidence, there are 63,360 inches in a mile. So if Sol was an inch from Terra, Alpha Centauri would be four **miles** away. Our farthest probes would be 100 inches away, traveling three inches per year. In this miniature model, light takes 8 minutes to move an inch. Space is vast, but it’s not beyond our comprehension.

Another tidbit: Distances to nearby stars are measured using parallax. One parsec is defined as the distance at which an object has an arc-second of parallax. Alpha Centauri’s parallax is 0.8 arc-seconds. That’s equivalent to looking at an object 4 miles away and moving your head two inches. The best instruments today can measure parallax as small as 20 micro arc-seconds, giving accurate measurements for objects thousands of light-years away.