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.