We can measure interstellar distances, but can we really grasp them? The distance to the nearest stars is so immense that even the scientists who study such things have resorted to homely comparisons. The most charming to my mind is that of the English astronomer Sir John Herschel (1792-1871), the son of the famous William Herschel who discovered Uranus. A wizard at mathematics, Herschel became a leading expert on double stars and the measurement of stellar distances through parallax (the apparent change in position of a nearby star against background stars due to the Earth’s changing orbit around the Sun).

When it came to the distance to the Alpha Centauri stars, Herschel saw things in terms of ocean voyaging, thinking himself standing on shipboard dropping peas into the water. As he once wrote, “. . . to drop a pea at the end of every mile of a voyage on a limitless ocean to the nearest fixed star, would require a fleet of 10,000 ships of 600 tons burthen, each starting with a full cargo of peas.” Herschel’s fascination with double stars led to a catalog of such stars published in the Transactions of the Royal Society, and his later work on determining their orbits around a common center of gravity was honored by the Royal Society in 1833.

But back to the long road to Alpha Centauri. The useful HyperPhysics site at Georgia State University has a page that puts the Centauri distance in more abstract terms than Herschel. Imagine the Sun scaled down to the size of a period on a printed page. On this scale, the distance to Alpha Centauri is 13.6 kilometers (8 miles).

Now change the perspective again. Suppose this time it’s the Earth that is the size of a period on a printed page (which is, by the way, about 0.5 mm). On that scale, the Sun would be a little smaller than a tennis ball at a distance of some 5.9 meters (19 feet). The Alpha Centauri A and B stars would lie roughly 890 miles away.

Now think about this one: the parallax of Proxima Centauri, the M-class red dwarf that apparently orbits Centauri A and B at about 10,000 AU, is about equivalent to that of a dime at a distance of 6 kilometers. Proxima is so faint that astronomers on a planet orbiting either Centauri A or B would probably not realize it was nearby until they took measurements of its own parallax. GSU provides a helpful backgrounder on parallax, noting its use in measuring the distance to the few stars that are close enough to the Sun to show a measurable parallax. The limit of this measurement is about 20 parsecs, which includes some 2000 stars.