We had no idea how far away the stars were until Bessel measured the parallax of 61 Cygni in 1838. For rhe first time we knew the true distance to an external stellar system, accurately determined with simple triangulation methods–11.4 light years away. Until that time, we had no idea how far away the stars were. The size of the universe, or at least the size of our immediate stellar neighborhood, was a complete mystery.
Parallax is simple triangulation, the technique used by surveyors to determine the distance to distant landmarks. You simply measure the location of a nearby object relative to its position against distant objects much further away. Its why we have two eyes, so we can determine distance to nearby objects. Try threading a needle with one eye shut!
The principle is simple, but the distances to even the nearest stars are so immense that it required a lot of technology to measure the tiny angles caused by parallax. You have to note the position of a nearby star against those of more distant ones from opposite sides of a baseline. And in astronomy, the biggest baseline available is the earth’s orbit itself. Measurements (or photographs) taken six months apart allow you to see the parallax shift against the distant stellar background. The parallax shift (in seconds of arc) is the reciprocal of the distance in parsecs (1 pc = 3.26 ly). So a star with a parallax of 0.1″ (one tenth of a second of arc) is ten parsecs away. These angles are so tiny and difficult to measure that parallaxes have been worked out for only a few hundred of the very nearest stars. All the other distances you see in astronomy books are just educated guesses and estimates built on a towering inverted pyramid of educated assumptions. The parallax is the fundamental measurement on which everything else rests.
The earth’s orbit is, by definition, an Astronomical Unit in radius, about 93,000,000 miles. But by using photographs taken from the New Horizon spacecraft, now cruising the outer solar system over 40 AU away, we can extend that baseline considerably.
Behold these images of two of our stellar neighbors, Proxima Centauri, and Wolf 359. against the background of far more distant stars. This corresponds to parallaxes 0.769 and 0.414 seconds of arc. Note how none of the background stars in the images reveal any shift at all.
Bessel chose to measure the distance to 61 Cygni because its extraordinarily large proper motion suggested that it was very close to earth. But he really had no idea how far away it actually was. After doing his parallax measurement, he knew, and he knew it was not an estimate or an educated guess–it was a measured quantity. Can you imagine how he must have felt, leaning back from a desk full of thrice-checked calculations that he was the first human being to actually measure the distance to a nearby star? Until that moment, no one had any idea how far away those little lights in the sky were. Even the ancients knew that the universe was big, because they believed the stars were so far away they had no measurable parallax, they did not have the technology to measure tiny, tiny angles. Even a good marine sextant can only measure with an accuracy of about 30 seconds of arc. And since no star (except the sun) is closer than a parsec to earth, no parallax can exceed 1 second. For Proxima and Wolf, the distances are 4.24 and 7.87 ly, respectively.
I have no idea how it happened, but I like to think Bessel finished his calculation, divided it into one and stared for a long time at the final result and muttered under his breath “Motherrrrr FUCKER”.