In the early 19 century, when telescopes had finally reached a stage at which astronomers could discern thousands of distant stars, one of the first things they wanted to do was determine how far away they were. Many astronomers made valiant efforts at this problem, but the results never agreed with one another sufficiently to be of scientific certainty.
Then in 1838 the German astronomer Friedrich Wilhelm Bissel reasoned that it could be done using the simple trigonometric concept of parallax—that is, sighting an object from two different positions just the way artillery range was calculated.
In the case of stars, however, the two positions had to be quite far apart, further than could be done on this planet—unless the Earth itself were in two different positions when the two sightings were taken.
So that’s precisely what Bissel did. He selected the bright star 61 Cygni, and took one sighting of it. Six months later, when the Earth was half way around in its orbit, he took another sighting. The distance he calculated was approximately 11 light years. He continued refining his measurements and techniques on 61 Cygni and other stars. Today, we know the distance to 61 Cygni is 11.4 lys.
Basic to Bissel’s technique is dividing the celestial sphere into arcs of degrees, minutes and seconds. At stellar distances, we are looking at a second of arc, or an arcsecond, in calculating the parallax (actually, even to the nearest star, it is a fraction of an arcsecond). Early in the 20 century, astronomers combined these two words to form a third astronomical measurement—the parallax arcsecond, or parsec, abbreviated pc.
A parsec is about 3.26 lys. That gives us yet an easier number to manipulate when dealing with stellar and galactic distances.
The distance to 61 Cygni in parsecs is 3.5 pcs; to the Andromeda galaxy about 800 kiloparsecs (kpc).
Due to atmospheric distortion, ground based telescopes cannot measure objects more than about 100 pcs away. The Hubble, however, can use Bissel’s method to determine distance to all but the most distant objects.