Sky Watch: Measuring the Cosmos

Astronomers quickly become used to dealing with huge numbers.

For example, in any one lecture, I might declare that the sun emits some 400 billion trillion watts of power each second, that our Milky Way Galaxy contains hundreds of billions of stars or that the distances to even the nearest stars are measured in trillions of miles.

With each increasing number, I can see listeners' eyes rolling farther back into their heads. And that's understandable; other than astronomers, the only people who throw around numbers this large are politicians.

And, while audience members don't often vocalize it, I'm sure each is thinking the same thing: "How can you possibly know that?" A natural question and, in the case of cosmic distances, the answer -- at least in principle -- is surprisingly simple.

Astronomers use a technique we all learned in elementary school and one we use every day of our lives to navigate our world by foot or by car. It's called parallax, and you can demonstrate it like this.

Close one eye, hold out your thumb at arm's length and align it with an object on the other side of the room. Without moving your thumb, close the open eye, and open the other one. Notice how your thumb appears to have "moved" relative to the more distant background object? Now bring your thumb closer to your eyes and try again. What happens now?

This apparent shift against the background is a measure of your thumb's parallax, and it's determined by the separation between your eyes (the "baseline") and their distance to your thumb. Since stars are considerably farther than our thumbs, astronomers must use powerful telescopes and much longer baselines to measure their parallaxes.

The first ever to do this successfully was Friedrich Wilhelm Bessel (1784-1846), who used a telescope at the Konigsberg Observatory in East Prussia to measure the position of the faint star 61 Cygni relative to the more distant background stars. Six months later -- when the Earth was on the opposite side of its orbit -- he made the same measurement.

And it was in 1838 that he proudly announced that his observations of the star showed a parallax of a mere 0.314 arcseconds -- the width a U.S. dime would appear if held at a distance of more than two and a half miles!

What Bessel achieved was truly stunning: not only did he determine this star's parallax, he calculated its distance as well. With this long baseline of some 186 million miles, Bessel determined that the star 61 Cygni must lie about 66 trillion miles from Earth -- pretty darned close to the 68.2 trillion miles we measure with modern equipment.

Go out on the next clear dark night and look high in the western sky for the famous three bright stars that form the Summer Triangle. Nearly overhead you'll spot the bright star Deneb, the tail star of Cygnus, the swan, and just behind the swan's easternmost wing, the faint star known as 61 Cygni. Though it looks like most others, it was the study of this star 174 years ago that made it possible for humans to begin measuring the cosmos.