Week of Aug. 4-10, 2019

Guests on my popular Borrego Night Sky Tours quickly become accustomed to hearing large numbers.

For example, on any night, I might explain that our Milky Way galaxy is home to hundreds of billions of stars, that our sun produces some 400 million billion megawatts of power each second, or that the distances to even the nearest stars are measured in trillions of miles.

With each increasing number, I can see my guests' 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 stargazers don't often vocalize it, I'm sure each is wondering the same thing: "How can you possibly know that?" This is a natural question and, in the case of cosmic distances, the answer — at least in principle — is surprisingly simple.

Astronomers use one technique we all learned in elementary school and one we use to navigate our everyday world. 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 shifted relative to the more distant background object? Now bring your thumb closer to your eyes and try again. What happens now?

This apparent shift you see 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 from your thumb. Since stars are considerably more distant, astronomers must use telescopes and much longer baselines.

The first ever to do this successfully was Friedrich Wilhelm Bessel, 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 around the sun, he made the same measurement.

And it was in 1838 that he proudly announced that his observations of the star showed a tiny 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 61 trillion miles from Earth — pretty darned close to the 67 trillion miles we measure with modern equipment.

Next time the moon isn't in your sky, head outdoors to look overhead for the three bright stars outlining the famous "Summer Triangle." There, you'll spot the bright star Deneb, the tail star of the swan, Cygnus — also known as the Northern Cross — 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 181 years ago that made it possible for humans to begin measuring the cosmos.

On a night without moonlight, be sure to look for 61 Cygni behind the Summer Triangle stars.

Visit Dennis Mammana at dennismammana.com. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.