61 Cygni

The star system 61 Cygni has a couple of distinctions. It moves across the sky faster than all but about a half-dozen other stars. And it was the first star to have its distance accurately measured.

The system is in Cygnus, the swan. The constellation is high in the eastern sky at nightfall, marked by Deneb, the swan’s bright tail. 61 Cygni is to the lower right of Deneb. Under dark skies, it’s just visible to the eye alone.

61 Cygni consists of two stars. Both are smaller, lighter, and cooler than the Sun, and much fainter. They’re far apart, so it takes more than 650 years for them to complete a single orbit.

More than two centuries ago, Giovanni Piazzi discovered that the system moves across the sky in a hurry. This quick motion suggested that it’s close by. So astronomers started trying to figure out how close.

They looked at the star at intervals of six months, when Earth was on opposite sides of the Sun. That slight change in perspective caused nearby stars to move back and forth a bit compared to stars that were farther away.

Early attempts to measure that angle didn’t work — the equipment of the day just wasn’t good enough. But in 1837 and ’38, Friedrich Bessel used a new instrument that provided a sharper view. It told him that 61 Cygni was 10.3 light-years away. That’s just one light-year off the true distance. So Bessel’s work provided the first good measurement of the distance to any star other than the Sun.

Script by Damond Benningfield

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