June 26, 2011
The experienced stargazer will recognize the star numbered “61” in the constellation Cygnus, the Swan, almost immediately. The beginner seems to have absolutely no reason at all to seek it out. This relatively faint point of light is, after all, not one of the constellations on the imaginary lines that identify the Swan’s shape. Why bother?
Sixty-one Cygni turns out to be of critical importance to the history of astronomy because it was the first star used to measure the distance to the stars.
If I say that the star 61 Cygni is 11.1 light years away, it’s natural to wonder how anyone can know the distance to something so far away. After all, a single light year is equal to about 5.9 trillion miles. Nobody makes tape measures 64.5 trillion miles long. I’m certain that 61 is that far away because one of the greatest astronomers of all time determined a very clever way of measuring the distance to some of the closer stars in our Milky Way galaxy. In fact, to this day, surveyors use the same method today to measure the distance to far-away objects.
Fredrich Bessel’s method depends on a quality of stars called parallax, the tendency of near stars to shift their position with respect to farther away stars if we change the position from which we observe them.
Sound complicated? A simple experiment will illustrate how it works. Close one eye, and hold out your index finger at arm’s length. (Please use your index finger. Others in the room may misinterpret the gesture if you use another finger.) Line up your finger with some object on the other side of the room. (I’m doing it with a lamp right now.) Without moving your hand or your finger, close the open eye and open the one that was closed. Notice how your finger is no longer lined up with the object? Since your eyes are not in exactly the same place, you are observing your finger from slightly different locations. The apparent shift of your finger against the background is a measure of your finger’s parallax. Now bring your finger closer to your head and do the same thing. Your finger seems to shift more against the background because it’s closer.
Clearly, you could determine the distance from your head to your finger by measuring the distance that your finger seems to shift against the background. A bit of simple trigonometry, and you’re there.
The problem with this method is that even the closest stars are very far away. We’d have to move our eyes really far apart to see any motion in a star against the background of even farther stars.
Now here is the genius of Bessel’s method. He measured the position of close stars by observing them at six-month intervals — from either side of earth’s orbit. That’s one heck of a baseline. Earth is 93 million miles from the sun, so the diameter of Earth’s orbit is 186 million miles. Besel picked a star he thought might be nearby, a faint, naked eye star in Cygnus called (you guessed it) 61 Cygni, marked on the accompanying star map. Besel took careful measurements of the position of the star against fainter background stars, and then did the same thing six months later when Earth was on the other side of its orbit. Lo and behold, it had moved. Besel became the first person to measure a star’s parallax shift.
He determined the distance at 11 light years, which is pretty darned close considering that Besel was using 1838 telescope technology. Soon thereafter another astronomer measured the distance to the closest star to the sun, Alpha in the constellation Centaurus. His figure, 4.2 light years, was right on the money.
The star called 61 Cygni looks like any other star in a telescope or binoculars, and some might say it is hardly worth observing. But as you gaze at that tiny point of light, consider that with the study of that star, the mysterious became commonplace and humans for the first time could measure out the universe.
Venus and Mars are very low in the east during morning twilight. Bright Jupiter is still pretty low in the east just before sunrise. Saturn is still close to the faint star Porrima in Virgo. Right after dark in the evening, look southwest for a fairly bright, yellowish point of light.
Tom Burns is the director of Ohio Wesleyan University’s Perkins Observatory. He can be emailed at email@example.com.