Fredrich Bessel measures the universe

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.

Plan­ets

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 tlburns@owu.edu.

Posted by Tom Burns on Jun 26 2011. You can follow any responses to this entry through the RSS Feed. Comments can be made below.