Measuring distance to the stars


Tom BurnsStargazing

It isn’t much of a crater, really.

The moon has hundreds that are larger and brighter.

It’s positioned almost in the middle of the three large lunar “seas,” those dark patches at the top of the moon. Near the center of the middle gray oval is a beautiful streak of light. At about the center of that streak is a tiny crater, visible only in a telescope.

The story of that obscure, little crater begins almost two centuries ago.

In the first decade of the 19th century, Friedrich Bessel was a young man with a mission. He wanted to get rich, which in those days meant becoming a merchant.

To get really wealthy, merchants bought and sold the exotic goods that came from faraway lands. They traveled for many months aboard ships that sailed the oceans.

And to be a world traveler, one had to know something about navigation. The oceans were vast, with nary a landmark to guide the way, so sailors used the stars to mark their passage across the endless water.

Through this unlikely pathway, Bessel became an astronomer. By 1810, he was carefully measuring the positions of stars in the night sky. In that year, the ancient Albertus University in Konigsberg chose him as the director of their soon-to-be-built observatory.

Konigsberg was in Eastern Europe, far from the trade centers and bustling ports to the west. Gone were the visions of gold; Bessel’s hobby had now become his profession.

He built the new observatory and set himself to accurately plotting the locations of stars, a task that he single-mindedly pursued well into the 1830s.

His accurate stellar positions turned out to be of critical importance to the act that made him famous: He was the first to measure the distance to a star. He used a technique called triangulation, which is used by surveyors, to measure the distance to far-away objects.

First, you must establish two observing stations at some distance from each other. The distance between the two observing stations is called the baseline. When you look at a distant object from the two stations, the direction of that object seems to change. The angle of that change allows you to calculate the distance to the object.

If all that seems confusing, you can verify it with a simple experiment. Hold your finger a few inches from your eyes. Now look at your finger with just your left eye. Then look at it with just your right eye. Repeat the process several times, rapidly opening and closing your left and right eyes.

Notice how your finger seems to change position with respect to a more distant object, like a wall, when you blink your eyes?

Repeat the process with your finger held at arm’s length. Notice how your finger doesn’t seem to move as much? Because your finger is farther away, it seems to move around less with respect to the wall.

Bessel decided to measure the distance to a close star with respect to the other stars in the sky, which he assumed were farther away, like the wall of your room is more distant than your finger.

He chose the star numbered 61 in the constellation Cygnus, the Swan, which sits high in the east right now and is visible to the unaided eye from dark, rural skies. He hoped it was a close star, but he couldn’t be absolutely sure.

He measured the tiny change in position of “61” as it is seen on either side of the earth’s orbit around the sun. His baseline was thus the diameter of the earth’s orbit, or about 200 million miles.

Even with such a large baseline, the change in position of “61” was tiny. But it indicated that “61” is 60 trillion miles away, or an astounding 600,000 times the distance of the earth from the sun!

Even at the incredible distance, “61” is still one of the closest stars to the earth, the fourth closest of those visible to the naked eye.

“61” is in fact not one star, but two. Even a small telescope will split this faint point of light into two beautiful orange stars very close together.

These stars are orbiting each other in the same way that your thumbs move around each other when you twiddle them. But they are doing so very slowly — it takes 650 years for them to complete one “twiddle” — one circuit around each other.

Bessel never got rich. However, in 1841 he got the gold he had craved as a youth. In that year, the Royal Astronomical Society awarded Bessel its gold medal, the highest award in astronomy at that time.

And the crater? Like I said, it ain’t much. But astronomers named it after Bessel, the man who first accurately measured the distance to the stars.

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Tom BurnsStargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.