Let’s try a demonstration that we all enjoyed in elementary school.

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, blink your eyes back and forth slowly, and you’ll notice that your thumb seems to shift relative to the more distant background.

Now bring your thumb closer to your eyes and try again. What kind of shift do you see this time?

The apparent shift of your thumb’s position against the background is called “parallax.” Because your brain knows the separation of your eyes (the “baseline” of a triangle formed by your eyes and any object), it can estimate an object’s distance pretty accurately.

Anyone with binocular vision (two functioning eyes) uses this technique constantly to navigate our everyday world. It’s what keeps you from tripping over the cat or driving into the car ahead of you.

But what about the stars? Might this same technique work to measure their distances? Of course, the best way to answer that question is to try it, but I’ll save you some time. You can blink your eyes at a star until you pass out; you’ll see no shift in its position. The fact is, the stars are just too far away.

What if we could use a baseline that is longer than the spacing between our eyes? Suppose, for example, we put two telescopes on opposite sides of the Earth and again measured the position of a star? Would that make a difference?

Again, the short answer is no, not even for the nearest of stars.

OK, what if our baseline were even longer: say, the diameter of the Earth’s orbit around the sun? If we could observe a star’s position in August, for example, when the Earth is on one side of the sun, and then observe the same star six months later when we’re on the opposite side of the sun, we’d have a baseline of some 186 million miles.

Well, that question was first asked by Greek thinkers two millennia ago, and, despite their best efforts, no one ever noticed a bit of parallax. Nope, the first time this was ever done was in 1838, when Friedrich Wilhelm Bessel (1784-1846) 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 — he made the same measurement.

That he detected any shift at all is quite remarkable, since it was only two hundred-thousandths of a degree — about the width a pizza in San Diego would appear if seen from New York City!

Since Bessel’s day, technology has improved even more, and we now have parallax measurements (and, therefore, direct distance measurements) for more than a billion stars!

The next time you show your child or grandchild how to blink their eyes while viewing their thumb, remember, it’s not just a childish game. It’s one of the fundamental techniques that astronomers use to measure the cosmos!