If I ask Google to tell me the distance to the nearest star, I get an immediate answer: 4.22 light-years.
Light takes four years to travel to Alpha Centauri, more than 250,000 times the 93 million miles between us and the Sun.
How on Earth do we know that?
I have a retired friend who wasn’t satisfied with Google and, looking for a new project, wanted to make the measurement himself.
He started with an ancient Greek experiment, first performed by Eratosthenes around 240 B.C. My friend planted a stick in the ground on the summer solstice and measured the length of its shadow at noon. He then waited an entire year, traveled a little way south, and made the same measurement at the same time on the same day. Since he was closer to the equator, the sun was a bit higher and thus the shadow a bit shorter.
The difference in length between those two shadows plus some high school algebra and trigonometry told him how many degrees of latitude he’d moved south. By measuring the distance he’d actually traveled, he discovered the number of miles per degree of latitude and thus, by extrapolation, the circumference of Earth.
With that in hand, all he needed to do was wait until the next lunar eclipse. As the moon passed through Earth’s shadow, careful measurements of the event gave him an estimate of the size of the moon compared to the size of Earth. Knowing the circumference of Earth, and, again, a little algebra and a little trig, gave him the distance between us and the moon.
After that, he just waited until the moon was next at its first quarter phase, when the sun’s rays intersected the moon at exactly an angle of 90 degrees. A careful measurement of the angular distance between the moon and the sun on the sky gives an answer that is almost, but not quite, a right angle itself. That slight difference with — you guessed it — more algebra and more trig gave him the distance between Earth and the sun.
But how do we get from there to the nearest star?
For that he needed something called parallax.
You can try an experiment yourself: hold your thumb out in front of your eyes. If you alternately close one eye and open the other, you’ll see your thumb jump back and forth with respect to objects in the distant background. Hold your thumb closer, it moves more, hold it farther away, it moves less. If you measure this, you’ll find your thumb jumps back and forth half as much when it’s twice as far away.
This works for nearby stars, too. All my friend had to do was take a picture of the nearest star through his telescope, wait six months as Earth moved halfway through its orbit, and then take another picture. Provided he knows the size of Earth’s orbit, something he’d just measured, he could work out the exact mathematical relationship between how much the star appears to move in the sky with respect to the background stars and its distance from us.
This is an extremely difficult measurement, but with dark skies and a good telescope, something anyone can do. Especially if you have a lot of time on your hands.
The first stellar parallax — of the star 61 Cygni — wasn’t made until 1837, with measurements of Alpha Centauri coming a few years later.
It took more than 2,000 years for astronomers to make that first measurement, something my friend could replicate with some off-the-shelf telescope components and a few years of his time. Admittedly, that last measurement is a challenge, so it might take him a bit longer.
But every time I search for something on Google, and get an answer within milliseconds, I’m reminded of the effort spent collecting these seemingly trivial facts. Someone dedicated their life — sometimes hundreds or thousands of people — to add one single number to the total of human knowledge.
Faced with this, I’m reminded that knowledge is a garden that needs tending. Like seeds, it must be planted, grown and harvested periodically to keep it viable. We must tell these stories of how we found out what we know, and share the skills of observation, of algebra and trigonometry, so everyone knows something of the struggle it took to learn this stuff.
And, occasionally, someone needs to go do it again. To prove, even if just to themselves, that they can.