The Moons of Jupiter

What sort of evidence does it take to get you to change your mind?


This is a story about the first time I saw Jupiter’s moons with my own eyes, a few weeks ago.

But it’s also about a bigger question: What’s the power of evidence?

What sort of evidence do you need to change your mind about something huge?

First, the moons! This all started when my son got a telescope for his birthday, and we took it onto the sidewalks of Brooklyn to look at the sky. We checked out the craters on Earth’s moon, which are pretty impressive, then we hunted around for some stars. You can’t see very many in a city like New York — the sky is too polluted with light — but we spotted a single bright one rising high in the south, so I pointed the scope over and peered in.

I saw one huge, bright dot, with three other tiny pinpoints of light nearby, all lined up in a row (just like the image at the top of this story). Holy moses, I realized; that’s no star. That’s Jupiter! And those are the moons of Jupiter!

I’m a science journalist and a space buff, and I grew up oohing and aahing over the pictures of Jupiter sent back by various NASA space probes. But I’d never owned a telescope, and never done much stargazing other than looking up in the night unaided. In my 45 years I’d never directly observed Jupiter and its moons myself.

So I freaked out. In a good way! It was a curiously intense existential moment. Being able to pick out not just another planet, but that planet’s moons, while standing on the sidewalk outside my house, made me feel suddenly rooted to the clock-like machinery of the solar system. A modern person walks around generally understanding that yes, sure, we live on a huge orb that revolves around a burning star and all that — but given the magic of inertia and frames of reference and one’s general solipsism, this cosmic staging isn’t exactly something you think about while pondering which revolting sugary cocktail to buy at Starbucks; and even if you did think about it, it doesn’t seem terribly real.

But when I realized I was looking at another planet and its moons, 520 million miles away, with my own eyes — man alive, the structure of the solar system suddenly felt physical, intimate, even tactile. So I started yelling like a crazy person for everyone to come and look. Kids, kids! Wife! Hey you, random stranger on the way to buy milk! Check it out! It’s nuts! Jupiter’s actually real! So are its moons!


I would feel embarrassed by my overreaction, except that it’s pretty much how Galileo reacted too.

The night I saw Jupiter and its moons, I downloaded a translation of Siderius Nuncius, the book Galileo published in 1610 describing his celestial discoveries. (You can get it here yourself for free.) Galileo was among the first scientists to use this newfangled technology called “the telescope” to start looking at the sky. He was the first to report that the Earth’s moon was covered with mountains and “vast protuberances, deep chasms, and sinuosities”. He discovered that the stars we see with the naked eye are only a tiny fraction of the total — indeed, that in reality the stars were “so numerous as to be almost beyond belief.”

But his really earthshaking discovery occurred when he pointed his telescope at Jupiter the night of January 7, 1609. He noticed that the planet had three little stars very close to it, “arranged exactly in a straight line,” with two on one side and one on the other. Galileo assumed they were normal, far-off stars. Except the following night — quite by happenstance — he happened to look at Jupiter again, and this time, “a very different state of things.” The stars appeared to have moved: All three were now on one side of Jupiter. How could stars shift their position so curiously and swiftly?

“My surprise began to be excited,” Galileo wrote. He waited for the next night “with the most intense longing,” wondering if they’d moved again, but damn: “I was disappointed of my hope, for the sky was covered with clouds in every direction.” The next day the clouds parted, and he saw that indeed they’d again changed position: Only two were visible, with one having apparently moved behind Jupiter. When the next day he found they’d moved once more, he realized what he was seeing. Jupiter had bodies moving around it just as the Earth had the moon.

You can practically feel him vibrating with excitement as he writes this stuff.

For the next two months, Galileo carefully recorded the location of Jupiter’s moons, producing a set of drawings that looked like this:

From the 1880 translation of Siderius Nuncius, by Edward Stafford Carlos; these illustrations aren’t Galileo’s own, but based on his originals.

(Put together, the drawings look like an extremely slow-mo animated gif.)

Siderius Nuncius was a powerful piece of evidence that Copernicus was right: The Earth wasn’t the center of our solar system. The sun was, and the planets revolved around it. Astronomers had been gradually warming up to the idea, and even some church authorities had accepted the Copernican system as a mathematical theory. But by showing that Jupiter had its own moons — that a planet could be a mini-system of its own — Galileo offered something rather more: Electrifying proof of the Copernican idea. You could argue endlessly (and people did) about the geometry and math of various systems explaining how the stars moved through the sky. It was just conjecture.

But proof — that’s different. Once people put their eyes to the telescope and saw those moons circling Jupiter, they had the same whoa-dude reaction that I had on the sidewalks of Brooklyn. The solar system got real. So real, in fact, that the church began to panic; and since Galileo went on to use his telescope to amass even more evidence against geocentrism, including the phases of Venus, religious authorities eventually stepped in and demanded he recant, or else.


The reason I’m fascinated by Siderius Nuncius — which means “Starry Messenger” — is that, as evidence goes, it’s so simple and blunt.

Galileo doesn’t do much analysis. He doesn’t pull out any complicated math or intellectual bank-shots. He doesn’t need to. He carefully observes what he’s seeing and, boom, there it is. He shifted humanity’s cosmological mooring just by noticing something — by pointing something out.

I suppose I’m drawn to this because it’s comparatively rare, today, for a single piece of evidence to have that sort of impact.

That’s partly because science itself has gotten more complex. When it comes to physics, plumbing the nature of reality can’t be done as easily by one person any more; our most audacious efforts now require, as Craig Mod wrote last week, particle colliders of insane expense and size. Meanwhile, a lot of contemporary disputes in the social sciences often hinge on sophisticated arguments about what type of statistical analysis you’re applying to a corpus of data; two people are staring at the same pile of data, the same slice of reality, but seeing utterly different things.

On top of that, when it comes to arenas of science like climate change, things get even messier, because there’s all manner of politics and “motivated reasoning” involved. Can you imagine any single piece of evidence that would convince doubters that climate change is anthropogenic? Or the converse?

In a sense, I’m craving that bolted-to-the-wireframe-of-the-world feeling I got when I first saw the moons of Jupiter: The sheer simplicity of that proof, and that sense of elegant reveal you get from Siderius Nuncius.

Of course, I’m being romantic. Things were hardly so simple back in the 17th century, either. Galileo may have found direct and simple evidence of the Copernican system, but people still argued for centuries about whether it was true, or if true, what it implied. There was plenty of motivated reasoning back then, too.

And even some of those who were persuaded and thrilled by Galileo’s evidence used it to jump to some rather hilarious conclusions. In an essay he wrote about Siderius Nuncius, the astronomer Johannes Kepler argued that only “intellectual cowards” would fail to be persuaded by Galileo’s observations of the mountains on Earth’s moon. Then he went on to add:

Nothing is more certain than that the southern parts of the moon teem with mountains, very many in number, and vast in size; and that the northern parts, inasmuch as they are lower, receive in most extensive lakes the water flowing down from the south.

The lovely, flowing mountain-streams of the moon!


Still, when it comes to the heavens, the vistas we can see with our own eyes haven’t lost their power. That evidence is still striking, even to us blasé folks of the future. A telescope is a sufficiently strange sight on the sidewalks of Brooklyn that as I stood there peering into it, a woman walking her dog stopped and asked what I was doing.

I’m looking at the moons of Jupiter! I gushed.

Huh, she said. Can I take a look?

So she did, and as she squinted into the eyepiece she exhaled, a cloud of her breath floated away in the freezing air:

Oh, cool.