See Part II of this series here.

Can you see Jupiter’s moons with the naked eye? On the night of Sept. 25-26, your chances of seeing them are pretty much as good as they get. And whether you see them or not, in just trying you will be observing a phenomenon that has multiple connections to Church history.

On Sunday night (Sept. 25) and early Monday morning (Sept. 26) Jupiter will be close to the Earth — about four times more distant from us than is the sun. (Astronomers would say that Jupiter will be about 4 astronomical units, or AU, from Earth). By contrast, in January Jupiter was six times as far from us as the sun (6 AU). The changing distance is caused by Earth’s orbital motion carrying us around the sun. As we orbit, we draw closer to and farther from Jupiter each year. 

On that same night, Jupiter’s two outer moons, Ganymede and Callisto, will appear as distant from that planet as they get. Moreover, because the Jovian system will be close to Earth, these moons will appear just about as bright as they can be. So if you can ever see these moons, this will be the night to see them. And while any time during the night will be good, the absolute best time will be at about 1:30am Eastern Time on the 26th.

Arguably, seeing the moons should be no big deal. The Jovian system is big. The orbits of the moons span a significant fraction of the apparent diameter of the Earth’s moon in the sky. The separation between Ganymede and Jupiter is comparable to the apparent width of some of the dark spots, or “maria” (Latin for seas) on our moon. Those spots make up the “man in the moon.” If you can see the spots on our moon with your eyes (even if you can’t quite visualize the man in the moon) should you not be able to see Ganymede standing off Jupiter?

Moreover, the separation between Ganymede and Jupiter is just about half the separation between the stars Mizar and Alcor, the famous “horse and rider” at the bend of the handle of the Big Dipper. People with good eyes can easily see Alcor standing off Mizar, so why not Ganymede standing off Jupiter? Ganymede is almost as bright as Alcor, too.

Callisto is considerably fainter than Ganymede. It requires dark skies and very good eyes to see a star as faint as Callisto. But on the night of Sept. 25-26 Callisto will stand off Jupiter even more than Ganymede, and in that way be even easier to see for those whose eyes can pick up such faint objects.

With all this said, it is generally thought that people cannot see the moons of Jupiter without an optical aid like a telescope or binoculars. After all, Galileo discovered these four moons with the newly-developed telescope, publishing his first observations of them in 1610 in his famous book The Starry Messenger. They were unknown before Galileo. 

(If you know something about Galileo, you might be starting to see the possible connection to Church history. However, the history connection goes all the way back to St. Augustine of Hippo and St. Thomas Aquinas! But the Church history is for Part II of this series.)

If Callisto is as bright as Alcor, and stands off Jupiter by the width of one of the lunar maria, why can’t people see it? It turns out that small, bright objects do weird things with our vision. When you look at our moon, you see its actual body; you see its size for what it is. But when you look at Jupiter or a star, that dot you see is not the body of Jupiter or of the star — that dot is just the light of Jupiter or the star, shaped by your eye, the atmosphere and the laws of optics. That dot’s size is a kind of illusion. 

But it is an illusion shared by everyone who has good eyes. (People with poor vision either see Jupiter and stars as fuzzy balls rather than dots, or they do not see them at all.) Skilled observers from ancient times up to the invention of the telescope in Galileo’s time have consistently looked at Jupiter and estimated its size, not knowing that size to be illusory. They consistently estimated its size to be just under one-tenth the diameter of our moon; they estimated the brighter stars to be a little smaller than Jupiter. Spend some time looking at Jupiter, our moon and the stars, and you will see that what those historical observers said makes sense. 

Jupiter’s big “tenth-of-our-moon” size leaves less room for seeing Jupiter’s moons. It pretty much eliminates any chance of seeing Jupiter’s inner moons. And the brightness of Jupiter tends to overwhelm the eye, washing out any fainter things nearby, like Ganymede or Callisto. Plus, most of the time those moons are not standing out at their maximum distances from Jupiter, and most of the time Jupiter is not at its nearest point to Earth. So even if someone in history saw one of Jupiter’s moons, they would probably not have thought of it as anything other than a faint star, which they did not see again. And thus Galileo discovered them with his telescope.

But on Sept. 25-26, Ganymede and Callisto will be standing out from Jupiter, and Jupiter will be close. If the skies are cloudy on that night, then the nights of the 24th-25th and the 26th-27th will not be bad, either (although Ganymede, which orbits Jupiter more rapidly that Callisto, will not be so well placed on either of those nights). So go out and look for Jupiter. At 1:30am Eastern Time it will be due south as seen from the Eastern United States. If you go out earlier, or live further west, it will be in the southeast. Look for the bright dot — you know, the one about one-tenth the diameter of our moon.

And when you see that dot, you are seeing something that concerned Augustine, Aquinas and the churchmen who fought with Galileo. You would not believe the connections those celestial dots have to Church history. That’s for Part II.