Every evening before retiring, I go outside with the puppies. They’re both spoiled rotten and if I don’t go into the backyard with them, they will simply hang by the door. Then I’m getting up in the middle of the night to let them out.
While I’m waiting for them to do what puppies do, I always look at the stars.
Night before last, Jupiter was high in the southern sky. I’ve read that Babylonion astronomers knew that Jupiter had moons (they didn’t phrase it like that, but they certainly knew it had companions). I’d also known an astronomer named Justin Dunlap who had such amazing eyesight he could see them. My wife can see them also.
My vision isn’t so great so I’m stuck using a telescope. But with glasses, I can see just as well as the next guy.
Most people would have scoffed at what the ancients said. It was left up to Galileo to point a crude telescope towards Jupiter and make the astonishing discovery that it was a world with moons of its own.
The moons Galileo found are actually rather bright and have an apparent magnitude of 4.6 to 5.6. Most people can see a star of 4.6 easily enough, and while a star that’s 5.6 is getting pretty close to the limits of naked eye visibility, it could still be seen.
The reason we don’t see the distant moons easily with the naked eye is that Jupiter is so bright. The light simply drowns them out.
So I decided to try a simple experiment. I’d blot Jupiter out. (Don’t mess with me. I can blot out planets.)
Actually anyone can do that. My neighbor’s roof made this easy. I moved till the planet itself moved just a little and was blocked by his roof. I looked closely and sure enough, there were two faint stars near where Jupiter had been.
When I went inside, I got online and looked up the current positions of the Jovian moons using Sky and Telescopes interactive site. Sure enough, it matched.
The following night I went out and repeated the experiment. This time I’d looked up their locations prior to going out. I blotted out the planet and sure enough, there they were.
You need a nice dark sky to accomplish this and some means of occulting (that’s the proper term for blotting out the light of a brighter object) for this to work.
One thing I should point out is that you have a good chance of seeing the outer two Galilean moons, Ganymede and Callisto. Io and Europa are too close, and the chances of seeing them are slim to none.
But it was more than possible for stargazers of old to see Jupiter’s moons.
Jupiter is one my favorite objects to look at in future Stars Like Dust posts, we’ll take a longer look.