At noon on Thursday the Sun goes down and we start the dark phase; that’s best of all. Jupiter’s colors really show and the other moons are easier to see. They can be almost anywhere and in almost any combination.
Jupiter and its satellites is sort of a miniature solar system; from Ganymede you have a front seat for the show. There is always something new in the sky. Besides the eleven “historical” satellites ranging in size from Ganymede down to Jay-ten or Nicholson-Alpha, which is a ball of rock and ice only fifteen miles thick, there are maybe a dozen more a few miles or less in diameter but big enough to be called moons and heaven knows how many smaller than that. Sometimes these little ones come close enough to Ganymede to show discs; they mostly have very eccentric orbits. Any time there will be several that are conspicuous lights in the sky, like the planets are from Earth.
Io, and Europa, and Callisto are always discs. When Europa passes between Jupiter and Ganymede it is as big in the sky as the Moon is from Earth. It actually is as big as the Moon and at that time it is only about a quarter of a million miles away.
Then it swings around to the far side and is very much smaller—more than a million miles away and less than a quarter as wide. Io goes through the same sorts of changes, but it never gets as big.
When Io and Europa pass between Ganymede and Jupiter you can see them move with your naked eye, chasing their shadows or running ahead of them, depending on the phase. Io and Europa, being inside Ganymede’s orbit, never get very far away from Jupiter, Io sticks within a couple of diameters of the big boy; Europa can get about sixty degrees away from it. Callisto is further out than Ganymede and goes all around the sky.
It’s a show you never get tired of. Earth’s sky is dull.
By six o’clock Saturday morning Jupiter would be in full phase and it was worthwhile to get up to see it. Not only was it the most gorgeous thing I had ever seen, but there was always the reverse eclipse, too, and you could see Ganymede’s shadow, a little round black dot, crawling across old Jupiter’s face. It gave you an idea of just how colossally big Jupiter was— there was the shadow of your whole planet on it and it wasn’t anything more than a big freckle.
Jupiter is ninety thousand miles across the equator, eighty-four thousand from pole to pole. Ganymede is only a little better than three thousand.
For the next couple of days after full phase Jupiter would wane and at Sunday midnight it would be in half phase again, the Sun would rise and a new light phase would start. One thing I expected but didn’t find was dim sunlight. Jupiter is a long way out; it gets only one twenty-seventh the sunlight that Earth does. I expected that we would always be in a sort of twilight.
It didn’t work out that way. It seemed to me that the sunlight was just as bright as on Earth.
George says that this is an optical illusion and that it has to do with the way the human eye works, because the iris of the eye simply shuts out light it doesn’t need. Bright desert sunlight back on Earth is maybe ten thousand foot-candles; the same thing on Ganymede is only four hundred foot-candles. But really good bright artificial light is only twenty-five foot candles and a “well-lighted” room is seldom that bright.
If you’ve got only a two-gallon bucket does it make any difference whether you fill it from the ocean or from a small pond? Sunlight on Ganymede was still more than the eye could accept, so it looked just as bright as sunlight on Earth.
I did notice, however, that it was almost impossible to get a sunburn.
11. “Share Croppers”
George got us a place to live when we had been there about a week, which was a lot better than most of the other immigrants did, but it didn’t suit him and it didn’t suit Molly and it didn’t really suit me.