The commander's voice sounded ringingly in all helmets: "Let's get to work, men. Jupiter will stay where it is. Look at it later."
For hours the ship was unloaded, equipment was set up, tents unfurled. Temporary air tights were prepared for possible use as oxygen-supplied headquarters outside the ship.
The men were not to be kept from watching the unusual sky, though. As it happened, all three of Jupiter's other large satellites were in the sky.
Europa was closest, appearing somewhat smaller than Earth's moon. It was a crescent, near the eastern horizon. Ganymede, appearing smaller still, was nearer zenith and half full. Callisto, only a quarter the width of Earth's moon, was nudging close to Jupiter and, like Jupiter, was some two thirds full. All three together gave not one quarter the light of Earth's full moon and were completely inconspicuous in the presence of Jupiter.
Bigman said exactly that.
Lucky looked down at his small Martian friend after having studied the eastern horizon thoughtfully. "You think nothing could beat Jupiter, do you?"
"Not out here," Bigman said stoutly.
''Then keep watching," said Lucky.
In Io's thin atmosphere there was no twilight to speak of and no warning. There was a diamondlike sparkle along the frost-covered top-line of the ridge of low hills, and seven seconds later the sun had topped the horizon.
It was a tiny seed-pearl of a sun, a little circle of brilliant white, and for all the light that giant Jupiter cast, the pigmy sun cast much, much more.
They got the telescope up in time to catch Callisto vanishing behind Jupiter. One by one, all three satellites would do the same. Io, although it kept only one face to Jupiter, revolved about it in forty-two hours. That meant that the sun and all the stars seemed to march around Io's skies in those forty-two hours.
As for the satellites, Io moved faster than any of them, so it kept overtaking them in the race about Jupiter. It overtook the farthest and slowest, Callisto, most rapidly; so Callisto circled Io's heavens in two days. Ganymede took four days and Europa seven. Each traveled from east to west and each in due turn was to pass behind Jupiter.
The excitement in the case of the Callisto eclipse, which was the first to be witnessed, was extreme. Even Mutt seemed to be affected by it. He had grown increasingly used to low gravity, and Norrich gave him periods of freedom during which he floundered grotesquely about and tried vainly to inspect by nose the numerous strange things he encountered. And in the end, when Callisto reached Jupiter's glowing curve and passed behind, and all the men grew silent, Mutt, too, sat on his swathed haunches and, tongue lolling, stared upward at the sky.
But it was the sun they were really waiting for. Its apparent motion was faster than that of any of the satellites. It gained on Europa (whose crescent thinned to nothingness) and passed behind it, remaining in eclipse for something less than thirty seconds. It emerged, and then Europa was a crescent again, with its horns facing in the other direction now.
Ganymede had plunged behind Jupiter before the sun could reach it, and Callisto, having emerged from behind Jupiter, was below the horizon.
It was the sun and Jupiter now, those two.
The men watched greedily as the seed-pearl sun climbed higher in the sky. As it did, Jupiter's phase grew narrower, its lighted portion always, of course, facing the sun. Jupiter became a "half-moon," then a fat crescent, then a thin one.
In Io's thin atmosphere the sunlit sky was a deep purple, and only the dimmer stars had been blotted out. Against that background there burnt the gigantic crescent in the sky, bulging out toward the relentlessly approaching sun.
It was like David's pebble hurled from some cosmic slingshot toward Goliath's forehead.
The light of Jupiter shrank still further and became a yellowish curved thread. The sun was almost touching.
It did touch and the men cheered. They had masked their face-plates in order to watch, but now that was no longer necessary, for the light had dimmed to bearable dimensions.
Yet it had not vanished entirely. The sun had moved behind the edge of Jupiter but it still shone murkily through that giant planet's thick, deep atmosphere of hydrogen and helium.
Jupiter itself was now completely blanked out, but its atmosphere had sprung to life, refracting and bending the sunlight through itself and around the curve of the planet, a smoothly bending film of milky light.
The film of light spread as the sun moved farther behind Jupiter. It curved back on itself until faintly, very faintly, the two horns of light met on Jupiter's other side. Jupiter's vanished body was outlined in light and one side bulged with it. It was a diamond ring in the sky, big enough to hold two thousand globes the size of the moon as seen from Earth.
And still the sun moved farther behind Jupiter so that the light began to fade and grow dim, and dimmer, until finally it was gone and, except for the pale crescent of Europa, the sky was black and belonged to the stars.
"It will stay like this five hours," said Lucky to Big-man. "Then everything will repeat itself in reverse as the sun comes out"
"And this happens every forty-two hours?" said Big-man, awed. ''That's right," said Lucky.
Panner approached them the next day and called out to them, "How are you? We're almost done here." He spread his arm about in a broad circle to indicate the loan valley, now littered with equipment. "We'll be leaving soon, you know, and we'll leave most of this stuff here."
"We will?" said Bigman, surprised.
"Why not? There's nothing living on the satellite to disturb the stuff and there's no weather to speak of. Everything's coated for protection against the ammonia in the atmosphere and it will keep nicely till a second expedition comes round." His voice was suddenly lower. "Is there anyone else on your private wave length, Councilman?"
"My receivers don't detect anyone."
"Do you want to take a walk with me?" He headed out, out of the shallow valley and up the gentle slope of the surrounding hills. The other two followed.
Panner said, "I must ask your pardon if I seemed unfriendly on board ship. I thought it better so."
"There are no hard feelings," Lucky assured him.
"I thought I'd try an investigation of my own, you see, and I thought it safer not to seem hand in glove with you. I was sure that if I only watched carefully, I would catch someone giving himself away, doing something non-human, if you know what I mean. I failed, I'm afraid."
They had reached the top of the first rise and Panner looked back. He said with amusement, "Look at that dog, will you? He's getting the real feel of low gravity."
Mutt had learned a lot in the past few days. His body arched and straightened as he lunged in low, twenty-foot leaps, and he seemed to indulge in them for the sheerest pleasure.
Panner switched Ms radio to the wave length that had been reserved for Norrich's use in calling Mutt and shouted, "Hey, Mutt, hey, boy, come, Mutt," and whistled.
The dog heard, of course, and bounded high in the air. Lucky switched to the dog's wave length and heard Ms delighted barking.
Panner waved Ms arm and the dog headed toward them, then stopped and looked back as though wondering if he did right to leave his master. He approached more slowly.
The men walked onward again. Lucky said, "A Sirian robot built to fool a man would be a thorough job. Casual examination wouldn't detect the fraud."