"Uh-oh, kids. Dad's in robot mode," Francine called from the bedroom. "What did Chris really say?" She followed him down the stairs and onto the lawn, where he pressed the telescope base into the soft grass and soil.

"That's what he really said," Arthur replied, dropping the big red ball of the reflector gently into the three hollowed branches of the base.

Gray, dignified Grant and lithe blond Danielle stood by the railing on the east side of the rear deck, overlooking the yard and the plum tree. "It's a lovely night," Danielle said, holding Grant's arm. Arthur thought they most resembled models in upscale real estate ads. Still, they were good people. "Stargazing?"

"It's not secret or anything, is it?" Francine asked.

"I truly doubt such a thing can be kept secret," Arthur replied, peering into the eyepiece.

"One of Jupiter's moons has disappeared," Francine called up to them.

"Oh," her sister said. "Is that possible?"

"We have a friend. An acquaintance, really. He and Arthur keep each other up-to-date on certain things."

"So he's looking for it now?" her sister asked.

"Can you see Jupiter from here? I mean, tonight?" Grant asked.

"I think so," Francine replied. "Europa is one of the Galilean moons. One of the four Galileo saw. The kids were going to—"

Arthur had Jupiter in view, a bright spot in the middle of the blue-gray field. Stars formed a resolving fog in the background. Two pointlike moons, one bright and one quite dim, were clearly visible on one side of the brighter planet. The dim one was either Io or Callisto, the bright probably Ganymede. The third was either in transit across the planet or in Jupiter's cone of shadow, eclipsed —or behind the planet, occulted. He tried to remember Laplace's law regarding the first three Galilean moons: The longitude of the first satellite, minus three times that of the second, plus twice that of the third, is always equal to half of the circumference ... He had memorized that in high school, but it did him a fat lot of good now. He murmured the consequences of the law to himself: "The first three Galileans—that includes Europa—can never be all eclipsed at once, nor can they all be in front of the disk at once. If Io and Europa are eclipsed or occulted simultaneously, or in transit simultaneously . . . Ah, hell." He couldn't remember the details. He would just have to sit and wait for the four to be visible all at once, or for only three to make an appearance.

"Can we see?" Marty asked.

"Sure. I'm going to be out here all night, probably," Arthur said.

"Not Becky," Danielle said.

"Oh, Mooommm! Can't I see?"

"Go ahead," Arthur encouraged, leaning back. Marty squatted next to the telescope and showed his cousin how to look into the eyepiece. "Don't knock it," Arthur warned. "Franchie, can you get me the field glasses?"

"Where are they?"

"In the hall cupboard, above the camping gear, in a black leather case."