"Oh, no," drawled Biggs nonchalantly. "It's still there."
"Still there!" I looked again, more closely, at the vision plate. It was as bare as a debutante's backbone at a ball. "What's still where, Lanse? Have you gone off your gravs, or are my optics myopic?"
"The large body," said Biggs blandly. "It's Themis' moon. It's there. Three points to starboard, and one degree to loft."
"Themis' moon!" croaked the Old Man. "What in Hades are you talkin' about, Lancelot! Themis is a moon!"
"I know," agreed Biggs. His larynx bobbled pleasantly. "That's the curious part of it. This is the first time in the solar system that any satellite has ever been found to have a satellite of its own! But we've located it, charted its trajectory, and cross-checked our calculations—haven't we, Dick?"
Todd looked up from the plot table.
"That's right," he said hollowly. "Themis has got a moon of its own. An—an invisible moon!"
"Invisible moon!" The skipper and I did a twin act.
"Yes," said Biggs. "You know, I believe that's why Themis—er—'disappears' periodically. It is circled by a large, opaque satellite with the peculiar property of being able to bend light waves around itself. Consequently, every time the moon, revolving around its primary, comes between Themis and observers, Themis is occulted—and disappears!"