FORBIDDEN WORLD

31.

“Golan,” said Pelorat, “Does it bother you if I watch?”

“Not at all, Janov,” said Trevize.

“If I ask questions?”

“Go ahead.”

Pelorat said, “What are you doing?”

Trevize took his eyes off the viewscreen. “I’ve got to measure the distance of each star that seems to be near the Forbidden World on the screen, so that I can determine how near they really are. Their gravitational fields must be known and for that I need mass and distance. Without that knowledge, one can’t be sure of a clean Jump.”

“How do you do that?”

“Well, each star I see has its co-ordinates in the computer’s memory banks and these can be converted into co-ordinates on the Comporellian system. That can, in turn, be slightly corrected for the actual position of the Far Star in space relative to Comporellon’s sun, and that gives me the distance of each. Those red dwarfs all look quite near the Forbidden World on the screen, but some might be much closer and some much farther. We need their three-dimensional position, you see.”

Pelorat nodded, and said, “And you already have the co-ordinates of the Forbidden World—”

“Yes, but that’s not enough. I need the distances of the other stars to within a percent or so. Their gravitational intensity in the neighborhood of the Forbidden World is so small that a slight error makes no perceptible difference. The sun about which the Forbidden World revolves—or might revolve—possesses an enormously intense gravitational field in the neighborhood of the Forbidden World and I must know its distance with perhaps a thousand times the accuracy of that of the other stars. The co-ordinates alone won’t do.”

“Then what do you do?”

“I measure the apparent separation of the Forbidden World—or, rather, its star—from three nearby stars which are so dim it takes considerable magnification to make them out at all. Presumably, those three are very far away. We then keep one of those three stars centered on the screen and Jump a tenth of a parsec in a direction at right angles to the line of vision to the Forbidden World. We can do that safely enough even without knowing distances to comparatively far-off stars.

“The reference star which is centered would still be centered after the Jump. The two other dim stars, if all three are truly very distant, do not change their positions measurably. The Forbidden World, however, is close enough to change its apparent position in parallactic shift. From the size of the shift, we can determine its distance. If I want to make doubly certain, I choose three other stars and try again.”

Pelorat said, “How long does all that take?”

“Not very long. The computer does the heavy work. I just tell it what to do. What really takes the time is that I have to study the results and make sure they look right and that my instructions aren’t at fault somehow. If I were one of those daredevils