"We do," objected Paul. "You see, if I took off with my telescope pointed along the axis of drive with the cross-hairs pointing at Alpha Centauri, I'd be heading for the star where it was four years ago. I intend to be on the way for nine days. So I'll want to point the nose of the ship at the spot where Alpha will be nine days in the future instead of four years in the past. Since Sol and Alpha drift in space, the motion and velocities of both systems must be taken into account, a correction-angle found, and then used to aim my ship. My telescope will angle away from the ship's axis by that correction-angle. Add to that the fact that I am taking off from earth, which will give me some angular velocity and some rotational velocity differing from an hypothetical take-off from Sol itself. Furthermore, I want Proxima Centauri instead of Alpha, and that must be taken into consideration too. "Then there is the question of velocity and time. We cannot see when we are exceeding the velocity of light. We must run at so many light-velocities for so many seconds. A minor error in timing or velocity will create a rather gross error in position at the end of the run." "Oh," said Nora, but her tone indicated a lack of comprehension. Paul smiled. "One second of error at one light-velocity equals a hundred and eighty-six thousand miles of error. One second of error at a hundred times the velocity of light equals eighteen millions of miles error. Not much as cosmic distance goes, but a long way to walk." Nora understood that, and said so. Grayson's data had been handed to him by Haedaecker. But apparently the long-range calculations that had been temporarily halted were still halted, for the operator was taking this opportunity of feeding some future flight-constant information to the big machine. It was not a complex calculation as some computations may go, but there were a myriad of factors. Terran latitude and longitude, the instant of take-off as applied to the day and the year, an averaging of previous dispersion-factors noted from previous trips, velocities, vector angles, momentum, and others, all obtained from tables and entered in the machine before the start lever was pulled. The machine mulled the information over, tossed electrons back and forth, chewed and digested a ream of binary digits, and handed forth a strip of paper printed with an entire set of coordinates in decimal angles. Paul showed Nora his folder of data. "The rest," he said, "is up to me." He led Nora from the computation room, intending to hit the dining room for coffee. Half-way across the lobby of the administration building he was hailed by the autocall. He went to the telephone. It was Stacey. "I've a couple of things to let you think over," said Stacey. "So?" "I've been on the job. I haven't much, but there are a few items you might mull over." "Shoot." "One. Your deceased thief was an ex-convict, escaped from the penal colony on Antarctica."