"Then let me show you the elecalc."
"What?"
"Elecalc. Short word for electronic calculator. I'm here to get an aiming point for my trip tonight."
"Now that I would like to see," said Nora, hooking an arm in Paul's.
[CHAPTER 5]
"I'm pointing for Alpha Centauri," said Paul. "And so that's what we calculate for."
Nora looked at the bays of neat equipment and shook her head. "Why not aim at it and run?" she asked. "Surely you do not need this billion dollars' worth of stuff to point out your destination."
"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."