Note 2 — Front matter
Reading edition
Navigation. Each ship has a reference sphere held rigidly by gyroscopes so that its great circle of galactic longitude is always parallel to the galactic equator. Its line of zeros is always parallel to the line joining Centralia, the central solar system of the galaxy, with the system of Vandemar, which is on its very rim. Thus, courses are expressed in galactic longitude and latitude, from 0 to 360 degrees in each circle.
Position is expressed in galactic coördinates of "x," "y," and "z." The origin is at Centralia, and the line of positive "x" is the above-mentioned Centralia-Vandemar line.
The position of the ship in the galaxy is known at all times by that of a moving dot in the tank. This dot is shifted automatically by calculating machines coupled inductively to the leads of the drives. When the ship is inert this device is inoperative, as any distance traversed in inert flight is entirely negligible in galactic computations. Due to various perturbations and other slight errors, cumulative discrepancies occur, for which the pilot must from time to time correct manually the position of the dot in the tank representing his ship.