If astronomical navigation is only required to give a ten-mile accuracy, as Mr. Parker has suggested in his paper, then its use will be mainly that of a last-aid in emergency. Radio, however, will give this degree of accuracy and, since a radio operator will in any case be carried, will naturally be more popular. On the other hand if astronomical navigation can be made to give a ½-mile accuracy it will become immensely more valuable than any other long-range aid. In the first place the signal received from the stars is completely reliable (at the heights which concern the air navigator, variable refraction can be ruled out) ; it is wholly independent of the ground; it provides a universal coverage which is free and is controlled by a non-political Body; and it can provide an accuracy greater than that of the long-range aids except the limited-purpose systems such as Oboe.