My first reaction on being invited by the Editor to contribute a ‘twenty-first birthday’ article on the above subject for the Journal was that there is little new to say. However, it is now 13 years since I prepared the presidential address on ‘The Place of Astronomy in Navigation’ (this Journal, 9, 1, 1956) and, apart from any other developments, space navigation has since become established as an operational science as well as a theoretical discipline. There have, in fact, been many developments and much progress in these 21 years; and, although these appear in the Journal and elsewhere mainly as gradual changes, a comparison over the whole interval reveals drastic changes, particularly in outlook and appreciation.