Published online by Cambridge University Press: 18 January 2010
In this note, we propose to enquire into how far it may in principle be possible to employ zenith-photography for astro-navigation in the air, also to touch on the question of whether the mechanical difficulties can be adequately overcome. The question of principle seems in fact quite clear, but the practical side will undoubtedly demand much care and study by engineers and by practical navigators if the method is ever in fact to be adopted; we hope to show that the method has advantages sufficient to make the problem worthy of attention.
* It should be noted that we are definitely projecting a sphere, not a spheroid ; degrees of declination are equal everywhere, and equal to degrees of equatorial right ascension. The terrestrial scale (about 1 : 4,400,000) is thus not quite constant as between maps at different latitudes, or as between different directions at the centre of any given map ; but the correspondence between points on the celestial sphere and on the terrestrial spheroid is of course exact, just as in any other method.