Longitude Without Time

Abstract

Specialized tables making possible time or longitude determination by lunar distances were eliminated from the Nautical Almanac in the early part of this century. However, in an emergency, the present day navigator's tables can readily be used for this purpose. Either the altitudes of the moon and another body or the arc between the moon and another body near the same vertical circle are observed. Examples are given which can be followed by the practicing navigator. These methods and a plotting method which Sir Francis Chichester recently described1 are analyzed for accuracy and convenience. Most of the observational accuracy can be preserved except that the plotting is difficult. It is shown that when the azimuths of the two bodies are close or when the bodies are near the same vertical circle, certain complicating observational errors and corrections and the great sensitivity to precise knowledge of latitude are minimized. There are “null” azimuths to be avoided. A sextant accuracy for the readings of 0.2′ and tables to the closest 0.1′ should at best give an RMS longitude accuracy of ±13′, and this is borne out in practice.