The Longitude Problem

  • F. G. Major


In our outline of the methods of celestial navigation in the last chapter, one essential input that is often glossed over is the precise time when the observations are made. The fundamental fact is that to navigate on a global scale, the only recourse up till very recently is to deduce positions on the earth by reference to observations of the celestial bodies. The modern global navigation satellite systems will be discussed in later chapters. On a human scale we cannot span the globe just by dead reckoning or some form of geodetic triangulation; we have to rely on celestial observations. This would not be so bad, were we not on a spinning observation platform and the celestial globe appears to be in constant rotation. Since an observer’s angular position about the earth’s axis is his longitude, a consequence is that a determination of longitude is inherently a matter of precisely tracking that apparent rotation in time. It is ironic that the very cyclical motion of the sun and stars which have historically been the means of keeping time is the same motion that requires us to have the ability to transfer time in order to navigate; that is, we need a precise clock. If the local time at some fiducial point on the earth (e.g., Greenwich) is available at any point on the earth’s surface as carried by a stable clock, then a measurement of the local time at that point will yield directly the longitude of that point. Since the earth turns through 360° in 24 h, it follows that every hour difference in local time corresponds to a difference of 15° in longitude.


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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  • F. G. Major
    • 1
  1. 1.Severna ParkUSA

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