before_revision: Noon sights obtain the observer's Latitude. It is impossible to determine longitude with an accuracy better than 10nmi by means of a noon sight. A noon sight is called a Meridian Altitude.Basic Principles of Marine Navigation by D A Moore Published by Kandy p81 While it is very easy to determine the observer's latitude at noon without knowing the exact time, longitude cannot accurately be measured at noon. At noon the sun's change of altitude is very slow, so determining the exact time that the sun is at its zenith by direct observation is impossible, and therefore it is impossible to obtain an accurate longitude at the moment of Zenith . However, it is possible to determine the time of zenith for longitude with a useful accuracy by performing a mean time of observation when the sun is on its ascent and descent prior to and following its moment of Zenith . By taking a sextant reading within 15 to 30 minutes prior to local noon ( zenith ) and noting the time, then leaving the sextant set to the same angle and subsequently observing the moment in time at which the sun passes through the sight tube on its descent from Zenith between a half-hour and hour later, the two times can be averaged to obtain a longitude sufficiently accurate for navigation (within 2nmi).American Practical Navigator (bowditch) by Nathanial Bowditch p253
after_revision: Noon sights obtain the observer's Latitude. It is impossible to determine longitude with an accuracy better than 10nmi by means of a noon sight. A noon sight is called a Meridian Altitude.Basic Principles of Marine Navigation by D A Moore Published by Kandy p81 While it is very easy to determine the observer's latitude at noon without knowing the exact time, longitude cannot accurately be measured at noon. At noon the sun's change of altitude is very slow, so determining the exact time that the sun is at its highest by direct observation is impossible, and therefore it is impossible to obtain an accurate longitude at the moment of culmination . However, it is possible to determine the time of culmination for longitude with a useful accuracy by performing a mean time of observation when the sun is on its ascent and descent prior to and following its moment of culmination . By taking a sextant reading within 15 to 30 minutes prior to local noon ( culmination ) and noting the time, then leaving the sextant set to the same angle and subsequently observing the moment in time at which the sun passes through the sight tube on its descent from its highest altitude between a half-hour and hour later, the two times can be averaged to obtain a longitude sufficiently accurate for navigation (within 2nmi).American Practical Navigator (bowditch) by Nathanial Bowditch p253
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