 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).
<clarity>  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).
 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 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).