A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Hanno Ix
Date: 2015 May 14, 10:06 -0700
by a number of repetitions.
Hanno, you wrote:
"In theory, can I determine my longitude by he time of the rising / setting of the moon at my location?"
In principle, it's a special case of longitude by lunar altitude instead of the standard "lunar" employing lunar "distances" from other celestial bodies. This lunar altitude method has been re-discovered numerous times and publicized as something that might make longitude by the Moon accessible to a broader group of navigators. I know of a paper from the 1840s describing this technique, and in the 1960s John S. Letcher invented a version which he wrote up in his excellent book "Self-Contained Celestial Navigation with H.O. 208". The trouble with using lunar altitudes instead of lunar distances is that it introduces the sea horizon into the problem, and the sea horizon is uncertain at a level of about 0.25' - 0.5' in good conditions and worse under difficult conditions. The rising or setting of the Moon could serve as a sextant-free surrogate except for one little detail: the Moon is almost impossible to see when it's right at the horizon. Even under the best of circumstances with a full moon and excellent sky transparency right down to the horizon, the lowest portion of the Moon becomes lost in extinction near the horizon. The rising or setting Moon could serve as a crude measure of longitude, but it's of no practical use (and was not even considered as a backup historically).
Conanicut Island USA