NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robin Stuart
Date: 2025 Dec 21, 11:48 -0800
David C,
Equal altitudes for longitude was the method used by Major John Wesley Powell during his 1869 expedition through Grand Canyon. Morning and afternoon sights were separated by about 4 hours because closer to noon the Sun was beyond the reach of his sextant using an artificial horizon. He took a large number (up to 20 sights) in the morning and afternoon (see attached).
I tried this method in 2020 with some success. Although I did not take as many sights as Powell with practice I was able to get within 0.1-0.2 NM in departure from my location.
The method has some advantages when it comes to sight reduction. Although it may seem that there is a lot of averaging to be done across the sights you really only have to deal with the seconds. There is no need to correct for dip, refraction or semi-diameter. The only thing that needs to be accounted for is the Sun's changing declination...but not today!
A fuller explanation can be found at https://navlist.net/Longitude-Equal-Altitudes-Stuart-jul-2020-g48277
Robin Stuart
