NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Long and Time at Sea
From: Lu Abel
Date: 2005 Jun 4, 13:56 -0700
From: Lu Abel
Date: 2005 Jun 4, 13:56 -0700
Gordon Talge wrote: > The "Noon Sun Shot" was a staple. However, due to the fact that the > Sun tends to "hang" in the sky on the meridian with little apparent > change in altitude means that finding longitude by noting the time > of passage, while in theory should work, in reality is not practical. > > One method has the observer, taking a shot at maybe 10 minutes before > LAN and noting the altitude and the watch time, doing a noon shot > and resetting the sextant to the altitude that was noted 10 minutes > before and watching the Sun until it is at this same altitude and > noting the watch time. Since the Sun must have been at LAN midway > between these two times, you then calculate that time and using the > equation of time from the NA get your longitude. I don't know how many on this list have actually taken a noon sight. I have. A noon sight is part of the requirements for US Power Squadron's Navigation course (advanced celestial nav) which I have taken. I found (at ~40N Lat, sight taken during the early summer) that not only did I get a spot-on latitude value, but by graphing a series of sights over about 10 minutes (5 min on either side of LAN) I got quite an excellent value for longitude. The sun being "nearly stationary" in the sky is relative. For an early navigator using a backstaff and lucky to get latitude to 1 degree, the sun truly appeared to be hanging in the sky at LAN. But looking at it through a modern sextant, I found the vertical movement around LAN quite perceptible. I also freely confess this sight was taken on dry land, so I wasn't dealing with trying to bring down the sun on a heaving ship's deck. Lu Abel