NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Old style lunar
From: Fred Hebard
Date: 2004 Dec 9, 17:24 -0500
From: Fred Hebard
Date: 2004 Dec 9, 17:24 -0500
On Dec 9, 2004, at 3:29 PM, Ken Muldrew wrote: > Longitude (from lunar distance): > 1800 17-Apr 115?12'00" > 18-Apr 114?57'45" > 22-Dec 115?11'00" > 1801 17-Feb 114?57'15" > 28-Feb 114?52'15" > 28-Feb 114?59'45" > 1-Mar 115?11'00" > 18-Mar 114?44'15" > 17-Feb 114?39'00" > 24-Feb 114?36'00" > 24-Feb 114?13'00" > 25-Feb 114?28'30" > 25-Feb 114?26'45" > > Thompson's average position: .... > 114?48'20" > > true position of Rocky Mnt. House: .... > 114?58'50" > > You can see that the spread of lunars covers a full degree but his > final > position was pretty close. As far as I have been able to see in his > journals, Thompson always updates his account when he takes a latitude > or > longitude reading. In addition, he updates all the entries in his > account > log proportionally since the last reading to correct for a systematic > bias > in his reckoning. He took this latter step because he was intending to > map > everywhere he travelled, but he clearly put more faith in his celestial > observations than did many of the ocean navigators that Frank has > written > about. > Actually, I think the spread of positions in the data illustrates precisely why sailors could not put great faith in lunars while underway: they weren't in the same place twice. But did Thompson measure more than one distance for every observation, so that each point is the average of several distances? Also, perhaps Thompson was not the best of lunarians. Kieran Kelly last year or so posted a series of lunar observations taken by an Australian explorer that were exquisite. I can't imagine that the distance cleared from those would have been in error by more than 0.1 or 0.2' of arc, given an accurate sextant. Fred