NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Position by compass variation
From: Frank Reed CT
Date: 2006 Nov 27, 00:21 EST
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Frank Reed CT
Date: 2006 Nov 27, 00:21 EST
Jack Ganssle wrote:
"And, how, especially long ago, did mariners determine variation with any
sort of precision?"
One of the most common daily astronomical observations from two centuries
ago has completely vanished from the navigator's modern toolkit. It was the
measurement of the Sun's amplitude. At sunrise or sunset, the Sun's azimuth can
be read off to the nearest degree or two from the compass. For a known latitude
and a rough estimate of longitude (to get the declination right), the actual,
true azimuth of sunrise or sunset can be calculated rather accurately,
though refraction complicates things. Compared with the magnetic azimuth, the
difference between observation and calculation is the variation (landlubbers
like geologists, and just about everyone except navigators, call this difference
in direction the magnetic "declination", which proves that two different
groups of users can invent equally bad names for the same phenomenon
<g>).
Records of magnetic variation in old logbooks often provide good examples
of excessive precision. It's not uncommon to see something like "Var. per Sun
Ampl.: 14d 22' ". They carry the calculation to the minute of arc because they
don't realize they shouldn't.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---