Next message: Frank Reed: "Re: Latitude and Longitude by "Noon Sun""
Robert E you wrote:
"Other entry arguments include the relative bearing of the object being
sighted. I believe that some time ago, I posed a question to this list on whether
or not there is a similar set of tables for the more sedate speeds as
experienced by the sea-going navigator."
It's simple enough in this case that you really don't need a table. You can
get the N/S component of your vessel's speed by drawing a simple triangle on
a chart or a piece of graph paper or by doing a basic trig problem (I prefer
drawing it). And the rest is just proportioning out the correction: a tenth
of a minute change in altitude per knot for every six minutes away from noon
(or other arbitrary starting point).
And:
"But this also begs the question: does this correction really matter for the
surface navigator who is not likely to be moving faster than 7 - 10 knots?
Having myself, taken numerous sights at sea, including noon sights (just in
case George questions my experience!) it seems to me that this factor is not
terribly significant. Or is it? "
Let's suppose the Sun's altitude is 60d 10' twenty minutes before noon and
60d 30' at noon. If we're motionless then it will be very close to 60d 10'
twenty minutes after noon (could be as much as 0.7 minutes different close to
the equinoxes). Now if I'm sailing towards the Sun's latitude at 10 knots, then
these altitudes will shift by (nearly) 6.6 minutes of arc because that's how
far we sail in 40 minutes at that speed. That is, we would measure 60d 7.7,
60d 30', and 60d 13.3. So yes, it matters.
And:
"you'd been doing me a big favour if you could provide me (us) with a
practical example of technique which you have described, using some real numbers
and speeds. "
OK. I've got a bunch of simulations of this with varying degrees of "noise"
in the sight data that I've used for experimentation. I'll write it up when I
get a chance.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
