NavList:

   

Reply

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

   

Reply