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Hi again Renee

Recently I posted to the Nav List, in answer to your query:
> >  Peter -- I disagree on the solution to part 5
> >   You can't set
> > the Intercept to 0.
my response:
> My calculated intercept via one method, capable of resolving this to the
> nearest minute of arc, was zero.
> I've now checked this with another method (don't be lazy myself!) and
> agree with your answer.

I've had another look at this Question 5. The time given is within a couple
of seconds of local noon, so treating it as a meridian passage of the sun
gives a latitude of N39d 46' which is the DR latitude, so via a sight
reduction method a zero intercept, as my original answer. So I've redone the
sight reduction two ways and confirmed the zero intercept, meaning the
computed altitude and the (corrected) observed altitude are both 54d 58' (to
the nearest minute of arc).
What intrigues me now is how I got an identical answer to yours, quickly
using my nav. calculator. I can't get it again!

Here are my steps for a noon sight:

sextant altitude   54 48.1
dip                          -2.9
index error              -2.5
apparent alt        54 42.7
alt corr                  +15
observed alt        54 58 (rounded to nearest minute of arc)

                           89 60 (another way of expressing 90d)
                         -54 58
 zenith distance    35 02
declination        + 04 44
=  latitude            39 46

Could you check your calculations for Q5, perhaps also work the problem as a
 noon sun sight, and let me know the results? I'm still not sure I
haven't made a mistake somewhere.

It would be good to check our answers against the official ones, but despite
a posting by Dan Hogan (the author and webmaster) that "ssea.zip has been
upgrades to include Leg 83" I can't find a link on http://www.wa6pby.com/
that leads to the Silicon Sea archive. In any case, it apparently won't
include Leg 88. I've had a look for the answers on the Nav List archives and
can't find them there either. Were they ever posted?

Some more comments on your answers follow:

> > 1) DR #1 39d39.4' N 154d 42.4'W

This is identical (to the nearest tenth of a minute of arc) to the most
accurate method I have, using both meridional parts and distances. My answer
was rounded to the nearest minutes of arc.

> > 3)
> >   78.5d, 1652 nmi via the Rhumb Line
> >   67d, 1639 nmi via the Great Circle. (67d for first leg.)

By comparison, my answer to nearest tenth of degree or nautical mile
078.8d for 1652.2nm rhumb line
066.6d (initial) for 1634.1nm great circle
is slightly different, implying that you used a different method for these
calculations?

I note you are using mainly electronic solutions. You might like to consider
having, at least as a back-up that needs no batteries or internet
connection, a manual alternative that only requires pencil and paper. Its
good to be able to check results from any method with another independant
method.
If you're interested I have a suggestion as to a reasonably simple paper
alternative (including "celestial data") to electronic calculations.

In any case, for "a self-taught beginner" you're not doing too badly.

Peter Fogg

   

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