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Thanks again for the east longitude practice, I've had to totally
re-think how I normally work these problems (including stopping to
think at every step about how the DR position relates to Greenwich, and
whether to add or subtract the corrections to/from local time). But
that's A Good Thing (at least I think it is...  ;-)).

Anyway, at first glance this looks like a 2-part problem (guesstimating
a DR position for the ship at sunset, and then calculating a time for
sunset/twilights for taking the star sights)- but since the DR estimate
for 1900 falls nicely around the end of of Nautical Twilight I would
just use the DR position for that time as the basis for the
calculations.

And even on a TC of directly south (180deg T), the ship's position is
only changing about half a degree of latitude per hour - probably not
enough to greatly affect our calculated times (unless the teacher
insists on split-second accuracy...  ;-))

Using the handy Arc to Time converter in the NA, 145deg 22' E yields a
time correction of 09h 41m 28s.

Interpolating the Sunset, Civil Twilight, and Nautical Twilight times
for N 10deg and N 20deg latitude for the 3-day period including 7 May
2008 (at Greenwich):

  SS: 18:18 Z
  CT: 18:41 Z
  NT: 19:08 Z

Applying the Arc-to-Time correction to get local time:

  SS: 20:37 (Local)
  CT: 21:00 (Local)
  NT: 21:27 (Local)

And applying the ZD-10 correction to get Z time for this location:

  SS: 10:37 Z
  CT: 11:00 Z
  NT: 11:27 Z

Now we need to calculate LHA Aries for the time we're going to be doing
the star sights. Since the time for Civil Twilight falls neatly on an
exact hour, we can just look up GHA Aries from the NA daily page (and
don't have to do the correction for minutes/seconds)

  GHA Aries: 30deg 39.6'

Using an AP of 14deg N / 145deg 20.4' E, we add GHA Aries to our
Assumed Longitude to get:

  LHA Aries: 176deg

Now using Pub 249 Vol. 1 (Epoch 2005.0), for Latitude 14deg N and LHA
Aries 176deg we get this list of stars to shoot (ALL CAPS = first
magnitude stars, [x] =  stars suitable for a 3-star fix):

  Star      Hc         Zn
  ------------------------
  Dubhe     41deg 44'  354   [x]
  ARCTURUS  53deg 19'  077
  SPICA     54deg 24'  134   [x]
  ACRUX     12deg 25'  175
  Suhail    22deg 25'  210
  PROCYON   29deg 16'  268   [x]
  POLLUX    33deg 10'  295

There's also a correction for precession and nutation of 03' / 110deg T
to be applied to the LOPs and/or fix.

--
GregR


--- Anabasis  wrote:

>
> Exercise #9 Star Time.
>
> A ship is steaming South (180deg True) at 14.2 knots.  The navigator
> predicts a 1900 (ZD-10) DR position of Latitude 14deg 10'N, Longitude
> 145deg 22'E on 7 May 2008.  Determine time of Sunset, pick a star
> time; and select some bodies to observe for PM stars, giving altitude
> and azimuth for each.  (please include method used to select stars
> and/
> or planets).
>
> Jeremy
>
> PS The next exercise will be a star fix around this time.
> >
>


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