We ran into Pelican Pete (not literally) at one of the far flung ports of the world, 
 and we are doing a story on Celestial Navigation,
 so we asked him if he has any preferences on how he does it.

 Pete replied, “Everyone has their own favorite method, including me.
  Of course I wouldn't advocate my way, but sometimes it's interesting to see
  how the other guy does it, so here it is:”

  I will mention the Intercept Method first, since I use it most often.
  It is the most complicated, but also the most accurate,
  and versatile method.  

  For the new navigators, the Intercept Method uses spherical trigonometry
  to solve 2 triangles.  These are the Astronomical and Terrestrial Triangles.
  The celestial body is one apex of the Astronomical Triangle, 
  and the sextant is used to measure the angle of the body above the horizon.

 Normally I do my sights at morning and evening twilight. 
 This is when you can see the celestial body and the horizon, at the same time.  
 If you are not moving too fast, the times of twilight will not change
 much from one day to the next, so after a couple of days at sea,
 I know when twilight is.

 For economy, I use plastic sextants which go out of alignment depending on
 temperature changes and which way they are stowed.  
 So before a round of sights, I always set the Horizon and Index mirrors
 to 0 Index Error.  This way I don't need to mess with an Index Correction.  
 To further simplify things, I set my watch to GMT, 
 so that I don't need to do any time conversions.

 We will see the brightest body at the beginning of evening twilight, 
 and at the end of morning twilight.  In the evening, when the first 
 celestial body appears, I will be on deck with a watch set to GMT, 
 a sextant aligned to 0 Index Error, a pencil and paper. 
 I take a sight with the sextant, immediately look at the watch, 
 and write the name, sextant angle and time on the paper.  
 If I don't know what the body is, then I will also take a
 compass bearing of it, and write that down.  

 I repeat the process until I'm done taking sights.
 Sometimes I will continue taking sights, all around the horizon until 
 twilight is over.  In the evening twilight, the window of opportunity for
 taking sights fades when the horizon becomes too dark. 
 In the morning twilight, the stars and planets will fade until they are
 not bright enough to take sights.

 During morning twilight, the window of opportunity for 
 taking sights starts when the horizon is sharp enough to
 get a good sight.  Sometimes the Moon is bright enough to
 illuminate the horizon, especially just before dawn twilight, 
 so sometimes you can get a head start taking sights
 in the morning. If I'm in the middle of the ocean,
 far from rocks, I don't care about my exact position,
 so I'll probably sleep through dawn twilight. 


 The number of sights I take depends on the situation and the conditions.  
 By the time you realize that you need more sights, the twilight window is gone
 and you may not have an opportunity to get another fix until the next twilight,
 so I try to take more sights than I think I need.

  It's no surprise that when you need a fix the most, the conditions will be the worst.
  You can get a good sight on a dim star if you have a distinct horizon, 
  but you can't get a good sight on a bright star if the horizon is obscured.
  So the most detrimental condition to the sight accuracy is an indistinct horizon.

 For each celestial sight we get a Line Of Position (LOP), so we are going to need 
 at least 2 LOPs' to get a fix.  These should be as close to 90 degrees apart as possible.
 For example, if you took a sight somewhere to the north or the south, then you should
 take another one to the east or west, if possible.  In good conditions you can get star
 sights all around the horizon so that your plotting sheet looks like a star wheel.

 Naturally the conditions are not always that good.  Sometimes you can only see one
 bright body, occasionally through the clouds, or only one area of the horizon is visible.
 In these cases, you can use NavPak to do a running fix using the Advance LOP function.

Then we asked Pelican Pete:Can you give us a hypothetical situation of how you would use NavPak on an ocean passage?”

 Of course (no pun intended), Pete replied.  Before stepping on the boat, I would store
 the Lat/Lon of the departure and destination in NavPak, then with a couple of taps,
 I can get the Great Circle distance and direction of the voyage. 
 Every time we get a fix and update the Assumed Position, we get the updated
 direction and distance to the destination.

 In this hypothetical example, we depart into 5 days of head winds and heavy fog.
 Since there is no possibility of taking sights, we will tack onshore during
 the day and tack offshore during the night.  This is where we need a 
 traditional maneuvering board or NavPak to keep a reasonable Assumed Position.
 As we are beating to windward in the fog, we are estimating our average course and speed.
 Each time we tack, we use this info in the DR Calculator to update our Assumed Position.

 As you can see, the Assumed Position enters into the equation frequently.
 This is the traditional way to do Celestial Navigation.  In summary we use our
 Assumed Position to do a round of sights, then the Fix we obtain from the sights
 becomes our new Assumed Position.

 The DR Calculator projects a position relative to your Assumed Position using the
 estimated distance and direction traveled.  To minimize entering numbers,
 the relative position can be saved as the new Assumed Position with one tap in
 the DR Calculator.  Also the Fix obtained on the Plotting Sheet can be saved as the 
 Assumed Position.

 Using NavPak Android, I would enter the first Assumed Position at the exit of the harbour,
 since we are in heavy fog.  This is our last known fix.  From here, we can use the
 DR function in NavPak and all we have to do is enter the estimated course and
 distance of each tack or leg to maintain a best known or Assumed Position.

 After 5 days of tacking in the fog, our best guess Assumed Position may be way off,
 but it is the best we have, so it doesn't get any better than that.
 When the fog clears enough to take sights, your Assumed Position is already stored in
 NavPak, so all you have to do is enter new sights and plot a new fix.
 This then becomes your latest Assumed Position and the process repeats.

 Pelican Pete!  You're all over the map with your scatter brained responses. I see, I guess that's why I keep ending up in far flung ports.”

 Can you give us something coherent for our audience?Yes, good idea” , Pete replied.

When you leave the harbour, you may not reach your destination, 
whether it be because of weather, mechanical breakdown or other acts of God,
 so be prepared  with extra charts and supplies.

   To be continued...