GREAT CIRCLES & WAY POINTS

I saw Pelican Pete at the Russel Yacht Club* Pot Luck dinner.  He was 
hungry and just sitting down to eat, so this seemed like a perfect 
time to ask a question about NavPak Android/Kindle edition.

“Hey Pelican Pete!  There are no way points in your Android app.  Most 
 navigation programs will spit out a list of way points on your 
great circle course.  What's up (Wusup) ?”

“I only use one way point and call it the destination (DEST).  Once the 
boat drifts sideways off the great circle course then that course is 
trash.  What you then need is a new great circle between your Assumed 
Position and the destination.  There is only one shortest distance to 
your destination and that is the great circle that you want.  It would be 
counter productive to try and make it back to a way point on the original 
great circle.

So our real great circle to the destination changes every time the boat 
moves sideways off course.  What we need is a quick way to get the 
great circle course every time we get a fix or update the Assumed 
Position (AP).  This is easy to do with NavPak.  Just store your 
destination then every time you update your position, you can get a new 
distance and direction to your destination with just a couple of taps.

Before the computer age, the navigator would draw a great circle on the 
chart or plotting sheet using the Sight Reduction Tables.  This was a 
tedious process where each vertex would need to be manually reduced in the 
same way as a celestial sight, or he could use a Gnomic Projection chart 
to figure the vertices.

In either case, it was complicated to change the great circle everyday,
  so the voyage would start with a pre-planned great circle and hopefully 
the ship could follow it all the way across the ocean.  Now that we have 
computers to do the boring tedious work, we can have a new great circle
 when ever we want it.
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* Russel is in the Bay of Islands, New Zealand. During this time of year, 
the Bay of Islands probably has the biggest conglomeration of foreign 
cruising yachts anywhere in the world, as they all try to get here by 
November to escape the Cyclone season further north in the South Pacific.
Most of these yachts will stay in NZ until April or May which is the end 
of Cyclone season in the southern hemisphere.

   To be continued...