ASSUMED POSITION

I spotted Pelican Pete walking back to his boat from the Super Market
 with a box of groceries.  You can always tell the cruising sailors  
 in town because they pay cash for everything and walk everywhere they go.

“Pelican Pete!  I've been trying to figure out how to use NavPak Android 
for celestial navigation, and I noticed that the Assumed Position is
 required everywhere.  I thought celestial navigation was the art/science 
of finding your position.  Does this imply that I need to know my
position to find my position?”

Pete managed to reply, “Sort of.” as he tripped over a line on the
 dinghy dock.

“The Assumed Position (AP) only needs to be an estimate within 2 or 3 
degrees”.  Navigation using the Intercept Method is a game of comparison.  
The process is to measure the actual Altitude of the body with the sextant,  
and then calculate the theoretical Altitude from the AP.  Then compare
 the two Altitudes and get the Intercept.”

“What if your AP is more than 2 or 3 degrees off?”  I asked.

“Then your Lines of Position will start falling off the edge of the
 Plotting Sheet in the same way that a ship will fall off the edge 
of the world if it goes too far offshore.  If this happens, the Azimuth 
will point to which way it went.

When you open the Plotting Sheet in NavPak Android, it will be preset 
to 10 x 10 degrees with your AP at the center.  That's about a 600 x 600 mile 
area at the equator.  If your Intercept is more than 300, or less than -300,
then the sight may fall off the edge of the plotting sheet.

If you solve and save a sight and it does not appear on the Plotting Sheet  
then the AP is probably way off.  The Ocean Maps in NavPak make it 
easy to estimate your AP within 2 or 3 degrees.

AP is also used to set the area of the Sky View and for the pre-computed 
Azimuths and Altitudes in the Almanac function.  The Star Chart shows your 
AP.  This is the point straight overhead.  And the Globe View shows the AP.

There will always be distortion between the flat plotting sheet and the 
spherical Earth, so you should try to keep the AP reasonable to 
minimize this error.

When you start calculating your sights, you will see excessive Intercepts 
and which direction they are off, so that you can adjust your AP accordingly.”



To be continued...