BAROMETRIC PRESSURE & TEMPERATURE CORRECTIONS

We were strolling through a boat yard in Carins Australia,
 where we noticed the Pelican up on the hard for a bottom job.
 Pete's first mate was jumping ship with all her suitcases
 (apparently this was not the first time she has done this),
so we thought this would be a perfect time to ask Pelican Pete
about temperature and pressure corrections.

“Pelican Pete!  You didn't mention anything about temperature and
 pressure corrections.  Please don't tell me that you have been 
drifting around the Seven Seas without applying these corrections 
to your sights.”

“Yes, that's right, I don't use them”, Pete replied.
“For those who are interested in why, here's my opinion:”

The temperature and pressure corrections only have a significant 
effect on low Altitude sights below 10 degrees above the horizon. 

 These corrections relate to Atmospheric Refraction which is also
significant below 10 degrees.  So it is better to avoid taking sights 
below 10 degrees.

The Atmospheric Refraction and pressure/temp corrections are big
 and uncertain for low altitude sights.  You can see this at sunset.  
When the sun is more than 10 degrees above the horizon it looks normal,  
then it swells as it gets closer to the horizon, then just before it  
goes down, all kinds of optical distortions can happen, such as  
Japanese Lanterns and Green Flashes.  This huge distortion just 
before sunset and just after sunrise will make your sight nearly useless,
with or without temperature and pressure corrections.


Sometimes the only sight you have may be 1 or 2 degrees Altitude 
(just above the horizon).  In this case, you can combine the 
corrections to your Sextant sight (Hs, Height sextant) as explained
in the Nautical Almanac, but keep in mind that this sight may be way off.


I was shocked that Pete was throwing these corrections overboard, 
when they seem to be somewhere in the middle of most navigation books.

So I replied: “Pelican Pete!  People like you are dangerous. You could be 
mis-leading people onto the rocks.  Do you make this stuff up, just to 
hear yourself talk?”

“No.” Pete replied.  “I got that from my uncle who was a submarine Captain 
in the US Navy.  He spent much of the Cold War below the polar ice cap.  
He would surface every now and then to get a celestial fix to check the 
Inertial system.  He said that they taught him at the Naval Training Academy 
to avoid sights below 10 degrees and disregard temperature and pressure 
corrections.”

To be continued...