HP-41 Navigation PAC question



#9

Hi all,

To the sailors out there, I've been asked if there is any need to update the HP-41 Navigation PAC programs with new Nautical Almanac data of Declination and Sideral Hour Angle.

I've been told that these data slightly change along the years.

Since a have little-to-no idea about what these terms may mean, I'd really appreciate any knowledgeable advice.

Thanks for your attention and best wishes.

Diego.


#10

Not really a sailor, but an astronomer!

Due to the precession of the Earth, the apparent position of stars in the sky with respect to the Earth-based lat/lon system changes slowly and systematically as the years go by. The annual change is around 1 arcminute per year - comparable to the accuracy that a really good sextant user might hope to achieve.

So, if you are worried about running aground at the nautical mile level, you do need current star positions.

I have no idea - does the Nav Pac apply precession? If it does, it should more-or-less stay up to date for quite a while.

There is also proper motion of stars (they move around in space), but this is at the arcsecond per year level (generally, much less) so you shouldn't have to worry about this for a century or more!

#11

I'm not a sailor but I have used a nautical almanach and a sextant on a layman's basis. As far as I know the Nav Pac has an eternal almanach programmed in its ROM. For instance, the manual says on page 39 "What is the declination of Polaris on 1 Jan 1980? What will it be on 1 Jan 2080?"

See TOS for details on the Nav Pac manual.


Edited: 23 Feb 2012, 3:41 a.m. after one or more responses were posted


#12

Hi, Dave, Alexander,

Thanks for your comments, I've sent them to my friend and hope they clarify his doubts.

Best wishes from Hispaniola island.

Diego.

#13

Hi Diego,

I can only second what Dave and Alexander posted. However, I once compared the star positions computed by the eternal almanach of the NAV-Pack to those in a current release of the "Nautical Year-Book", the german version of the alamanach for celestial navigation. I noticed that they deferred by a couple of arc-minutes.

Regarding navigation accuracy that may not be too significant, at least not on open sea. Since star positions vary only slowly, a position fix computed using multiple star sightings should not be off by too much. It may be different once you get close to shore though.

However, I'm just an "armchair" celestial navigator doing some backdoor sun-sightings for fun. Never managed to do star sightings with my do-it-yourself sextant since the city backlight is just too brigth and it does not have a telescope. So maybe it is a good idea to verify this by someone who is more experienced with using celestial navigation out on the open sea...

Best regards,
Timo


#14

Hi Timo,

you're using a DIY cardboard sextant like I did? I purchased that thing from AstroMedia some years ago and it was fun!


#15

Hi Alexander,

yep, that's the one I've meant. ;)

Was really fun to build and it is good enough to produce a usable position using the Noon-Sighting method. Unfortunately, it is quite unlikely that I will ever come to test my skills out on the ocean...

But then again - maybe my stomach will be quite thankful for that!

Cheers,
Timo

#16

Quote:
However, I once compared the star positions computed by the eternal almanach of the NAV-Pack to those in a current release of the "Nautical Year-Book", the german version of the alamanach for celestial navigation. I noticed that they deferred by a couple of arc-minutes.

That seems rather big. Assuming the initial star positions (probably for either 1950 or 2000) are correct (can you compare those? Bright star positions are all known to MUCH, MUCH better than an arcsecond), almost any precession algorithm should be good to well under an arcminute with a few simple equations. (To do better than about 20 arcseconds for an instantaneous star position, you also have to account for the (non-circular) orbit of the Earth and how that affects the aberration of starlight.)

Does anybody have a description of the precession formulas?


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