Celestial Navigation Qs. Simulated sights, long term almanac

[Paul Howard]

All the babel about electronic methods are distractions from the fact you are looking for a reliable back up to failure of GPS.

Educate yourself on how to do the manual method for position finding first for greater understanding of position finding and self-confidence when electronics fail

[SeanPatrick]

Originally Posted by Paul Howard

All the babel about electronic methods are distractions from the fact you are looking for a reliable back up to failure of GPS.

Educate yourself on how to do the manual method for position finding first for greater understanding of position finding and self-confidence when electronics fail

As a celestial navigation enthusiast, I am all for the recommendation of learning the "manual" [non-electronic] method. However, the reality is that the best backup to GPS is not celestial. It is another GPS - or two or three. The likelihood of multiple GPSs failing (especially if one or more are kept in shielded containers) is extremely low.


What celestial is really good for is being another source of information with which to cross-check what your GPS, radar and other systems are telling you. As has been said countless times: the prudent navigator uses all available sources of information. Anyway, even if one does learn the manual method, what is the harm of keeping a celestial calculator of some sort available to ease the workload? Whether you calculate by hand or with a calculator/program, you are still using an independent system.

Originally Posted by Gruz

I do not use any software, I am old school, paper and pen so not sure. It looks more or less ok, (couple of miles here and there), but I will still stick to my paper calculations just in case and I think that your HS is still wrong. 22°59'.5 it should be 22°57'.3

Well, that's not exactly true ... is it?

Originally Posted by Gruz

I have program that can give me/generate sextant readings anywhere in the world, any time (GMT) any date between 9999 BC to 9999 AD.

And Almanac within that program can be used to produce conventional Nautical Almanac pages for any date between 9999 BC to 9999 AD.

It is obvious from that thread that you do [at least] use AstroNav.


But, I believe a more accurate Hs [using the given parameters] would be 23°00.3'. The reason I believe this is because I created a spreadsheet specifically for figuring out what Hs would produce a given Ho using [mostly] the formulae from the Nautical Almanac.


Here are the calculations:


Hs: 23°00.3'

• Index error = -1.4'
• Dip (in arc-minutes) = -0.97∙√HoE in ft. = -4.2'

Ha = 22°54.7'

• Refraction = -0.0167/tan(Ha+7.32/(Ha+4.32)) = -2.3'
• Parallax in altitude = HP[54.6']∙cos(Ha) = 50.3'
• SD = 0.2724∙HP = 14.9'

Ho = 23°57.6'


23°57.6' exactly matches the figure for Hc one would arrive at using the N.A. formulae for sight reduction:


2019, March 29 0800 UT
Moon GHA & Dec.: 13°33.7' ; S21°32.5'
DR: 40°55.0' N, 9°31.6'E
LHA = 23°05.3'

• Hc = asin(sin(Dec.)∙sin(Lat.)+cos(Dec.)∙cos(LHA)∙cos(La t.) = 23°57'34.9" or 23°57.6'
• Z = acos((sin(Dec.)∙cos(Lat.)-cos(Dec.)∙cos(LHA)∙sin(Lat.))/cos(Hc)) = 156.5°
• If LHA<180, Zn = 360-Z otherwise Zn = Z
• Zn = 203.5° or 204°

Now, using an Hs of 23°00.3', Celestial by Navimatics yields an intercept of 0.1' away and a Zn of 204°. I can only suppose that the 0.1' intercept is a result of not taking into account the "augmentation of the semi-diameter of the Moon" into account [0.3∙sin(Ha) = 0.1']. None of this is surprising given the fact that the Navimatics app "Utilizes the USNO NOVAS C3.0 software for astrometry predictions." and that the Nautical Almanac is jointly produced by the USNO and the United Kingdom Hydrographic Office.

Originally Posted by Black Mate

Navimatics has always given me greater differences than Interceptor.
Probably as for StarPilot they have not adjusted the ephemeris, so even Navimatics have not updated it since 2015.

I'm almost certain that the Navimatics app was updated in 2017 - at the time of the last leap second. The reason I am so sure is that I corresponded with the developer by email about this very issue (although I am unable to locate this specific email at this time - perhaps because I deleted it). The developer assured me that the app would be updated to account for the leap second - and so it was. And that is the very issue at hand - the only reason to update an ephemeris (all of which should be calculated using "terrestrial time" [TT] and adjusted for leap seconds) is to account for the [unpredictable] slowing of the Earth's rotation - which is the reason for the insertion of leap seconds in the first place.


This is why I pointed out that it is beneficial that the Navimatics app allows for a custom value of Delta-T (the difference between TT and UT1). It allows for more accurate results, even if the app has not been updated.

[Black Mate]

I was not looking for the perfect condition, but through a simulation I would compare different instruments in the result.
I agree with the manual resolution of Navigation with Sextant.
It's also necessary to maintain adequate supervision by electronic means.
When you are on the bridge of a ship, you do not rely on electronic instruments, but you control them, because these are aids to navigation.
CelNav through the different methods and tables (HO 249, HO 229, HO 208, S-Table, Long Term Alm, etc.) remains the only possible backup for me.
I'm very pleased to see that in this forum there are many competent people at a high level. Thanks.

[Gruz]

Originally Posted by SeanPatrick

Gruz

Well, that's not exactly true ... is it?

I do not use, don't have any software for celestial navigation when I am "out there" on the boat,(it is on my home computer), just sextant, winding pocket watch (patek philipe), paper, pen, ho 229 and norie's tables.
Even at home. I can jump from my bedroom in the sea, so I am using sextant even at home. This program that I have, I am using it occasionally in cases like this one, just to check your work.

About that astronav or whatever, did not know that it even exists before I red about it here. Program that I am using is developed for merchant navy colleges in Croatia for celestial navigation training and it is not commercially available.It will give you just sextant reading and nothing else, does not calculate instead of you. you have HS rest is up to you, take almanac, tables pen and paper and calculate.

So do not try to be sherlock holmes when you are obviously not.

[Black Mate]

ezAlmanac is another very useful software to avoid having with you the HO 229 and ephemeris.
Very practical and easy to use for table interpolations.
The software is available for iPad and Windows 10 (I have both).
In addition to the tables described, the program can solve all CelNav calculations including Lunars.
The programmer is using the latest vector astronomy software (NOVAS 3.1) and JPL ephemeris which have evolved over time and both are more accurate than they were back then. It's really accurate.
Let me know what you think, even if the smell of books is always better.