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Old 12-04-2014, 19:36   #1
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Sextant sight reduction software/app.

What is your favorite and why?
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Old 12-04-2014, 21:30   #2
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Re: Sextant sight reduction software/app.

An obsolete Palm Pilot app simply called "Sights". Part of a set by Charles Manson. Fast, simple, reliable, and at the time, there were no smartphones so my Palm was King.

In many ways it still is. A much more loyal and faithful servant than the smartphones, even if it is not everything they are.

The app is simple, fast, clear. The batteries are cheap and long-lasting and easily replaced. The display is easy to read in full sunlight. What else can you ask?

(I know, for an Android version!)
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Old 12-04-2014, 22:16   #3
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Re: Sextant sight reduction software/app.

AP 3270 Air Tables (3 Volume)

and a pro-forma which a friend and I jazzed up on an Apple Lisa (precursor to the Mac) which is almost self-calculating; certainly no calculator required.

If you laminate it, it will last indefinitely for backup or emergency use; you can reuse it, with a chinagraph pencil.

I guess you could even take it with you if you have to adopt alternative and more cramped surroundings



PM if you would like the full sized scan.

OOops, just noticed the "software/app." appended to the thread title (was that there before?)

I guess you could call this an algorithm, which is the guts of an app.
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Old 13-04-2014, 04:22   #4
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Re: Sextant sight reduction software/app.

Quote:
Originally Posted by Jim Walsh View Post
What is your favorite and why?
I quite liked the Opencpn plug-in just with a very brief playground.


For tables these seem comprehensive and easy to use..
2014 sun almanac has increments at the start.
All the sight reduction tables are in there.

http://www.backbearing.com

Plus here for altitude corrections...
http://astro.ukho.gov.uk/data/na2003/bookmark03.pdf

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Old 13-04-2014, 05:00   #5
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Re: Sextant sight reduction software/app.

I've had an unexpectedly high demand for the pro forma.

Firstly

I'm not sure if everyone realises that

a) It relies on Vols 2 and 3 the AP3270 Air Tables (which apparently are now the same as HO249, available online at Maritime Safety Information) (scroll to "Sight Reduction Tables for Air Navigation": You only need the first pdf for each volume, ie "Pub249 - Volume x". The other files are just the constituent parts)
There were errors in these tables discovered about five years back but these have presumably all been corrected
It's much more efficient to work from the hardcopy books; they're reasonably priced and you don't have to buy new every year. If you have to abandon ship, you'd have to be pretty unlucky to be around 39 to 40 deg of latitude, and have to take both volumes ...

(Vol 1 is for stars: my proforma is only for sunsights)

b) THIS IS A PRO-FORMA, NOT AN APP !

It does not require electrons, just neurons.
You will need to perform addition and subtraction, and look up figures in a book.
You can use an Ipad to keep the book open at the right page, if you like. It won't matter whether the batteries are charged or not.

If these provisos don't put you off, please remember to tell me your email address when you PM me.
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Old 13-04-2014, 14:19   #6
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Re: Sextant sight reduction software/app.

Answers to a couple of questions from perceptive recipients :

1) Significance of shapes and borders for various data boxes:

a) Rounded ended boxes: these denote the data we provide from our observations.

b) Thin-bordered boxes denote intermediate calculation results which are used immediately, being added or subtracted from the running total as instructed (ie they will NOT need to be copied into a box elsewhere).

c) Non existent boxes (with vertical dividers only) denote "Corrections", which are found from the Sight Reduction Tables, and used immediately. Directions for where to find corrections will be found to the right on the pro forma.

d) Heavy solid borders denote intermediate calculation results which are NOT used immediately: they will either need to be copied into a box further down the page, or be needed as a 'lookup value' when consulting the Sight Reduction Tables.

e) Heavy speckled borders denote final results which we will use to plot the Line of Position, using the hints at bottom right of the form.


Misc Questions:

i) "Where do I find the 'Approximate True Bearing of the Sun' and what is the purpose of recording it".

This is an optional estimate (most easily made either by mentally dropping perpendicular from the sun to the horizon and using a hand bearing compass for a rough bearing, or by using the vertical pin which some domed pedestal steering compasses have, which will cast a shadow on or near the rim of the card.
If the sun is very high, neither method will be useable, particularly the first.

The idea is to compare this estimate with Zn, as a check on the sight reduction to make sure a big error has not been made in the calculations

ii) "What is Hc?".
Sorry: Tabulated Altitude. I've revised the pro-forma accordingly to show both.

iii) "Where are the yellow pages".
These are in the Nautical Almanac. Possibly they are not yellow in your Almanac, particularly if it's from an online source. I don't have a current almanac to check, and if I did it would not be a US version, so YMMV

iv) What's with the funny looking 'd'?
This is for the Almanac-derived 'd', to differentiate it from the Sight Reduction Tables 'd', which relates to a different part of the process.

v) What's with the "Same/Contrary" (hemisphere as declination)?
The idea is that you circle whichever of these words is true, so that when you go into the Sight Reduction tables, you have all the entry criteria at your mental fingertips.


This pro-forma is not designed to TEACH sight reduction, but to assist, with prompts and hints, someone who once knew how to do it.



Thanks for the queries ! - I'm grateful for the opportunity to make changes which improve the comprehensibility, not least because it may be that next time I get out a sextant I've forgotten details of the procedure, or I'm not feeling that flash, and will need it to be as self explanatory as possible.

Anyone I've sent it to can request a fresh revision when this thread has died down (or if it doesn't pick up): I've already done a few minor changes, simply moving a few of the hints so it's clearer what they're attached to, and adding an explanatory label or two (such as "Tabulated Altitude".)

In the meantime, can I encourage anybody with further questions to post them on this thread rather than PM me, so that others can decide whether (and if so when) it's worth asking for a revision?
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Old 13-04-2014, 14:43   #7
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Re: Sextant sight reduction software/app.

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Originally Posted by Andrew Troup View Post
Answers to a couple of questions from perceptive recipients :



iii) "Where are the yellow pages".
These are in the Nautical Almanac. Possibly they are not yellow in your Almanac, particularly if it's from an online source. I don't have a current almanac to check, and if I did it would not be a US version, so YMMV

I think Andrew would have been using a British Hydrographic Office Almanac.
The yellow pages are the Increment pages, and are towards the back of the Almanac.
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Old 13-04-2014, 15:00   #8
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Re: Sextant sight reduction software/app.

I am thinking that if you must rely on electronics to use your sextant then why not just sell your sextant a buy a handful of GPSs ? Would you rather have a $600 sextant or six handheld GPS ?
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Old 13-04-2014, 15:44   #9
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Re: Sextant sight reduction software/app.

To the OP's question, I have never kept up with sight reduction software because it never made much sense to me. I learned celestial with the Power Squadrons and practiced the various table approaches and calculator solution. These days, with GPS/GLONASS/GALILEO I haven't used celestial in years, and with the multiple levels of backup aboard I doubt that I ever will need it again (although I might practice just for fun). If the time came that I did need celestial as my backup then it is very unlikely that I would have a functioning computer to solve the problem (otherwise I'd plug in a GPS dongle and be done with it). Currently my Air Tables (HO 249) are out of date; otherwise they would be my preferred method. The Marine Tables (HO 229) are never out of date so I would probably take them if I needed something. The compact tables are compact, but more prone to making errors so I prefer to avoid them. I still have a scientific calculator or two aboard which I might use as an alternate solution and to get back up to speed.

I also carry a sounding lead, swung compass, and a Walker log with the mounts on the taffrail. I haven't needed the log or lead either, but find comfort in having them. Going without an adjusted compass is just bonkers IMHO.

Greg
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Old 13-04-2014, 16:01   #10
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Re: Sextant sight reduction software/app.

Why would HO 249 tables go out of date?
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Old 13-04-2014, 16:33   #11
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Re: Sextant sight reduction software/app.

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Why would HO 249 tables go out of date?
Vols 2 and 3 don't go out of date.
Vol 1, Selected Stars, does after about ten years... its to do with 'precession and nutation' of the stars....

Don't think it would sink your ship though, unless your Vol 1 was very very old.
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Old 13-04-2014, 17:30   #12
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Re: Sextant sight reduction software/app.

Nice presentation Andrew , good looking too, better then something I knocked in excel a frw years ago and printed out as a template.

I've never used " software" for celestial, I don't see the point.

Dave


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Old 13-04-2014, 17:49   #13
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Re: Sextant sight reduction software/app.

Sorry, I was a bit sloppy re: HO 249. Personally I think Volume 1 is a gem, as it tells you which are the best stars/planets to use for the position/time, as well as precomputing the numbers. Volumes 2 & 3 are basically just a lower resolution version of the marine tables, and it can be argued that the increased accuracy is not justified given the quality of a sight taken on a small boat at sea. IIRC there is a way to extend the Volume 1 numbers for a few years (the volume is updated every 5 years for a new "epoch"). Again, loss of accuracy by using a recent epoch may not be worth worrying about, unless you are grading navigation papers. (Don't tell the Power Squadrons I said that )

I would like to know what the OP wants to do with a celestial app. Is he expecting to use it for actual navigation, or an aid to learning, or ? From a learning POV the use of a form can be a good start; programs can be very educational or just pop the numbers out.

Greg
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Old 13-04-2014, 17:52   #14
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Re: Sextant sight reduction software/app.

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Vols 2 and 3 don't go out of date.
Vol 1, Selected Stars, does after about ten years... its to do with 'precession and nutation' of the stars....

Don't think it would sink your ship though, unless your Vol 1 was very very old.
The only thing I know further to that is that the perpetual almanac parts of Vols 2 and 3, in the online version at least, are only good til 2016

However these are immaterial to the process of sight reduction; they're just intended as a backup should you have lost access to a formal almanac.

I hope the fact that they've not been updated doesn't mean the tables have run their course, though.

Does anyone have a recently published hardcopy of Pub 249 or AP 3270, to check Table 4? In the current online version of 249, the date at the top reads "GHA and Declination of the Sun for the Years 1981-2016"
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Old 14-04-2014, 02:00   #15
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Re: Sextant sight reduction software/app.

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Volumes 2 & 3 are basically just a lower resolution version of the marine tables, and it can be argued that the increased accuracy is not justified given the quality of a sight taken on a small boat at sea.

Greg
I never saw any of these tables used until I went from UK to foreign flag.... then the only ones I ever saw used were the Air Tables together with those green ( the print not the paper) american plotting sheets.

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