Celestial Nav. - Sight Reduction with Calculator

[Snowpetrel]

Originally Posted by Dsanduril

The "traditional" way to handle this (at least for us northern hemisphere folks, don't know how they handle this down under) is to always have southerly latitudes/declinations be negative, that way the formula stays the same. Again, just as Snowpetrel pointed out.

Ha, us southern hemisphere bunch get used to always being the negative/down lot. doesn't bother me, except on some computers that insist on defaulting to Northern hemisphere ALL the time...

For what it's worth I normally use the concise tables at the back of the almanac. Easy (and quick) to use once you understand the weird rounding rules and double entry. Sure beats paying for, and lugging around sight reduction tables, and Norries with the logs and haversines is very cumbersome.

Just to clarify the rules I use with calculators, If DR lat is south I call it negative, if dec is south it's also negative so they automatically cancel out. if DR lat is south it's neg, and if dec is north it becomes + giving me the negative in the calculation that I need.

[Gos2015]

Thanks Snowpetrel, got it, Lat and Dec the same then you add; Lat and Dec different then Dec is treated as a negative number.

Learning the concise tables is my next goal. They look confusing but it's just like everything else, once you do it a few times, the mystery soon disappears.

What part of the Southern world are you in?


Sent from my iPhone using Cruisers Sailing Forum

[sailr69]

Been a long time since I did celestial but when I did use it I used the Marc St. heliare method using the Norries tables. did not like using the air navigation tables. all can be done without any calculator, plus the plotting is much easier. I did try to program a Casio once and found it more of a hassle. I would guess that by now there are a platitude of computer programs to reduce sights. But why not just get a GPS or two. much more accurate and quicker, without errors.

[Snowpetrel]

Originally Posted by GoSailing

Thanks Snowpetrel, got it, Lat and Dec the same then you add; Lat and Dec different then Dec is treated as a negative number.

Learning the concise tables is my next goal. They look confusing but it's just like everything else, once you do it a few times, the mystery soon disappears.

What part of the Southern world are you in?

I'm in Tasmania, Australia. Not a bad part of the world to live, though not quite enough snow for my liking, But some great sailing, walking and kayaking, look up Port Davey or Wineglass Bay.
Snow Petrel I in Port Davey

The way you deal with north/south works fine but is slightly different to how it's more commonly done. your results will be the same but if you are comparing notes you will have slightly different workings. I don't look at if dec or lat are the same or contrary, I just call Lat and dec negative if they are south, and positive if they are north, and let the maths do it's work.

EG your system: dec N, lat S you would call dec- and Lat+ whereas I would call Dec+ and Lat-. End result the same, but it's easier to spot a mistake if you are more consistent with rules across all calculations. This was more important in the "good" old days when we did everything by hand/calc. Nowadays with gps and chartplotters it's not so critical, so you find older professional navigators are very systematic and use rules and follow them religiously, not because they can't take the shortcuts, but because it helps prevent mistakes, and makes it easier to spot them when they occur (and they still do! the difference between a novice navigator and a experienced one is not about not making mistakes but in the ability to spot them). The N/S convention is one of these Rules and is how I spotted your initial mistake.

If you get stuck on the concise tables send me a PM, the examples in the amanac aren't the best to work off. The tricky part is splitting degrees and minutes. You Round the whole degrees up or down, but leave the minutes the same. It seems weird to do, but it works. Also the DRLong needs to massaged to give a whole LHA, so you can get some pretty large intercepts. A useful Proforma is here Though I prefer not to use proformas for my workings, they are handy for learning.

Cheers

Ben

[Gos2015]

Okay so I will start a new thread, I will put up the first Celestial Nav. problem, (one I made myself - I say that as a qualifier), and we will see if there is enough interest to keep it going for awhile.

Thanks for everyone's input here.

Les