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Old 20-04-2015, 14:28   #46
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Re: Help with (fictional) celestial navigation

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Originally Posted by FamilyVan View Post
You quoted me in the response so I think it was directed at me.
Please explain if Dead Reckoning is not the method one would use to correctly select the appropriate position. Navigation is a science as well as an art, you can't just say that you know where you are, you have to be able to back up that knowledge with a methodology.

Are you arguing that method is an EP and not a DR?

Not that any of this is the least bit important to me. I am extremely unlikely to relearn celestial navigation because I have multiple GPS on my sailboat and my shipping days are behind me, I now spend my days happily in an office managing other sailors and going home at night.

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In my first post, #10 on this thread I commented about circular LOPs but I did not directly quote your post. So again, I was not directing that comment specifically towards you.

So here is my logic about dead reckoning and celestial navigation.

Dead Reckoning is by definition using your compass course and boat speed, corrected for leeway, currents, etc to determine how far and in what direction you have gone. Here is the definition from Wikipedia. Same definition in other books but this is easiest to cut and past.

In navigation, dead reckoning or dead-reckoning is the process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time and course.

Now perhaps our discussion on this is based on different understandings of exactly what dead reckoning is or perhaps how we are using it in the discussion.

Based on the above and commonly accepted meaning of the term if you do not know where you are, dead reckoning will not determine where you are. Dead reckoning will only tell you how far and in which direction you have gone in a specified time.

EP or Estimated Position is of course part of celestial as you do need to know roughly where you are to use the proper tables in your calculations and the EP is usually established by using DR from a previous known position or fix. Perhaps this aspect of celestial is what you're referring to? If so, I have been misunderstanding your meaning.
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Old 20-04-2015, 14:35   #47
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Re: Help with (fictional) celestial navigation

The OP is exactly right - plot five stars (which can readily be done on a clear, calm night) and you end up with a messy polygon on your plot sheet. Any error (from your deck watch time being out or your index error added instead of subtracted maybe) - any small, consistent error gives a larger and increasingly messier 'polygon', or more likely, lines crossing every which way (this happened to me when my watch skipped 29th Feb in a leap year!) When your fictional character realizes that the lines of position can be given a direction (marking the ends of each LOP with arrows indicating the direction TOWARDS each star), then suddenly all those jumbled lines make perfect sense, as the constant error on each becomes quite obvious and a fairly accurate fix can be plotted from what was once chaos.

Those comments about 'circles of position' and 'two possible locations thousands of miles apart' are irrelevant to the practical business of plotting a fix from the stars - true in theory but never form any part of practical navigation - LOP are always plotted as straight lines, and even my wife can tell if we are in the Atlantic or Pacific, North or South Atlantic, Gulf of Mexico or Mediterranean! Those questions/problems just do not arise and your book would be very, very silly if such ideas entered into it.
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Old 20-04-2015, 14:44   #48
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Re: Help with (fictional) celestial navigation

Yes, that is what I meant. If you chose to split hairs between the terminology I used saying dead reckoning rather than Estimated Position found using dead reckoning, then I think you have a more text book approach to navigation then I do.

And yes I hold an STCW navigation license and yes I spent over a decade sailing as 2nd mate. So my methods definitely work on some level. Celestial Navigation on ships is not used where I sail, not in the last 20 years any way, so I was trying to remember details from Astro II which I wrote back in the mid 90s.

I apologise. Estimated Position using Dead Reckoning, not Dead Reckoning position.

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Old 20-04-2015, 15:05   #49
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Re: Help with (fictional) celestial navigation

Ok so I accidentally hit reply.

Cameras won't have linear relationship between pixels and angle but they can shoot a whole mess of stars at varying distances between one another to come up with a curve (input the data into Excel and plot it). The locations of all the Navigational stars in the shy are documented for earth. Their relative positions will be the same as seen from Mars so you can calc distance between them using spherical trig. Given that they are on Mars there will only be justification for all the work of coming up to Mars centric celestial coordinates and a Mars almanac if there is a fairly large population. Mars centric coordinates become important later when you want to start locating yourself there.

Two givens I forgot to include:
If they are on Mars all their non-satellite based technology should still work or they will be having more pressing problems than where they are like breathing, freezing to death or even talking to each other between space suits. This means they will have access to things like calculators to crunch the numbers for them, cameras to measure angles with and access to stored data that they can crunch.

Second they need some sort of downloaded data repository. The future's Wikipedia or they won't be able to get the source data and the formulas they need to make this work. I vaguely remember talk of some people downloading Wikipedia or big chunks of it for use offline when away from internet access. That will need to be a plot point to set up prior to this point.

The biggest sticking point I see is how to justify them having access to a Mars centric coordinate system and a Mars almanac. An almanac tells you the exact point on earth or Mars a celestial body is directly over. The almanac is only going to be assembled if there is demand for it which implies significant population. Ok so Mars has a large population then, how does it come to be that they have an almanac saved on board or the formulas to calculate celestial positions.

It maybe that a better story line if they have communication with the outside still and can get specialized help that will talk them thru taking sights with a camera and somebody on the outside crunches the numbers.

Ok so they have used their cameras to shoot between stars and have calibrated so they can measure angles, you still have to be able to measure the angle between the star and a local reference to horizontal or vertical and you need to know your election with respect to some local vertical or horizontal. Since there is no open water here is no useful horizon. If you could sit right under a very long pendulum in windless conditions you could measure against vertical. The easiest would be getting a pan of water and shooting the body against its reflection. Because of the field of view of the camera you would need to shoot bodies low in the sky since the angle is double what it would be between the body and the horizon.

I think the atmospheric pressure is high enough that water would remain liquid and not immediately boil off, but the temps would probably be so low you'd need to shoot one or two stars then scrape the surface the ice rime forming on top.




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Old 20-04-2015, 15:16   #50
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Re: Help with (fictional) celestial navigation

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Originally Posted by FamilyVan View Post
Yes, that is what I meant. If you chose to split hairs between the terminology I used saying dead reckoning rather than Estimated Position found using dead reckoning, then I think you have a more text book approach to navigation then I do.

And yes I hold an STCW navigation license and yes I spent over a decade sailing as 2nd mate. So my methods definitely work on some level. Celestial Navigation on ships is not used where I sail, not in the last 20 years any way, so I was trying to remember details from Astro II which I wrote back in the mid 90s.

I apologise. Estimated Position using Dead Reckoning, not Dead Reckoning position.

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OK, I will confess to splitting hairs on definitions of terms.

Not sure if I'm a text book navigator or not. I did learn from a book but only ever did it on boats. Never crossed the Atlantic but did sailboat deliveries from TX to Maine, South America and the VI. For the first 5-6 years celestial was my only means of getting around.
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Old 20-04-2015, 18:16   #51
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Re: Help with (fictional) celestial navigation

I'm sticking with my statement that sighting 2 stars will give the kids a good fix on Mars. Someone mentioned using the moon,,, I don't think so,, way too complicated mathematically and I would be surprised if it is even visible from Mars. The biggest obstacle, as previously stated, would to have access to the sight reduction tables for Mars. By the time of your story, celestial navigation would have been dead for 100s of years. Maybe they can Google the task and have the math adapted to work on the planet Mars. The actual shooting of the stars could be done with a smart phone or helmet device that has an integral bubble level.
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Old 20-04-2015, 18:31   #52
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Re: Help with (fictional) celestial navigation

using the earth's moon for navigation on mars would be ridiculous, it shouldn't be too big of a problem to calculate star tables for mars, basically just replacing earth's rotation with mars rotation. In fact I think Stelarium planetartium star program that runs on windows allows you to veiw the stars and planets from Mars or any of the other planets, so the calculations are already in my computer.

A bubble level would be totally unsuitable for navigation, using it you would be lucky to have 60 mile accuracy.
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Old 20-04-2015, 19:10   #53
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Re: Help with (fictional) celestial navigation

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..... A bubble level would be totally unsuitable for navigation, using it you would be lucky to have 60 mile accuracy.
Remember we are navigating on land on Mars! Using a cheap, plastic bubble attachment (it attaches to some marine sextants for practice on land) my last two sights of sun and moon gave intercepts of 5.0 mi towards and 4.9 mi towards the GPS location (looks like my bubble correction, applied just like index error, may be at fault). My early results were within 15 to 20 miles of the GPS location and they get better with practice - on land of course.

Star sights with the plastic bubble attachment are harder - quite tricky to see the stars when the bubble is illuminated - but I guess the ones used on aircraft solve this problem.
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Old 20-04-2015, 19:19   #54
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Re: Help with (fictional) celestial navigation

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A bubble level would be totally unsuitable for navigation, using it you would be lucky to have 60 mile accuracy.
Since bubble sextants used to be the standard for air navigation I would think they are a bit more accurate than that. It's what my father used on a B-17 in WWII.
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Old 20-04-2015, 19:35   #55
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Re: Help with (fictional) celestial navigation

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.... Star sights with the plastic bubble attachment are harder - quite tricky to see the stars when the bubble is illuminated - but I guess the ones used on aircraft solve this problem.
I imagine so as pre fancy electronic nav stuff they only crossed oceans at night. As recently as 1970 ( ) I flew from Sydney to Mexico with Qantas...left Syd in the PM... arrived Tahiti via Fiji just after breakfast and had a day at a resort near the airport at Qantas expense before taking off again after dark bound Acapulco.. which was at the time the longest overwater flight that planes did.

Same same Australia - South Africa via Cocos and Gan.
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Old 21-04-2015, 05:00   #56
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Re: Help with (fictional) celestial navigation

Check with

Reed Navigation in Mystic CT.
Frank Reed is one the foremost authorities, holds classes and seminars, and is a great teacher. He'll have all you need
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Old 21-04-2015, 05:14   #57
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Re: Help with (fictional) celestial navigation

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Absolutely correct that the term derived from deduced reckoning, however it seems like "dead" has become the accepted spelling instead of the more relevant "ded".

I just went back to see if my memory had failed on this and all the references I checked: Chapmans, Wikipedia , Encyclopedia Britannica and a couple of old navigation books including Mary Blewitt all spelled it "dead".

I just checked an old book I had found on Gutenburg, The Sailor's Wordbook written by a British Admiral and published in 1867 which also spells it "dead reckoning".

CORRECTION.

After reading The Sailor's Dictionary I did a little google research and found an article refuting the origin of the term from deduced which is what I had always thought. However there was no alternate theory on the origin of the term offered only a claim that "dead" had
been the spelling since the 1700s.

So a mystery. What is the origin of the term dead reckoning?
OK, I see I'm swimming against the tide with ded. Nevermind, I don't mind being a minority as long as there is one or two like minded "ded" folk still around.
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Old 21-04-2015, 07:12   #58
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Re: Help with (fictional) celestial navigation

I have made many trips across the Atlantic and Pacific as a navigator in C-130s using a bubble sextant. Depending on the amount of turbulence , I could expect fixes of 5-10 miles. Not bad considering that the plane is traveling at 5 miles a minute. Night time observations were best with a greater choice of bodies to shoot. Though flying from Calf. to Hawaii I was sometimes lucky to have the sun fore or aft and the moon off the wing to give an excellent fix. And towards sunset Venus would appear for a three body fix. Don't underrate the accuracy of the bubble sextant.
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Old 21-04-2015, 17:52   #59
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Re: Help with (fictional) celestial navigation

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OK, I see I'm swimming against the tide with ded. Nevermind, I don't mind being a minority as long as there is one or two like minded "ded" folk still around.
'Ded' for me...maybe we need a poll.... ded/dead/dunno what you are talking about..... I'd reckon #3 would win.
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Old 21-04-2015, 17:56   #60
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Re: Help with (fictional) celestial navigation

and I just did a google search for ded' reckoning...plenty of sites say that is the correct and original form.... such as http://www.straightdope.com/columns/...uced-reckoning
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