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Old 20-04-2015, 10:08   #31
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Re: Help with (fictional) celestial navigation

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Originally Posted by rhubstuff View Post
It has, of course, occurred to most of us that celestial navigation on Mars would require a completely different set of ephemeris data, and different Prime Meridian, time system, etc. Plus the fact that Mars has no water, so the horizon would be cratered and mountainous, and height of eye indeterminable. Without accurate HE and a good horizon, celestial navigation is impossible. (OK, an artificial horizon could work.) But how would you compute the Equation of Time or LMT, even if you had a Martian Almanac? And you'd need to find the body's GHA and declination, and then sight reduction tables. (You could use basic spherical trig, I guess...). But there is no coordinate system in place...no equator, no Prime meridian. There can be roughly 40 steps in a typical sight reduction. Most of them would be different on Mars than on Earth, and no such computations have ever been done.
All very knotty problems.

However, easily solved by the author stipulating that all this will have been done XX years in the future when the events in the book occur.
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Old 20-04-2015, 11:21   #32
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Re: Help with (fictional) celestial navigation

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Not sure why you are picking my post apart. Its been nearly 20 years since I studied nautical science, the guy asked a question I noticed nobody had answered him, so I gave him the best answer I could so his thread didn't get buried. I'm pretty sure every thing I said was technically correct, except for misspelling of ded reckoning (thank you for pointing that out Wotname).

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I learned the basics from a transatlantic sailor around 1980. I bought the book he recommended by Hewett Schlereth which was really simple and straightforward and included an "eternal" sun almanac. So of course I only did sun sights.
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Old 20-04-2015, 11:32   #33
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Re: Help with (fictional) celestial navigation

Misshenry (and others) - Two LOPs give one intersection, but no indication that a mistake or inaccuracy has been made, therefore no means of generating confidence. Three LOPs will result in a "cocked hat" - a triangle. The size of the "cocked hat" is an indication of the accuracy of the sights and the reduction. But in a larger "cocked hat" you know that at least one LOP is the result of a significant error - but which one? Four LOPs will usually result in three "cocked hats". If the three are reasonably small there is a good degree of confidence. But if they give two small "cocked hats" that aren't close to each other all bets of confidence are off. Five LOPs will usually give a very good indication of which LOPs are not to be trusted and which three of the five give the best confidence. I know that this does involve significant work, but having made the passage between the mainland and Hawaii singlehanded I often did celestial navigation just to have something to do.
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Old 20-04-2015, 11:45   #34
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Re: Help with (fictional) celestial navigation

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Originally Posted by ceswiedler View Post
Hi! I'm writing a novel which involves celestial navigation, and I have just-slightly-over-a-landsman's understanding of how it works. Fortunately, I don't need to do any _actual_ navigation, I just need to make sure my plot and descriptions aren't ridiculously inaccurate. I'm hoping that you guys can help.



My main question is this: I know there are lots of different ways of doing celestial navigation. I believe the one I want my protagonist to use is sight reduction of stars. It's important that he is able to plot a position, but not be terribly accurate about that position.



Is it correct to say that if a reasonably-practiced, but certainly non-expert, navigator took sights of 3-5 stars, and then did the correct calculations, that they could end up with 3-5 lines which didn't intersect, therefore only giving a position that was as accurate as the polygonal shape of those lines?



If that were the case, what would the inaccuracy come from? Is there any other method that could be used (moon or sun sights or something like that) to improve the accuracy? I'm looking for the character to have a series of initially-inaccurate plots, and then later realize "oh, all i need to do is XXX" and then get a better position.



Thanks in advance for any help and explanation!

There a whole litany of things that can be done wrong or forgotten that would lead to an inaccurate position some of them don't make it obvious that the answer you get is wrong.

The way I would write this is that he/she shoots a round of 3 stars and the moon. Stars are relatively easy to run the calcs for and the moon is the hardest.

You can have the protanonist redo the calcs for the moon several times without fixing the problem then wonder if the time is wrong.

Once time is checked, error determined, and time for each celestial body corrected protagonist redoes the calcs for ALL the bodies since time was wrong for all of them. Calcs can be redone the next day or even 3 days later. More than that and you will need to start doing error rate calcs and I don't think you want to try explaining that.

Maybe some kid messed with ship's clock making it 20sec off at last port of call on day of departure and this the first opportunity to get a fix since getting underway and protagonist hasn't checked clock since day before leaving. Navigator would have chef clock when in port.

If the protagonist is using celestial then the story is set in the early 1990s or before. Correcting time would involve tuning in to a time broadcast on a shortwave receiver. There are a very small number of makes and models that were widely used up until the mid-80s. "TransOceanic" was one model by Zenith(?) I think.

For scale of error indicate stars give 12nm circle for likely position with the moon falling another 5nm away.and in weather like this 7-10nm is normal accuracy.

Have your protagonist be only moderately skilled at this.

If you want to go for more truthiness, pick the time of day (just after sunset or just before dawn), time of year (to the nearest month or half month), pick your latitude and then pick real stars to name.

There are 57-100 stars normally used for navigation. They need to be 20-65 degrees above the horizon and they need to be 30-150 degrees apart horizontally. With 2 stars 90* is optimum and with 3 stars it's 60* or 120*. If you want help picking stars I might be able to help.





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Old 20-04-2015, 11:46   #35
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Re: Help with (fictional) celestial navigation

As a professional merchant mariner who used to be in charge of celestial navigation (now it's all GPS) at sea, I will tell you that the comments are accurate. 3-5 stars is considered adequate for a fix and no fix, even if all LOPs intersect, is accurate to more than 5 miles. But, since you are at sea and out of sight of land, that's fine. I will say that one can use Venus , and sometimes Jupiter, in addition to stars. Additionally, if you're good and the vessel is stable, you can cross planets with the moon, although the moon moves very quickly. A round of stars on a ship from start to finish - no electronics - would take me about 15 minutes.
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Old 20-04-2015, 11:57   #36
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Re: Help with (fictional) celestial navigation

Quote:
Originally Posted by ceswiedler View Post
Hi! I'm writing a novel which involves celestial navigation, and I have just-slightly-over-a-landsman's understanding of how it works. Fortunately, I don't need to do any _actual_ navigation, I just need to make sure my plot and descriptions aren't ridiculously inaccurate. I'm hoping that you guys can help.

My main question is this: I know there are lots of different ways of doing celestial navigation. I believe the one I want my protagonist to use is sight reduction of stars. It's important that he is able to plot a position, but not be terribly accurate about that position.

Is it correct to say that if a reasonably-practiced, but certainly non-expert, navigator took sights of 3-5 stars, and then did the correct calculations, that they could end up with 3-5 lines which didn't intersect, therefore only giving a position that was as accurate as the polygonal shape of those lines?

If that were the case, what would the inaccuracy come from? Is there any other method that could be used (moon or sun sights or something like that) to improve the accuracy? I'm looking for the character to have a series of initially-inaccurate plots, and then later realize "oh, all i need to do is XXX" and then get a better position.

Thanks in advance for any help and explanation!

Mistakes can happen if you fail couple of seconds when taking a sight, Then you will be miles off course-Your Hero will realize this and will say "Oh, lets do the Sun and forget the stars" Its always easier to take a sight of Sun than a sight of a star.- First you should be absolutely sure which star is what - not an easy task for a newbie

So stay with the Sun, Either the noon sight which is the simplest or a running sight of the Sun which is not that complicated as it sounds - use also the dead reckoning position and you are there

Mistakes that the Hero could have done are
The time _ Incorrect reading of his stop watch as couple of seconds can through him miles off course
The tilt of the sextant
The incorrect sight of Sun's Lower or upper limb due to waves and motion of the vessel
Wrong time keeping in regards with the UTC ( GMT) time
Any of these "mistakes" he can find easily and go back and re do successfully the job
Please tell us when you have the book ready
I love this kind of adventures
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Old 20-04-2015, 11:59   #37
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Re: Help with (fictional) celestial navigation

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Misshenry (and others) - Two LOPs give one intersection, but no indication that a mistake or inaccuracy has been made, therefore no means of generating confidence. Three LOPs will result in a "cocked hat" - a triangle. The size of the "cocked hat" is an indication of the accuracy of the sights and the reduction. But in a larger "cocked hat" you know that at least one LOP is the result of a significant error - but which one? Four LOPs will usually result in three "cocked hats". If the three are reasonably small there is a good degree of confidence. But if they give two small "cocked hats" that aren't close to each other all bets of confidence are off. Five LOPs will usually give a very good indication of which LOPs are not to be trusted and which three of the five give the best confidence. I know that this does involve significant work, but having made the passage between the mainland and Hawaii singlehanded I often did celestial navigation just to have something to do.
Celestial navigation is an art form. After awhile, you know which LOPs to trust more just because you have done it so many times that you know when you have a bright, fat star right on the line and a dim flickerer on the dark side of the horizon. Stars are, of course, at twilight. The best LOP of all is Local Apparent Noon, historically the most important sight of the marine day.
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Old 20-04-2015, 12:34   #38
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Re: Help with (fictional) celestial navigation

There are many corrections that one needs to apply to get a good LOP. DIP comes to mind (Height of eye).

Is sailing it is presumed that the distant horizon and the water under your boat are in the same plane and thus the height of eye (sextant) above the water needs to be corrected for (to bring the sextant to the same level as the plane of the horizon).

The higher (or lower) the sextant is above or below the plane of the horizon the greater the correction needed.

In your story your observers could be quite a ways above the plane of the horizon. Put them on a plateau with the distant horizon much lower than their feet, put them in a depression so that the distant horizon is quite a way above the plane of their feet.

have them apply height of eye correction as if the distant horizon and their feet are on the same plane. Poof, instant error that can be made quite large so that the plotted LOP are miles and miles from intersecting each other.

Once they figure out the correction for the height of the horizon (referenced to their position) the LOP can move much closer to each other and thus a much more accurate fix.

For what it is worth....
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Old 20-04-2015, 12:45   #39
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Re: Help with (fictional) celestial navigation

The easiest sight is Local Apparent Noon. A child can do it. It gives you a very accurate latitude line. No chronometer needed. In the olden days, before chronometers (needed for stars and sun lines) allowed the determination of longitude, sailing ships leaving England would "sail down" to the latitude of their destination and then sail west. The noon sight gave them latitude.
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Old 20-04-2015, 13:08   #40
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Re: Help with (fictional) celestial navigation

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Originally Posted by FamilyVan View Post
Not sure why you are picking my post apart. Its been nearly 20 years since I studied nautical science, the guy asked a question I noticed nobody had answered him, so I gave him the best answer I could so his thread didn't get buried. I'm pretty sure every thing I said was technically correct, except for misspelling of ded reckoning (thank you for pointing that out Wotname).

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Was not trying to go after you. It is a discussion and that involves some give and take and voicing different opinions. If my reply was less than polite I apologize.

My original post about circular LOPs was not targeted at you since there were 2 or 3 similar comments on the thread. However you responded to that post with the comment about dead reckoning which I still believe is incorrect. If I am not I welcome your logic on how it is correct.

One statement you made in your first post is absolutely not correct and I think misleading to the OP. A fix from celestial navigation is not to within 100 miles, it is typically within 5-6 miles even on a bad day.
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Old 20-04-2015, 13:15   #41
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Re: Help with (fictional) celestial navigation

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The easiest sight is Local Apparent Noon. A child can do it. It gives you a very accurate latitude line. No chronometer needed. In the olden days, before chronometers (needed for stars and sun lines) allowed the determination of longitude, sailing ships leaving England would "sail down" to the latitude of their destination and then sail west. The noon sight gave them latitude.
Another easy one, at least in the northern hemisphere is latitude from Polaris. Also simple calculation and no need for a chronometer.
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Old 20-04-2015, 13:23   #42
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Re: Help with (fictional) celestial navigation

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Was not trying to go after you. It is a discussion and that involves some give and take and voicing different opinions. If my reply was less than polite I apologize.

My original post about circular LOPs was not targeted at you since there were 2 or 3 similar comments on the thread. However you responded to that post with the comment about dead reckoning which I still believe is incorrect. If I am not I welcome your logic on how it is correct.

One statement you made in your first post is absolutely not correct and I think misleading to the OP. A fix from celestial navigation is not to within 100 miles, it is typically within 5-6 miles even on a bad day.
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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Old 20-04-2015, 13:54   #43
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Help with (fictional) celestial navigation

Ok so since this is on mars ignore my previous post.

So basics:

A solar flare is not going to wipe out their GPS system, but a direct hit by a CME (Coronal Mass Ejection) might. The mean time between major hits on earth is on the order of several hundred years. Mars averages 1.66 times as far away so odds of a hit are decreased and it would need larger to have the same effect at Mars so let's say its a 7000-1000yr event. I would try bank robbers hacking the system for a heist or an accident hat puts ground control out of action and it's backup has not been properly maintained due to sparse population and lack of manpower.

Marian "day" is different than ours and it's called a "sol".

Mars' molten core froze some billions of years ago so magnetic field is gone. This means compasses won't work there but gyrocompasses will.

They won't have sextant a for measuring angles, breaks all credibility if they did. Surveying equipment would would but coincidentally having that on a stolen rover would put credibility at the breaking point.

What they would have is cameras. With a 10Mp or so camera you can count pixels from one star to the horizon, between stars or between a star and it's reflection and get accuracy approaching 1arc-min.
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Old 20-04-2015, 14:04   #44
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Re: Help with (fictional) celestial navigation

Yes this is very reasonable. A practiced but non-expert navigator can take several shots of stars in very little time (withi, say, 2 minutes) and then reduce them at their leisure to get several lines that may or may not cross over. I used to do 5 as a general rule, depending on visibility of course. Now i'm lazy and look at a GPS.....

Stars are nice because they don't move very quickly (unlike the sun, and certainly the moon) and you can get several of them in a very short period of time so there's no need to advance a position line, estimate speed etc. as there is with, say, sun lines. Plus, because they are essentially a point of light you don't need to do corrections for upper/lower limb (you want to know the position of the centre of the sun, or the moon, but you can't just guess, so instead you use the upper or lower edge and then do corrections to calculate the altitude of the centre).

There are loads of potential sources of error that would suit your storyline.

Index error is an easy one. It's due to mis-alignment of the sextant components so it's a constant and gives constant error for each measurement.

Time errors are an obvious one too and there are lots of options for you. Converting GMT into local time, daylight savings, misinterpretation of times given in the conversion tables, inaccurate timepieces. The list could go on for a very long time.

Dip error is an error due to the fact that your height of eye is not level with the horizon, but a few feet above it (or many thousands of feet if you are navigating an aircraft).

Another one is the error caused by refraction of light as it passes through the atmosphere. The effect is more pronounced the lower on the horizon an object is, because the light reaching you from those sources has passed through more atmosphere. (In fact, you have never seen a sunset in real time. By the time you observe the sun disappearing underneath the horizon it has already happened. The reason you can still see the sun at all is because of refraction.)

Parallax error is another option, but this is only really a problem when you're doing moon sight (the moon is a general pain in the arse for celestial navigation) so I don't think i need to go in to that do i?

A nice error if your guy were doing sun sights would be something like adding the declination of the sun when you should be subtracting it (or vice versa) (because the sun isn't on the equator, it moves either side of it (relative to us) depending on the time of year). This is a good one because it is an error that is very common, particularly among novice navigators. But, if you want to do stars it's not an issue.......

Finally, some of these corrections need to be done in a specific order because they are dependent on one another. For instance, you don't want to correct for atmospheric refraction before you've corrected for index error, otherwise you'll be doing a correction based on a value that was wrong to begin with.

Hope you can get something from all our ramblings! Good luck with the book.
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Old 20-04-2015, 14:11   #45
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Re: Help with (fictional) celestial navigation

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Two kids living on Mars ~100 years in the future steal a rover and run away. A solar flare destroys all of the GPS satellites, leaving them with no way to navigate. One of the kids has a decent amount of training, and has been taught how to sight stars. I'm handwaving the measurement stuff with sci-fi magic: for example I'm saying that gizmos in their helmet will help them measure the angles of stars, and it's easy enough to say that they can do this without actually seeing the horizon.


chris
did anybody mention they don't have GPS satelites on Mars, I'm sure 100 years in the future there will be some other system that they will implement for navigation when we colonize Mars. Maybe they will make some artificial north pole and a similar east pole and time share, turn them on and off and use a compass based computer system to locate their position. That system could fail, that might be more believable than GPS.
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