41 replies to this topic

[Cavelamb]

A while back someone here names a book on Celestial Navigation written by a woman -
supposed to be one of the best primers available.

I've tried to find that thread, The search engine turned up lots of noise - but no signal.

Anyone have a link to that book?

[Cavelamb]

A while back someone here names a book on Celestial Navigation written by a woman -
supposed to be one of the best primers available.

I've tried to find that thread, The search engine turned up lots of noise - but no signal.

Anyone have a link to that book?

Ok, Amazon found it...


Celestial Navigation for Yachtsmen
by Mary Blewitt


http://www.amazon.com/gp/offer-listing/0070059284

[B.J. Porter]

A while back someone here names a book on Celestial Navigation written by a woman -
supposed to be one of the best primers available.

I've tried to find that thread, The search engine turned up lots of noise - but no signal.

Anyone have a link to that book?

Ok, Amazon found it...


Celestial Navigation for Yachtsmen
by Mary Blewitt


http://www.amazon.com/gp/offer-listing/0070059284

I heard good things about it too, so I bought it though I've not had much chance to actually use it yet.

[Presuming Ed]

Quite a woman.
http://www.telegraph.../Mary-Pera.html
I think that there are newer introductions to celestial
http://www.amazon.co.uk/Celestial-Navigation-Edition-Tom-Cunliffe/dp/1898660751
I really fancy one of Stokey Woodall's courses.
http://internationaloceanservices.co.uk/celestial-courses/4558930559

[Cavelamb]

Quite a woman.
http://www.telegraph.../Mary-Pera.html
I think that there are newer introductions to celestial
http://www.amazon.co...e/dp/1898660751
I really fancy one of Stokey Woodall's courses.
http://international...rses/4558930559

Thanks Ed.
I'll grab a copy of Tom's book too.

I'm in over my head (again), of course.
But this is something I always wanted to learn.

[Presuming Ed]

Me too.

[Paps]

Quite a woman.
http://www.telegraph.../Mary-Pera.html
I think that there are newer introductions to celestial
http://www.amazon.co...e/dp/1898660751
I really fancy one of Stokey Woodall's courses.
http://international...rses/4558930559

Thanks Ed.
I'll grab a copy of Tom's book too.

I'm in over my head (again), of course.
But this is something I always wanted to learn.

Lamby, I found myself in a position where I had to learn CN very fast. We were at sea, 12 or so days from our destination when it became obvious the owner and his girlfriend had no fucking idea! They would both take a sight, do the calcs and could not get within 100 miles of each other. They did have on board though a decent library so I set to it.

I had real difficulty "getting" it and was trying to process way too much information at once. The physical actions, the geometry, the reductions and corrections etc. Just when I thought I can never do this I picked up a little paperback which was basically CN for dummies and the penny dropped. After that it was just practice and getting to know the calculations. The book recommended using the sight reduction tables for air navigation rather than the nautical ones and they are indeed easier to use.

In my case I just couldnt get my head around the basic concept of what I was sighting, adjusting and calculating for! I am one of the types that has to understand the way things work rather than taking things at face value. If I want to understand how something works, simple, pull it apart!! Many Xmas presents ended up in pieces.

The message from the book that cracked it wide open for me was something like this. LOP, imagine the sun is a duck and the sextant is a gun. You shoot the duck and record the time. Now imagine the duck falls straight down and where it splashes in the water is the suns GP. From the sextant you know the angle between you and the duck. From the almanac you know the suns GP @ that exact time. So you have an exact point being the suns GP and an angle which gives you a circle around that point you must be on. Progressive sights give you intersects of those circles and voila you can narrow down which part of the circle you are on. These are called running sights or line of position (LOP).

Once I got that gist the rest was easy by comparison. Perfecting the actual sextant shots, developing a regimen around correcting the time piece and getting comfortable with the calcs and corrections. We got to Tokyo more or less and from there I hit LA 6000 mi away +/- 30 miles after 3 days of overcast.

Sorry this has turned into an essay but the message is, get the basics and dont let the technics bog you down.There is way too much information to take in all at once. Baby steps.

[BillB]

The best courses in CN are given all over the country at low cost by the United States Power Squadrons. Check out: Find a Course
Select Advanced Courses, get Junior Navigation from the list (it's the first of the two courses) and plug in a zip code.
If nothing pops up in your area (USPS Squadrons usually take the summer off) try contacting your local Squadron for their upcoming schedule.Find a Squadron

[Nessun Dorma]

Paps, that is a funny story about the light bulb going on. My Dad taught my brother and me how to take noon sights using that same analogy, except being Navy he used airplanes, pilots with parachutes and anti aircraft guns as his proxies. So I did the same with my kids.

You're right though, for most people it "clicks" all at once, sort of like when we learned to ride a two wheel bike or swim. And,like aything else, the more you practice it the easier it is and more fun is.

For those who want to learn,I always suggest taking a class or finding someone to teach you. Most who kow are all too happy to teach. Self teaching isn't easy but there are better books than the Blewitt classic (not knocking it, it's just dated;I have a well-loved 1952 hardback that smells like old mold .. ). Celesteial Navigation in the GPS Age is a goodbook written five or six years ago by J. Karl that teaches well and makes the practice more relevant to a 2012 sailor. I've used it to teach 4 or 5 others and they all seem to like it.

Quite a woman.
http://www.telegraph.../Mary-Pera.html
I think that there are newer introductions to celestial
http://www.amazon.co...e/dp/1898660751
I really fancy one of Stokey Woodall's courses.
http://international...rses/4558930559

Thanks Ed.
I'll grab a copy of Tom's book too.

I'm in over my head (again), of course.
But this is something I always wanted to learn.

Lamby, I found myself in a position where I had to learn CN very fast. We were at sea, 12 or so days from our destination when it became obvious the owner and his girlfriend had no fucking idea! They would both take a sight, do the calcs and could not get within 100 miles of each other. They did have on board though a decent library so I set to it.

I had real difficulty "getting" it and was trying to process way too much information at once. The physical actions, the geometry, the reductions and corrections etc. Just when I thought I can never do this I picked up a little paperback which was basically CN for dummies and the penny dropped. After that it was just practice and getting to know the calculations. The book recommended using the sight reduction tables for air navigation rather than the nautical ones and they are indeed easier to use.

In my case I just couldnt get my head around the basic concept of what I was sighting, adjusting and calculating for! I am one of the types that has to understand the way things work rather than taking things at face value. If I want to understand how something works, simple, pull it apart!! Many Xmas presents ended up in pieces.

The message from the book that cracked it wide open for me was something like this. LOP, imagine the sun is a duck and the sextant is a gun. You shoot the duck and record the time. Now imagine the duck falls straight down and where it splashes in the water is the suns GP. From the sextant you know the angle between you and the duck. From the almanac you know the suns GP @ that exact time. So you have an exact point being the suns GP and an angle which gives you a circle around that point you must be on. Progressive sights give you intersects of those circles and voila you can narrow down which part of the circle you are on. These are called running sights or line of position (LOP).

Once I got that gist the rest was easy by comparison. Perfecting the actual sextant shots, developing a regimen around correcting the time piece and getting comfortable with the calcs and corrections. We got to Tokyo more or less and from there I hit LA 6000 mi away +/- 30 miles after 3 days of overcast.

Sorry this has turned into an essay but the message is, get the basics and dont let the technics bog you down.There is way too much information to take in all at once. Baby steps.

[SemiSalt]

Y'all know this, but to articulate for anyone who doesn't, CN requires 1) spherical trigonometry plus 2) how to use a sextant plus 3) how to correct the sextant sight you get for the ideal sight you need. The 2 & 3 are pretty much the same for every method, but a lot of methods were created to simplify the math for the common man.

The point being that any one book on CN is probably focusing on one method. So you want to pick the book you want based on the sight reduction method you want which you don't know until you've read the book.

I think the most common method is HO214 which consists of volumes of tables which reduce the math to addition and subtraction. However, with a programmable calculator from CVS and the forumlas in the appendix, you can do without the books.

[DDW]

If you buy the program AstroNav for your iPhone, with this (and nothing else) you can take the sight, identify the body, reduce the sight, plot the sight, then check it against your GPS position .

[pogen]

If you buy the program AstroNav for your iPhone, with this (and nothing else) you can take the sight, identify the body, reduce the sight, plot the sight, then check it against your GPS position .

I think the ideal would be to do it with no electronic gizmos. As if one had lost all powered devices through a lightning strike or something. I guess the government could turn off GPS and then you would have you gizmos working.

We have 4 GPSs on board so as long as the satellites work we should be OK!

[VALIS]

Take a look at "Step-by-Step Sextant Users Guide", by Andrew Evans (Singlehanded Transpac "Foolish Muse", anarchist "Foolish"). This used to be posted on the MiniTransat site, but that link is dead. Blewitt's book is great, and for that matter William F. Buckley did a nice instructional video (so I am told), but Andy's guide is very well written, and free.

[Nessun Dorma]

I dont think its so much the fear that the GPS systems will crash or we'll all be out there with 5 non-functioning backup, battery operated GPS; I think (for me at least) it has more to do woth the satisfaction that comes with doing something challenging that puts us in touch with our environment and the larger planet. There's something special about making landfall without a GPS that's hard to explain unless you've done it.

If you buy the program AstroNav for your iPhone, with this (and nothing else) you can take the sight, identify the body, reduce the sight, plot the sight, then check it against your GPS position .

I think the ideal would be to do it with no electronic gizmos. As if one had lost all powered devices through a lightning strike or something. I guess the government could turn off GPS and then you would have you gizmos working.

We have 4 GPSs on board so as long as the satellites work we should be OK!

[DDW]

I agree that it is the dabble into the arcane that keeps us interested in celestial. I wonder though: where do you get your time reference? Off the GPS ?

Probably the easiest small format table for sight reduction are the S tables. Still need a current almanac though....

[Cavelamb]

I agree that it is the dabble into the arcane that keeps us interested in celestial. I wonder though: where do you get your time reference? Off the GPS ?

Probably the easiest small format table for sight reduction are the S tables. Still need a current almanac though....

Time reference would be from the ship's Timex - checked for millisecond accuracy with a noon shot?



I built a "ship's bell" system based on a P8x32 (Propeller) processor.
The first cut just plays a wave file of the bells (which is up and working!), so with that and the real time clock driver code there
isn't a lot of memory left to add other "bells and whistles".

So to set the time on the real time clock chip, I'm just using a single input bit with a push button switch.
Push the button at local noon - out pops eight bells.

Hey, it's "traditional"!

[kdh]

If you buy the program AstroNav for your iPhone, with this (and nothing else) you can take the sight, identify the body, reduce the sight, plot the sight, then check it against your GPS position .

Very funny DDW. Once I get rid of my dumb phone I'll do this. Of course, the smart phone will have a GPS.

The sextant just gets you the angle between the thing in the sky and the horizon in its direction, no? So with one sight you are somewhere on the planet that is at that angle. Seems like that would be a circle, given a spherical model of the earth. Knowing at what point the celestial body would fall to the earth and your distance from it are sufficient to define that circle.

Two sights would be required then to get a position, but I thought noon sun sights were all that was needed.

Please explain.

[VALIS]

Two sights, yes. These can be two different astronomical bodies (Sun, Moon, Venus, Antares, etc), or they can be the same body (sun, usually) with the sight taken at different times. This is the "running fix", and you have to also plug your course and distance run from the first fix to the next into the plot. As you say, a single sight gets you a circle, called a "line of position" since the radius is usually so large that the arc you are interested in is essentially a straight line.

It isn't necessary that any of these sights be noon sights, but the noon sight is a convenient way to establish your latitude. Polaris sight is even easier, as long as you can see the horizon. You had better take everything I say on this topic with a grain of salt, since I've only ever made a handful of celestial fixes.

[edit] And as Cavelamb points out, with the noon sight and knowledge of GMT, you can indeed calculate both lat and lon. And here I was recently raving about the "Longitude" book and movie!!! I wonder about the accuracy though, as the point where she sun reaches maximum height is hard to get exactly -- it's a gradual arc. In the movies they seem to agree within a few seconds, so I guess it's pretty good.

[DFL1010]

My prefered method is to have one primer and one authoritative text. For my money, it's Tom Cunliffe's (see Presuming Ed's link above) and a copy of BR45(2). This is available from the Nautical Institute (nautinst.org) but is £60. In my opinion Bowditch is almost as good but it is free. I like this method because one will give the bare bones and the other will give insane detail for if it doesn't full sense (eg a full chapter on different types of time). One more point to bear in mind is that the simpler books will give one method of doing the sums and one way of describing them; what is obvious to some is not to others. Finally, American publications seem to prefer (free online) sight reduction tables (229 and 249 tables) whereas British pubs prefer the intercept method (Marcq St. Hilaire).

[Cavelamb]

If you buy the program AstroNav for your iPhone, with this (and nothing else) you can take the sight, identify the body, reduce the sight, plot the sight, then check it against your GPS position .

Very funny DDW. Once I get rid of my dumb phone I'll do this. Of course, the smart phone will have a GPS.

The sextant just gets you the angle between the thing in the sky and the horizon in its direction, no? So with one sight you are somewhere on the planet that is at that angle. Seems like that would be a circle, given a spherical model of the earth. Knowing at what point the celestial body would fall to the earth and your distance from it are sufficient to define that circle.

Two sights would be required then to get a position, but I thought noon sun sights were all that was needed.

Please explain.

All the noon sight does is tell you when the sun peaks.
That's local noon.

Now, if we have Greenwich time on board then the difference between Greenwich time and local time is
our longitude.

Of course, working out that difference is left as an interesting exercise for the student...

[DFL1010]

1339980847[/url]' post='3755027']

1339979098[/url]' post='3754995']

1339947344[/url]' post='3754495']
If you buy the program AstroNav for your iPhone, with this (and nothing else) you can take the sight, identify the body, reduce the sight, plot the sight, then check it against your GPS position .

Very funny DDW. Once I get rid of my dumb phone I'll do this. Of course, the smart phone will have a GPS.

The sextant just gets you the angle between the thing in the sky and the horizon in its direction, no? So with one sight you are somewhere on the planet that is at that angle. Seems like that would be a circle, given a spherical model of the earth. Knowing at what point the celestial body would fall to the earth and your distance from it are sufficient to define that circle.

Two sights would be required then to get a position, but I thought noon sun sights were all that was needed.

Please explain.

All the noon sight does is tell you when the sun peaks.
That's local noon.

Now, if we have Greenwich time on board then the difference between Greenwich time and local time is
our longitude.

Of course, working out that difference is left as an interesting exercise for the student...

The other thing that Local Apparent Noon gives is an altitude at Meridian Passage: sun is at its max. altitude for that day and will bear due north or due south of the observer. Therefore the position line will run 90° to North/South, i.e. East/West, i.e. a latitude.

[kdh]

Cave, time from GMT relative to 24 hours is your longitude relative to 360*.

What do I win?

Edit: I've always known the expression as "left as an exercise for the interested reader."

[kdh]

All the noon sight does is tell you when the sun peaks.
That's local noon.

Now, if we have Greenwich time on board then the difference between Greenwich time and local time is
our longitude.

The other thing that Local Apparent Noon gives is an altitude at Meridian Passage: sun is at its max. altitude for that day and will bear due north or due south of the observer. Therefore the position line will run 90° to North/South, i.e. East/West, i.e. a latitude.

Got it. Time at local apparent noon gives longitude and the sight reduction gives a latitude line. Makes a sight at local noon sufficient. How does one know when the sun is due N on a boat to make the local noon determination?

Edit: I just looked it up. You just take successive sights with the sextant waiting for maximum altitude.

[DFL1010]

1339981870[/url]' post='3755050']

1339981276[/url]' post='3755035']

1339980847[/url]' post='3755027']
All the noon sight does is tell you when the sun peaks.
That's local noon.

Now, if we have Greenwich time on board then the difference between Greenwich time and local time is
our longitude.

The other thing that Local Apparent Noon gives is an altitude at Meridian Passage: sun is at its max. altitude for that day and will bear due north or due south of the observer. Therefore the position line will run 90° to North/South, i.e. East/West, i.e. a latitude.

Got it. Time at local apparent noon gives longitude and the sight reduction gives a latitude line. Makes a sight at local noon sufficient. How does one know when the sun is due N on a boat to make the local noon determination?

Almanac will give you the gmt time of mer pass, then apply your long as longitude in time (table in almanac or by calculator) - this relies on your longitude being accurate.
However, the absolute method of getting it cock-on is that the alt of the sun (or star or planet - same idea) will reach its max and start going down. Therefore get outside before you think it will be (bearing in mind that your long maybe wrong therefore est time of mer pass may be out) and keep shooting. When altitude starts going down, that's it.

[Cavelamb]

All the noon sight does is tell you when the sun peaks.
That's local noon.

Now, if we have Greenwich time on board then the difference between Greenwich time and local time is
our longitude.

The other thing that Local Apparent Noon gives is an altitude at Meridian Passage: sun is at its max. altitude for that day and will bear due north or due south of the observer. Therefore the position line will run 90° to North/South, i.e. East/West, i.e. a latitude.

Got it. Time at local apparent noon gives longitude and the sight reduction gives a latitude line. Makes a sight at local noon sufficient. How does one know when the sun is due N on a boat to make the local noon determination?

Not due north - but as high as it's going to get...

[kdh]

Thanks DFL and cave, I've learned a lot. Cool to know that with just a crude way of measuring angles, a watch and the sun (longitude), and Polaris (latitude), we can place ourselves on the planet.

[Cavelamb]

Two sights, yes. These can be two different astronomical bodies (Sun, Moon, Venus, Antares, etc), or they can be the same body (sun, usually) with the sight taken at different times. This is the "running fix", and you have to also plug your course and distance run from the first fix to the next into the plot. As you say, a single sight gets you a circle, called a "line of position" since the radius is usually so large that the arc you are interested in is essentially a straight line.

It isn't necessary that any of these sights be noon sights, but the noon sight is a convenient way to establish your latitude. Polaris sight is even easier, as long as you can see the horizon. You had better take everything I say on this topic with a grain of salt, since I've only ever made a handful of celestial fixes.

[edit] And as Cavelamb points out, with the noon sight and knowledge of GMT, you can indeed calculate both lat and lon. And here I was recently raving about the "Longitude" book and movie!!! I wonder about the accuracy though, as the point where she sun reaches maximum height is hard to get exactly -- it's a gradual arc. In the movies they seem to agree within a few seconds, so I guess it's pretty good.

I'm guessing a 500 to 1000 ton ship is a bit steadier platform than my 6000 pound sloop.

1 second arc = 1 NM (at the equator) so 10 or 15 seconds is plenty close enough at the moment.

But recall at the beginning of the movie when the captain is asking his officers for THEIR numbers.
They all agree - exactly - just before the entire squadron runs into the reefs.
(and the poor slob who was right is still hanging from the yard arm!)

In the modern navy the junior officers are the first to speak.
For exactly that reason.

[Cavelamb]

The other thing that Local Apparent Noon gives is an altitude at Meridian Passage: sun is at its max. altitude for that day and will bear due north or due south of the observer. Therefore the position line will run 90° to North/South, i.e. East/West, i.e. a latitude.

It wouldn't surprise me if I have a whole lot of misconceptions about this right now.
But I hadn't made that association.

Is the noon sight always due N or S ???
I'm trying to see that in my mind but the Shiner Bock is getting in the way.

[DFL1010]

1339984549[/url]' post='3755115']

1339981276[/url]' post='3755035']
The other thing that Local Apparent Noon gives is an altitude at Meridian Passage: sun is at its max. altitude for that day and will bear due north or due south of the observer. Therefore the position line will run 90° to North/South, i.e. East/West, i.e. a latitude.

It wouldn't surprise me if I have a whole lot of misconceptions about this right now.
But I hadn't made that association.

Is the noon sight always due N or S ???
I'm trying to see that in my mind but the Shiner Bock is getting in the way.

Yes. Meridian passage (which is what we're talking about here) is when the GP of the Sun (star, planet) passes over your longitude. Therefore it must be either north or south of you. Compare the body's declination with your latitude and bob's your uncle.

[Cavelamb]

The other thing that Local Apparent Noon gives is an altitude at Meridian Passage: sun is at its max. altitude for that day and will bear due north or due south of the observer. Therefore the position line will run 90° to North/South, i.e. East/West, i.e. a latitude.

It wouldn't surprise me if I have a whole lot of misconceptions about this right now.
But I hadn't made that association.

Is the noon sight always due N or S ???
I'm trying to see that in my mind but the Shiner Bock is getting in the way.

Yes. Meridian passage (which is what we're talking about here) is when the GP of the Sun (star, planet) passes over your longitude. Therefore it must be either north or south of you. Compare the body's declination with your latitude and bob's your uncle.

Yeah. I can see it now - sorta fuzzy like.
But yeah.
Shame we can't see Polaris during the day, huh?

Cool.
Another penny drops.
Only 99 more to go.

[DFL1010]

1339985756[/url]' post='3755137']

1339984938[/url]' post='3755122']

1339984549[/url]' post='3755115']

1339981276[/url]' post='3755035']
The other thing that Local Apparent Noon gives is an altitude at Meridian Passage: sun is at its max. altitude for that day and will bear due north or due south of the observer. Therefore the position line will run 90° to North/South, i.e. East/West, i.e. a latitude.

It wouldn't surprise me if I have a whole lot of misconceptions about this right now.
But I hadn't made that association.

Is the noon sight always due N or S ???
I'm trying to see that in my mind but the Shiner Bock is getting in the way.

Yes. Meridian passage (which is what we're talking about here) is when the GP of the Sun (star, planet) passes over your longitude. Therefore it must be either north or south of you. Compare the body's declination with your latitude and bob's your uncle.

Yeah. I can see it now - sorta fuzzy like.
But yeah.
Shame we can't see Polaris during the day, huh?

Cool.
Another penny drops.
Only 99 more to go.

The way I started to be able to visualise it was to sit down with a case of wine and keep drinking until it made sense. A more sensible option is to download something like Stellarium onto your computer and play around with your position and time until what it gives you kinda correlates with your sums/ what you think it should look like.
I prefer my method though.

[DDW]

A meridian transit of the sun will get you a longitude, but not a very accurate one. There is a good reason so much effort was sunk into the development of the chronometer. The altitude is a very slowly varying function at local noon. Slocum used Lunar methods to get time - they are described in Bowditch and seem somewhat complicated.

With AstroNav and the iPhone, you sight along the edge of the case. The built in inclinometer measures the angle, you don't need a horizon. It then does the solution to the spherical triangle using the almanac contained in the program (and time from GPS...). I've been able to get within 50 miles or so consistently with three stars. But a meridian transit isn't going to get you much closer.

[I'moutahere]

Two sights, yes. These can be two different astronomical bodies (Sun, Moon, Venus, Antares, etc), or they can be the same body (sun, usually) with the sight taken at different times. This is the "running fix", and you have to also plug your course and distance run from the first fix to the next into the plot. As you say, a single sight gets you a circle, called a "line of position" since the radius is usually so large that the arc you are interested in is essentially a straight line.

It isn't necessary that any of these sights be noon sights, but the noon sight is a convenient way to establish your latitude. Polaris sight is even easier, as long as you can see the horizon. You had better take everything I say on this topic with a grain of salt, since I've only ever made a handful of celestial fixes.

[edit] And as Cavelamb points out, with the noon sight and knowledge of GMT, you can indeed calculate both lat and lon. And here I was recently raving about the "Longitude" book and movie!!! I wonder about the accuracy though, as the point where she sun reaches maximum height is hard to get exactly -- it's a gradual arc. In the movies they seem to agree within a few seconds, so I guess it's pretty good.

I'm guessing a 500 to 1000 ton ship is a bit steadier platform than my 6000 pound sloop.

1 second arc = 1 NM (at the equator) so 10 or 15 seconds is plenty close enough at the moment.

But recall at the beginning of the movie when the captain is asking his officers for THEIR numbers.
They all agree - exactly - just before the entire squadron runs into the reefs.
(and the poor slob who was right is still hanging from the yard arm!)

In the modern navy the junior officers are the first to speak.
For exactly that reason.

That is the second time that mistake has been posted in a couple of weeks, and to make matters worse - nobody has noticed.


One MINUTE of latitude is one nautical mile.


I have found that an easy way to explain is that if you measure the angle to the top of a lamp post from any point on a circle around the lamp post, the angle will be the same - and the base of the pole is the GP of the object.


Time link

[VALIS]

And one minute of time = 360 degrees / (24 hours * 60 minutes) = 360 / 1440 = 0.25 degree = 15 minutes latitude = 15 nautical mile.

One minute time error puts you off by 15 NM north or south, or E/W at the equator.
Four seconds of time error gives one NM distance error.

Assume your visual horizon is about six miles. To spot that low atoll your chronometer should be within 24 seconds of true (making the unreasonable assumption that there are no other errors).

[stranded]

+1 Johnny --- lamp post

This was also how I was taught to understand the concept of angular altitude ( and solving from an assumed position )


Nobody has discussed graphing time / observed altitude ( either at noon or mid morning / afternoon ), so that clearly incorrect shots can be discarded, and can help to identify local noon / max altitude ( is in Blewett )


Nobody has discussed how to work to the LOP using a plotting sheet with mid lat of the sheet = or close to assumed lat ( is in Blewett )


- - - - - - - and there is an admiralty " chart " ( has an HO No. ) that is a graph that provides the same information as the air tables.





The i Phone app........ Can it work beyond range of mobile signal / wifi ?

[DDW]

The i Phone app........ Can it work beyond range of mobile signal / wifi ?

Sure - although as the clock gets off over time it will become increasingly inaccurate. That IPhone app (and many others) also allows punching in the altitude from a sextant and calculates the solution. Out of network range you won't get a nice plot on the map, either....

[Cavelamb]

Two sights, yes. These can be two different astronomical bodies (Sun, Moon, Venus, Antares, etc), or they can be the same body (sun, usually) with the sight taken at different times. This is the "running fix", and you have to also plug your course and distance run from the first fix to the next into the plot. As you say, a single sight gets you a circle, called a "line of position" since the radius is usually so large that the arc you are interested in is essentially a straight line.

It isn't necessary that any of these sights be noon sights, but the noon sight is a convenient way to establish your latitude. Polaris sight is even easier, as long as you can see the horizon. You had better take everything I say on this topic with a grain of salt, since I've only ever made a handful of celestial fixes.

[edit] And as Cavelamb points out, with the noon sight and knowledge of GMT, you can indeed calculate both lat and lon. And here I was recently raving about the "Longitude" book and movie!!! I wonder about the accuracy though, as the point where she sun reaches maximum height is hard to get exactly -- it's a gradual arc. In the movies they seem to agree within a few seconds, so I guess it's pretty good.

I'm guessing a 500 to 1000 ton ship is a bit steadier platform than my 6000 pound sloop.

1 second arc = 1 NM (at the equator) so 10 or 15 seconds is plenty close enough at the moment.

But recall at the beginning of the movie when the captain is asking his officers for THEIR numbers.
They all agree - exactly - just before the entire squadron runs into the reefs.
(and the poor slob who was right is still hanging from the yard arm!)

In the modern navy the junior officers are the first to speak.
For exactly that reason.

That is the second time that mistake has been posted in a couple of weeks, and to make matters worse - nobody has noticed.


One MINUTE of latitude is one nautical mile.


I have found that an easy way to explain is that if you measure the angle to the top of a lamp post from any point on a circle around the lamp post, the angle will be the same - and the base of the pole is the GP of the object.


Time link

Bless me father, for I have sinned.

And bless Johnnie the Saint for catching it and correcting it.

All I can say is that I must have been in a real hurry!

[stranded]

time signals are more accurate than " rating " a time piece

start a stop watch from wwv ( or its equivalent in other parts of the world ) at the start of a minute

before taking sights, then take time from the stopwatch - - - now we have multiple " splits " on many watches and phones

and check again after

was pretty accurate for me

I used a mechanical " no split " old time stop watch.


- - - - - and read the explanatory information in the front of the air tables

[Cavelamb]

Interesting site...


Rocks are for scotch
… not boats



Celestial Navigation - DOS programs
http://www.celnav.de/page3.htm



Navigation rules (international) - learn by multiple guess?
http://www.proprofs.com/quiz-school/story.php?title=navigation-rules-internationalinland



Navigation Rules - Lights only
http://www.proprofs.com/quiz-school/story.php?title=nav-rules-lights

[Cavelamb]

Mary's book came today.
I scanned through it, and yeah, I think I can follow it.
I guess it's time to buy a sextant...

Attached Files

•  med_sighting.gif   12.51K   17 downloads

[Paps]

Phone app?



We don't no steenkin phone app!!!!

[cap10ed]

I started with Power Squadron courses and realized I could handle the math. I used Mary Blewitts book to unravel the jargon given by my college prof. at Marine school. The book that fascinated me the most was by the American John Letcher. There is a great story how he thought he had devised Lunar Distance sights to get the correct time for his clock. If you get what Mary tells you then go and listen to John.
Self-Contained Celestial Navigation with H.O. 208