Sextant?

[mariner 40]

Hi
I'm looking at different sextants, metal and plastic. Is the plastic easier to use because of weight. Or, is the metal one more stable? Someone with experience please post!!
Mike

[safewalrus]

When plastic sextants where first produced they rapidly became known as "throwaway sextants - you take one sight then ditch it!" by the professional Navigators!:p

Couple of points to add to that,

for accuracy the proper metal (bloody expensive) full blown sextant has no equal, heavy, a joy to use and bloody acurate with the right use!:cool:

If your on the bouncing deck of a small vessel using GPS as your main means of deepsea navigation a plastic sextant whilst not as accurate should be able to give you a reasonable degree of accuracy (if the boat and thus horizon is going up and down like a roller coaster you probably won't be able to get much better accuracy anyway) :idea:

plus of course if in the use of this you either bash, bang or otherwise bend the damn thing (includes dropping it over the side) you wont cry so much when you buy a new one!:P

The other thing is if your new to sextants (good on you for even wanting to try) :idea: why not start with a cheaper plastic sextant to learn with (obviously the best you can buy, but still cheaper than a metal job) then if .......the above applies.:D

One last thing, IF you DO get a plastic sextant don't ever tell a big ships (Warship OR Merchant) Navigator or Captain / Master you have one! He'll likely wet himself laughing!:rolleyes: :rolleyes:

But seriously in the firast place I'd go for the plastic job until I have some esperience -I well remember the first time I used one (in college - THEN I was told the price AFTER I put it down, Good job too! nearly did brown stains somwhere!):D

best of luck

After taking a sight by sextant (and all the math - especially in my day - log tables and the like!) it was a wnderful feel of achievment, even better as a junior officer when you got a better fix than either The Chief Officer or the Master!!!)

[Robert Gainer]

Plastic is fine unless you want to do navigation as a hobby and you have extra money to spend. My first trans-Atlantic was with a Davis and I was only 1/2 mile north of Bishop Rock Light when I was aiming to be 1/2 mile south of the light. I now own a Plath among other models but that’s because I get a lot of enjoyment from using it and I can still use the plastic one and get very good results. By all means start with plastic and as your skill improves you can make an informed decision about upgrading later.
Robert Gainer

By the way, a sextant and taffrail log is still my primary means of navigation and my first offshore passage with a GPS was only last November during a trip to Bermuda from VA. My upcoming trip to Greenland will still be done using traditional means.

[safewalrus]

Robert

Whilst I applaud yor infinite skills I fear Columbus, Nelson, Frobisher Magellan and others of that ilk would find you a crazy person! By all means use the traditional means ( I for one mourn their passing) but please carry a modern method for checking and use in case of danger - a prudent navigator uses everything he has and can get - including and especially his brain!:cool:

[Robert Gainer]

Different people enjoy different hobbies. Some people still fly a biplane and in spite of its archaic design consider that great. Some people will climb a mountain “clean” and forgo the use of aid or protection. Others decide to sail rather then use a motorboat because they do not want an engine to intrude into their world. These people do not tell you what’s good or bad about your hobbies and demand that you do it there way.

I am not telling anyone that they should sail in my style or only use a sextant in preference to more modern methods. As a matter of fact I use the best available technology when it’s appropriate but in pursuing my own interests I may use older methods. I design and build traditional boats using wood and mostly traditional methods.

I sail very simply using my own skills without the help of modern technology for some of the same reasons that the Amish still farm and live without the modern conveniences and distractions. Does this make my “crazy”? You say that you mourn the passing of the older skills, well I teach the older skills and practice them as often as I can. You choose your path and I will not criticize you but let me decide how I will lead my own life.
Robert Gainer
Boat Building Program Director
Hudson Fisheries Trust
Beacon, New York
www.fisheriestrust.org

[Bergalia]

My worst nightmare realised. I have to agree wholeheartedly with Walrus.
Congratulations on your enthusiasm Mariner 40. Every - repeat - every would-be sailor (excuse the pun) worth his salt should know how to use a sextant, a compass and chronometer - if only for the joy of the exercise. Too many boaties take to the sea relying on Satnav and its ilk...too many boaties get lost and come to grief because the "computer let me down."

As Walrus - in a rare moment of sobriety - suggest: go for a plastic cheapie to begin with, then when you feel happier buy the "real thing." After all who takes driving lessons in a Ferrarri. ;)

[safewalrus]

By God Bergalia if this is sobriety I'm back on the beer! Can't handle the Bergalia being nice to me!

Robert I'm not knocking your lifestyle (you may not think so) you obviously are good at it, unfortunately when a learned person such as yourself states "I do such and such in the old style and it's fantastic" many others without your competance look at this and decide if HE can do it so can I. They then proceed to go ahead and kill themselves (their problem admittingly) this causes some problems to the poor devils that try to rescue them and their families both of who suffer!

Perhaps a better way would be to state "I do such and such in the old way, but I AM AN EXPERT in my field and will use other more modern technical things as back up should I need to!

Remember the Amish do what they do for all their lives which makes them experts in their field (no pun intended), whilst they shun modern ways they do it entirely from birth (though they may leave if they so wish), this is diferent from the guy who does it at weekends only - no matter the action! So please feel free to live your life as you deem fit, just don't ruin anybody elses in the process - a mere statement can accomplish this without you realising it!

[mariner 40]

Hello Guys
I have purchased a used Plath on ebay for the price of a new plastic sextant. My desire to have knowledge of use of a sextant is for backup to GPS system. I do not want to go to sea and the battery die and depend on someone else to bail my ass out!! I want to use both GPS and sextant together to improve my accuracy of in using a sextant. Then and If the GPS goes away, I won't be saying "WHAT NOW" "WHERE AM I"!! If I have learned the proper use of the sextant, I can keep right on. Thanks for the input guys!! Maybe some of you might refference a good instructional book on use. Better yet, maybe some of you old seadogs might want to venture out one day with me to places unvisited!!!
Mike

[BillyDoc]

Mariner 40,

Way to go! I learned on a Plath myself. Except I bought it new in Annapolis.

And don't let those wild stories about bouncing boats discourage you, I did my sailing on a 25 foot Contessa . . . and managed to find land every time using only my trusty sextant, a digital watch, and a nautical almanac. Which brings me to the point of this post.

The math for getting a position using stars is tedious, to say the least. Good to know, but tedious. For a start, I would concentrate on the venerable noon sight. This will give you your position in both latitude and longitude quite accurately (within a mile, usually), but is limited to, well, a noon position. It also involves nothing more than a nautical almanac, a chronometer (and a good modern digital watch works pretty well for that) and a sextant. The whole thing takes about 30 minutes. No fancy tables are involved, and the math is a couple of subtractions and a lookup in the almanac. With a small sailboat you aren't going anywhere all that fast anyway.

The other point I would like to make is that even if your chronometer fails you can find your port with your sextant and almanac alone, just like Columbus did. The almanac will tell you how high the sun will get on any given day, and thus the latitude you would be if it was directly overhead. Using the same math as for the noon sight you can therefore determine your latitude directly, even if you don't have a clue about the time. So, like Columbus you sail to the latitude of the port you are trying to find and simply sail East or West as appropriate until you run into it.

That sextant is a wonderful backup to the GPS. Electronics fail, but sextants are quite reliable . . . unless you drop it overboard! I kept a light line tied to mine at all times when I was on deck with it.

Fair winds!

Bill

[mariner 40]

Hello
I have a backup GPS, compass, watch, and leadline. I hope I will never need them but, it's worth the extra for comfort.
Mike

[safewalrus]

Billy you seem to know a lot about Columbus, you wearn't his Yeoman were you by any chance? (actually sextants were not invented in his day) and I bet you found the nearest land every time eh! Straight down!!!:cool:

Mike for books on the subject best have a chat with the ancient mariner your side of the pond, one Robert Gainer - he after all will know the in book on your side of the pond (actually as Bill will tell you there's not a lot of difference between methods [unless you use air tables!] but it's best to be talking the same language as the rest of the people in your area! That is if the salty old bugger hasn't gone of in a huff 'cos I popooed his use of gunpowder!:rolleyes: :P

[Wellydeckhand]

Plastic melt and break.......... sometime the marking is....... well for cheap asian version. We always get the reject.

Stick to the metal expensive item........ dont buy one in Indonesia unless u r the one selling it.:D

[BillyDoc]

Walrus,

Lately I've been feeling old enough to have been Columbus's yoeman, but, alas, I never met the man.

And you are definitely right about the sextant not being invented then, in fact, I believe a countryman of yours invented it, one I. Newton. But Mr. Newton's invention was an improvement in the instrument, not the method. Columbus would probably have been quite familiar with the Cross Staff or Back Staff, both of which measure the angle of the sun over the horizon just like a sextant, and darn near as accurately too. Of course you go blind using the Cross Staff, a slight problem.

Bill

[Robert Gainer]

I started out with instruction from someone that know what he was doing and used the book “Navigation the Easy Way” by Carl Lane and John Montgomery. That book is long out of print and you might try "Common Sense Celestial Navigation" by Schlerith instead. I have read that book and found it very usable. "A popular one is Celestial Navigation for Yachtsmen" by Mary Blewitt. I have not read this one myself but instructors whose opinion I respect very often recommend it. When you read an older book keep in mind that some of the terms are named differently now. Time was measured using GMT and now it is called UT. That means Greenwich Mean Time and Universal Time. For all practical purposes, they are the same thing. If the book is old enough you will see RA instead of HA. This is the change from Right Ascension to Hour Angle. They are not quite the same thing but today’s nautical almanac used GHA, which is Greenwich Hour Angle instead of the term RA, used in astrometry.

Navigation does not have to be complex and the noon sight is about as simple as it gets. You don’t need to know time and all you do is follow the sun as it reaches your local noon and record the greatest altitude. Correct that for dip, refraction, and index error. Subtract that number from 90 degrees and add the sun’s declination that you get from the daily pages in the nautical almanac and you have latitude as accurately as you can read the sextant. The dip is just a correction for the height of your eye above the water and refraction is a correction for the fact that light bends as it goes from one density to another. The air becomes denser as you get closer to the surface of the earth compared to higher up. The index error is how far off the sextant is and you find that by a simple test using the sextant and looking at an object more then 1 mile away. You cannot find your longitude with a noon sight. The sun or any heavenly body during a meridian transit hangs at the same altitude for several moments. This makes it imposable to get the time of the meridian transit unless you are on dry land with very precise equipment. Meridian transit is just the name for the time at which a body crosses your meridian of longitude, otherwise called noon.

Modern inspection tables have removed the need to use trig or log tables unless you are doing this as a hobby. The books Ho. 229 and HO. 249 are the old standbys but HO. 211 is a small booklet that will fit into you sextant case instead of the seven volumes needed by some other methods. The current Nautical Almanac also has a set of sight reduction tables in it that are not hard to use.

Try it and you’ll be surprised at how easy this is and you will get a sense of satisfaction that most other people will never realize. By the way, don’t go offshore and kill yourself. Offshore work after all is reserved for the privileged few who got there experience in mystical ways that are imposable to duplicate today.
Good luck and all the best,
Robert Gainer

[trouty]

Chop it out - I can't afford a sextant!:rolleyes:

Actually a guy just advertised a course on celestial nav using sextant - hoping to get enough students to run a class locally and I was very sorely tempted to go along - but am short on $ and time right now!

Safewalrus... re:

Billy you seem to know a lot about Columbus, you wearn't his Yeoman were you by any chance? (actually sextants were not invented in his day) and I bet you found the nearest land every time eh! Straight down!!!

Hey - I believe your likely right, about the invention of the sextant - BUT having said that AND watched the TV show bout the guy who invented the reliable timepiece for vessels (ships chronometer) so that longitude could be reliably determined at sea.. I figured I knew a little bout such things.

A while back I visited one of our matritime museums.., and they have relics from various sunk Dutch east indiamen discovered along our west coast!.

These seemed to have all sunk round Chris Columbus' time - when they would round cape whatever in africa (good hope / horn)?...and sail west on the roaring forties till they sighted good ol Oz and then turn left at alberquerky...and er, well - you get the idea arrive in the nightime and run aground! :rolleyes:

Now - this was apparently because they couldn't tell the time..:rolleyes: sounds like my missus when she's getting her war paint on to go out! :eek:

BUT - the point of my question is this.

Often one of the relics recovered from such Dutch east indiamen is a round brass instrument graduated with a long needle pivoted in the middle, that is usually referred to as an "astrolabe" and was apparently used in their primitive attempts at navigation??.

So - what was an astrolabe? how'd it work? and how is it different from a sextant?

I'd appreciate the encyclopedia britannica explanation - but would settle for the readers digest version!:D

Thanks in advance!

Cheers!

[jehardiman]

So - what was an astrolabe? how'd it work? and how is it different from a sextant?

I'd appreciate the encyclopedia britannica explanation - but would settle for the readers digest version!:D


An astrolobe was an arab invention used to measure the heights of the stars from the vertical (i.e. the azimuth). This gave them the ability to determine the approximate direction to Mecca, rather than just "over that way". It was basicaly an angle scale with a pivioting sight and a ring to hang it by. Some had the declination corrections on them or a mechanical declination correction. Used primarly on land, they were taken to sea, but because of roll and pitch, proved hard to use. This lead to the development of the cross staff, followed by reasons explained above to the back staff, the quadrent, octant, and finally, the sextant.

For a good text on navigation during the time of Columbus, there is always "The Mariner's Mirror", state of the art circa 1588. And there was a text published several years ago on Viking "sun stone" latitude navigation.

Mariner 40, one thing about used metal sextants, especially quality ones, that are for sale really cheap...if it has been droped it will not read correctly until realigned and may be for sale because it can't be realigned. At that point it is just a resturant decoration.

[mariner 40]

Hello
To show how green I am. I am not afraid to say when I screw up!! Yesterday, after I bought a "Plath" on ebay. I received several emails asking if I realized this sextant was made in India as a knockoff? That the seller was passing these as original. What is the best way to tell? I think now my only option is buying a new sextant! I really hate it when I feel so dumb!!

[Robert Gainer]

Nice description jehardiman, but I think azimuth is the direction or bearing and not the altitude of the body.
Robert Gainer

[Robert Gainer]

Hello
To show how green I am. I am not afraid to say when I screw up!! Yesterday, after I bought a "Plath" on ebay. I received several emails asking if I realized this sextant was made in India as a knockoff? That the seller was passing these as original. What is the best way to tell? I think now my only option is buying a new sextant! I really hate it when I feel so dumb!!
Sorry you got burned mariner 40, but you may be able to tell by looking at the picture if you have seen enough sextants. The other way is to look at the history of the seller. Very few people can sell more then one or two real sextants. If the seller is dealing in lots of brass and has many sextants and other similar types of things in his feedback you can say it’s a fake. Look in the feedback and hit on some item number. It shows you what he was selling in the last couple of months.
Robert Gainer

[jehardiman]

Nice description jehardiman, but I think azimuth is the direction or bearing and not the altitude of the body.

You are using the modern navigation definitions.

Azimuth come from as sumut: the ways or the directions.

Zenith comes from samt ar-ras (incorrectly read as senit by spanish scribes): the road above.

Altitude is a neo-latin word meaning "highness".

In my first post I intended that measurement or reading of the astrolobe was the azimuth, not the vertical. As I understand it, most translations show that the reading taken from the astrolobe was the as sumut and that only the direct vertical was the samt ar-ras. This is because the reading was used to determine the direction to Mecca, the altitude of the star was unimportant. Only when Copernicus began to map the heavens did azimuth shift from the reading to the direction and standardized altitude as the new word for the distance from zenith

Edit: I think a better way to say the above is that the altitude is the azimuth from the zenith.

[safewalrus]

Whilst you are correct 'JE' it is considered 'normal' (whatever that is) to use modern speach to talk to modern man; or would you have us believe that 'gay' means happy and 'faggot' is a piece of wood rather than the modern versions of both words which have come to mean 'sexual deviant whos desires are against the ancient ways of the bible - a homosexual'.

The general view is that we all speak from the same 'hymn sheet', it is hard enough with a forum like this whose members may have difficulty with the English Language let alone different forms of it (your Nation is a prime example, and as usual WE invented it, but you fu** it! That word itself can have many different connotations and is a 'minefield' in itself so can we please stick to the modern version of the language (incidentally the word 'faggot' on this side of the Atlantic still means a small piece of wood, normally used as fuel for a fire, and a 'fag' is a colliqual term for a cigarette!);)

[Robert Gainer]

Very interesting and I accept you definition of the words in the old sense. But now I am confused about its use. Both from early engravings and from holding the instrument itself I can only see how it measures altitude or zenith distance in the modern use of the word. The instrument seems to be always suspended from the top and the disk is free to rotate about the vertical axis. The engraving I would point to is in the work “Treatise on the Astrolabe” by Geoffrey Chaucer (the same Chaucer from The Canterbury Tales) from 1391 so it is not guaranteed to be correct if you are describing a non-European instrument. How did they measure a direction with this?
Thanks,
Robert Gainer

[Robert Gainer]

Edit: I think a better way to say the above is that the altitude is the azimuth from the zenith.

Jehardiman, this still does not give you a direction, at best if you have the ground point of the body you now have a distance.
Robert Gainer

[safewalrus]

The engraving I would point to is in the work “Treatise on the Astrolabe” by Geoffrey Chaucer (the same Chaucer from The Canterbury Tales) from 1391 so it is not guaranteed to be correct if you are describing a non-European instrument. How did they measure a direction with this?
Thanks,
Robert Gainer

If this be the case why would an arab need to know where Mecca was?

Consider the religion in Arabia before the onset of the Prophet! Before Mohammed nobody gave a fishes tit were mecca was, why should they?:cool:

[Robert Gainer]

Safewalrus, I think the Prophet Mohammed was born around 571 A.D. so Mecca was the spot before 1391.
Robert Gainer

[safewalrus]

Robert your right, I realised that after posting but unfortunately as I'm at work and have no points of reference with me I made a whoopsie! I'm sure the followers of Mohammed (all praise to his name) will excuse me this small indiscretion!

[jehardiman]

Jehardiman, this still does not give you a direction, at best if you have the ground point of the body you now have a distance.
Robert Gainer

All you had to do was know the date and the altitude of the sun at Mecca on that day. You knew you were east or west of Mecca by your lunars and north or south by your noons. Knowing the latitude difference would give you the altitude at noon Mecca. When the sun hit that altitude, you were pointing the astrolobe at Mecca. Won't work the whole world over, but good enough for them.

Try this website I googled up on the question. Also look for the "Toledo Tables" which started into the spherical trig.

http://muslimheritage.com/topics/default.cfm?TaxonomyTypeID=18&TaxonomySubTypeID=107&TaxonomyThirdLevelID=261&ArticleID=529

Edit; BTW I was wrong. the astrolobe was a Greek invention, not Arab.

[mariner 40]

Hello
I bought this on ebay. After purchasing it, I received an email by a guy stating this was a knock off from India. Seller says it's a real plath. Can anyone identify please?
thanks

5251

5252

5253

[safewalrus]

Mike can't see the gearing properly but try this - carefully and gently click open the bottom cam and swing the arm, let it go it should click into place with a smooth audible click, no fumbling, no rolling nice smooth action! Take hold of the wheel and turn it gently, smooth action yes? Check the different shades, all crisp with click type action - if it works like a curt smooth tic toc toy its the original German Jo any sloppiness and it could be a fake or just well worn! If that's the case take it to a maritme instrument makers and have him recalibrate it - he'll soon tell you if it's the real thing or a fake - you can tell by the loudness of his laugh! You could recalibrate it yourself but at your stage it's a bit much - as it is it will do you for the first couple of sights, until you get the confidence! I suspect you may have the real thing but until I get to pick it up/look at it in the room I wouldn't like to committ myself!

[mariner 40]

Thanks for the info. My butt is not hurting as bad now!!
Mike

[safewalrus]

Nice clean clicks huh! Even if it isn't it'll do the job and that's what matters!

[Robert Gainer]

Sorry, it's a fake.
Robert Gainer

[mariner 40]

Hello
I sent pictures to a sextant dealer/restoration shop on the east coast. He stated this was an early plath with the paint removed. The box is not original.
thanks
mike

[Robert Gainer]

Mike,
I am not an expert by any stretch of the imagination but I think your expert is even less so. You might ask him about the following observations.

A. The box is a given, it has felt and a wood block whose purpose was to carry a filter for the telescope’s ocular lens is missing. Heath used felt, but only on contact surfaces and the inside bottom and inside top. Not on all surfaces as seen in the picture. Plath did not use felt on any surface.

B. The frame is composed of a lattice that is too thick and chunky for the model. It is a reproduction and not as delicate as the original.

C. The horizon shades have a filter glass that is smaller then Plath. The frame around the glass is therefore too wide.

D. A similar problem with the index shades. They have an even greater width of metal around the glass.

E. The telescope is too long for the type and too heavy being made from a casting. The original was thin tube that was spun instead of cast.

F. The left end of the limb has the name C. Plath in the wrong font and is missing the "stick man" logo. See www.offsoundings.info/Plath-Logo.bmp for a picture of the font and logo.

G. The horizon mirror has the wrong type of screw for its adjustment. The holder obscures part of the Horizon mirror.

H. The telescope is attached to the rising post with screws instead of having a threaded ring on the scope that screws into a ring at the top of the rising post. The scopes were intended to be interchangeable between the sun scope and the star scope.

You will find that this is a heavy and inaccurate toy. I am sorry to be at odds with your expert but this is my opinion based on the pictures. If you don’t mind, what is the EBay ID of the seller. I would enjoy looking at the items he has sold in the last few weeks to see what’s up.
Good luck with this, I hope that my monitor has distorted the pictures I looked at and I am making a total fool of myself,
Robert Gainer

[BillyDoc]

Mr. Gainer,

I beg to differ about whether or not you can get your longitude from a noon sight. I've done it many times! But you are right about the sun “hanging there” when local apparent noon occurs (LAN) making the time of the exact transit difficult to determine. The trick is to guestimate when LAN will occur from dead reckoning and take a preliminary altitude 15 or 20 minutes prior to that guestimated time when the apparent movement of the sun is fast --- and note the time of this sight precisely. Then, catch the altitude at the maximum “hang” (which will give you your latitude, as you note) and then re-set your sextant to the first altitude and wait until the sun drops to that measure and again note the time. Subtract the first time from the second, divide by two and add this value back to the first time. This will give you the time of the transit (LAN) precisely enough to use the almanac for your longitude.

Bill

[Robert Gainer]

Bill, of course you are correct. I have even done it for fun. Although I would maintain that you can’t get the longitude with enough precision to make this method worthwhile. In this case, “worthwhile” is subjective and if you are happy with the method then it is worthwhile for you.

I would rather get two LOP’s from time sights instead. If the weather is overcast you can perhaps still get two sights but not necessarily at the right time for your method and even if the clouds don’t get in the way you can’t get the time of transit with enough precision to get the same type of results that you can get otherwise. But, you are right and I do stand corrected.
Robert Gainer

[Vega]

An astrolobe was an arab invention used to measure the heights of the stars from the vertical (i.e. the azimuth). This gave them the ability to determine the approximate direction to Mecca, rather than just "over that way".

The astrolobe is a lot older than that. The old Greeks were the inventors. It is believed that the Inventor was Hiparco (194 bc/120 bc).

The Arabs brought the knowledge (and made some improvements) to Iberia.
It was then improved to its final form by the Portuguese and Spanish.

They needed it for their discovery voyages. The Astrolobe, with the help of fairly accurate sun tables, was a reliable tool to find the boat’s latitude.
Each boat carried at least one.

Take a look at some original ones, recovered from shipwrecks of old Portuguese
Boats. (Look at the end of the page...sorry it is in Portuguese)

http://www.ancruzeiros.pt/anci-astrolabio.html

For the stars they used this one, the balestilha:

http://www.ancruzeiros.pt/anci-balestilha.html

[safewalrus]

going back to THAT sextant, what do you want to do with it? Take sights or hang it on the bloody wall to show how smart you are?

First you establish what inaccuracies there are in the sextant (check with your favourite book theres a couple that need to be done regularly! Once you have the inaccuracies and know how to apply them your sextant becomes accurate! No problem you can now take sights, OK they may not be dead accurate and tell you wether your on the fx'cle or the poop (on a forty footer does this matter?) but it will be accurate enough for your purposes! Ok your first few may be pretty rubbish mine were (much like everybody elses they just don't admit it!)

SO take a few on the beach at home and find where the beach is, you'll soon gt the hang of it (and don't worry if your slightly out your not surveying with the bloody thing - now that does need accuracy!)

Play and enjoy

[Wellydeckhand]

mmmmm..... just buy 2 GPS for trouble free cruise......... or 3 GPS up to your pocket........ sextant too clumsy for novice and amarture sailer...... easy to drop it in the sea and wait for help................. :):):)

[safewalrus]

Welly you tie a bit of string to it when you drop it in the sea it makes a great lead line!:D :P :D

[Bergalia]

Welly you tie a bit of string to it when you drop it in the sea it makes a great lead line!:D :P :D

Ah...now talking of string..........:D

[safewalrus]

Now HOW did we get back to that?



Don't tell me!

[gonzo]

An astrolabe has a sight tube and a pivoting rod. You sight the celestial body through the tube and look at the position of the rod on the graduated circle. That is the altitude.

[safewalrus]

A whisk(e)y bottle looks like a site tube. You sight the celestial body through the glass as your drinking the whisk(e)y. That is attitude. :rolleyes:

You may not find where your at, but it don't seem to matter to much anyway:!: :cool: :p

[Vega]

An astrolabe has a sight tube ....

:?: :?: :?:

This one is Binzantino (1062):

http://www.bresciascienza.it/cityline/cult/zani/astrolabio.gif

This one is Arab (1079):

http://www.legadoandalusi.com/legado/contenido/rutas/1024/BAA09777.jpg

A nautical one, like the ones used by the Portuguese and Spanish:

http://usm.maine.edu/maps/exhibit8/27.jpeg

It worked like that:

"In the Middle Ages, the instrument used for measuring the angular height of a celestial body above the horizon was the planispheric astrolabe. This instrument adapted to one more suitable for "taking the height" while at sea--the nautical astrolabe. Consisting of a perforated disc made of bronze or brass, which gave it weight, the astrolabe was suspended from a ring at the top. Affixed to the center of the disc was a sighting bar called the alidade, which could be turned in a complete circle. The navigator aimed the alidade at the heavenly body, either the sun during the day or the pole star at night, aligning it by sighting through holes or notches in plates at each end. He read the altitude in degrees directly off a scale inscribed around the circumference of the disc."

http://usm.maine.edu/maps/exhibit8/nrnav.html

[Robert Gainer]

Vega,
They are beautiful and I wish I had one in my collection. I have sextants, octants, quadrands, Ball recording sextant (aircraft sextant) and I even have a survey instruments called a Quintant but I don’t have an astrolabe.
Robert Gainer

[Vega]

I don’t have an astrolabe.
Robert Gainer

Robert, they are very rare and old. They cost a fortune. Only 80 survived, all have a name and are registed in the Maritime Greenwich Museum.

If you are not a very rich man the best you can have is a perfect copy;)

http://museu.marinha.pt/Museu/Site/PT/Loja/Replicas/Astrol%C3%A1bioN%C3%A1uticoPortugu%C3%AAsATOCHAIIIS%C3%A9culoXVII.htm

[Robert Gainer]

Yes Vega, I know about the price tag. I have been in the same room with some but have not been allowed to hold or use a real one. The chances are that I will not ever have one in my collection unless I am the diver who finds it and that’s not likely to happen.
Robert Gainer