Astrolabes

[JoshuaRed]

Well this thread certainly puts the A S S in Astrolabe, doesn't it?

[Captain Jacob Badger]

..Aye...

[Capt DaggezEber]

I have to admit that I envy you. I have been looking for a keychain (it is a keychain, right?) of this kind for quite some time but I found most to be too expensive. But I keep looking... 

Georgia RenFaire has a man selling notical equipment, yes mine can be worn as a neclace. Mine is rapped around me buckle. In me profile picture its just too small. It looks like a white shining circle between me gold hilt and the corner of me black pouch. I like the look, but the nut on the back rips me chest-hair out. If'n I be beter with HTML I would post a picture right here.

As for Captain Badger.......stop Badgering these men. Arrrr!

[Captain Jacob Badger]

Arrr......aye aye, ...right you are sah!.........the badgerin' will stop....but it were so much fun........(kicks over tin can).......

[Capn_Enigma]

Capt. DaggezEber:

I like the look, but the nut on the back rips me chest-hair out.

Ouch!

I would greatly appreciate it if you could show me a photo of your astrolabe. You could put it on the Gallery here at pyracy.com, for instance.

The code for posting pix is:

<img>http://whatever_your_image_address_may_be.com/image.gif</img> (but replace all the "<" with "[" and the ">" with "]" (no quotation marks, of course.))

[Gentleman of Fortune]

Enigma

is the one you made an astronomical one. When would you need to swap out the plates?

About how much heavier is a brass one than a wood one. Would a 5 lb wood one be more accurate than a 3lb brass one or would the larger size make it more of a target for wind?

I have no idea how to use these things but am very curious.

It is an interesting discussion... For once, I am not in the heated argument!

[Gentleman of Fortune]

Atstrolabes are not the only navigational tools that have been listed in this thread....

Could you knowledgable folks post pictures of the other things you have (or have made) and tell us how they were used?

Back staff.... octant

Please?

GoF

[Capn_Enigma]

When would you need to swap out the plates?

Gentleman Of Fortune,

when constructing a planispherical (astronomical) astrolabe, you first have to determine the latitude area where it will most likely be used (for the Caribbean that would be between 8°N (Panama) and 30°N (St Augustine)).

Then you calculate and manufacture the tablets in 2° to 3° latitude increments. A bigger increment would result in the astrolabe outputting incorrect data. Again, for the Caribbean that would be (in 2° increments): 8°N (1st tablet obverse), 10°N (1st tablet reverse), 12°N (2nd tablet obverse), .... , 28°N (6th tablet obverse), 30 °N (6th tablet reverse).

The number of tablets is, of course, limited by the thickness of the mater (a mater can only hold a limited amount of tablets).

To ensure that my astrolabes can be used on the entire world, I have come to use the so- called "Saphea Arzachelis", which is a projection of the globe onto a flat surface (, i.e. the surface of the astrolabe). With it, you can use the astrolabe at all latitudes from North to South Pole.

There are other tablets aside from the "normal" latitude tablets. The "Horizon Tablet" is used to determine the time of the sun or stars rising and setting ("We will attack Panama on sunrise. What hour will that be?"), while the so- called "Geographical Tablet" is used for determining the culmination of heavenly bodies above a geographical point ("At which time tonight will Altair reach its highest point here at Port Royale?").

Other scales on the astrolabe include the so- called Shadow Square, which could be used for a wild variety of trigonometric uses ("We need to build ladders to scale this fortress, but we don't know the height of the walls, and the walls are defended by musketeers. Find it out from the distance, so that our ladders won't be too short." or "How wide is this river?").

Would a 5 lb wood one be more accurate than a 3lb brass one or would the larger size make it more of a target for wind?

That depends on the diameter of the wooden or brass astrolabe and of the specific weight of the wood. If you made it from balsa wood, the diameter would be very large, thus giving the wind a large surface to attack. If made out of exotic wood, with the specific weight larger, the diameter would be less, but still much larger than that of a metal one. Thus, it would yield more easily to the wind than a metal astrolabe would.

Metal (i.e. mostly brass) Mariner's Astrolabes on average had a diameter of 7 inches and they were extremely thick. Unfortunately, the smaller the diameter, the more inaccurate the finding will be. But ancient mariner's were obviously willing to run that risk in order to get a decent reading at all (Any reading is better that no reading at all).

[Fox]

Good point GoF,

My living history pilot's kit consists of 3 astrolabes, a quadrant (2 if you count a minature), a davis quadrant, a cross staff, (for the later periods) a sextant, a terrestrial globe, a celestial globe, 2 sandglasses, a log line, a lead line (though that now lives in the boat), oh, 2 compasses (d'oh, how could I forget them?), a traverse board, a couple of books of charts and several loose ones, 2 books of tables, several sets of dividers, a couple of protractors, and probably other stuff I've forgotten.

For when I'm playing a surveyor on land I also have a thoedolite, a chain, and a pair of fathom sticks in addition to the above.

I don't have e-photos of it all to hand:

My traverse board and small sandglass

One pair of dividers, a book of tables and a protractor copied from the Mary Rose

My celestial globe, copied from a 17th century example in the NMM

[Capn_Enigma]

Very nice items that you got there, Foxe!

And while we're showing off , here's one of my most beautiful astrolabes in assembled state. The tablets, rete and rule are gilt, the star pointers are silvered.

And here's another popular instrument, the horometer. Amongst other things, it has a dial with which to tell the current phase of the moon:

EDIT: Though it has nothing to do with astrolabes, my last project was a telescope built to Galileo Galilei's exact optical specifications and in the style of an early 17th century telescope (unfortunately, also with such an instrument's deficiencies , but that's another story!).

[Gentleman of Fortune]

See we can all play nice!

Here are some more questions then.

Would the Navagator have more than one insturment for a particular task and use both? I assume yes but here is the real question.

If you had an Astrolab and an XX (what ever other insturment does the same thing) and you got two different readings, historically, would they average the two, go with just one or ???

"Well Captain, the Astrolab says we are here, but the Cock Ring says we are here so that means we are really screwed...."

???

GoF

[Capn_Enigma]

Assuming there is a variety of instruments aboard (as a matter of fact, on wreck sites, harldy ever more than two or three Mariner's astrolabes were found in one ship's remains):

In windy or stormy weather, a navigator would take his metal Mariner's astrolabe.

In a calm, and I mean a really dead calm, a metal or wooden quadrant may be used. The main advantage of a quadrant, its greater limb, is also its major disadvantage in bad weather: Quadrants have a plumb line which is prone to swinging in everything but the slightest breeze. However, when the weather is fine, a quadrant is to be preferred, due to the greater accuracy of reading because of the greater radius. The material of the quadrant doesn't really matter because it is held in the hand and does not need to be heavy.

In all other cases, astrolabes are to be preferred. A planispheric astrolabes may be used in a period "between" a weather when the quadrant cannot be used and a weather where a Mariner's astrolabe is not yet necessary.

As a navigator, I myself would never average readings. As the saying goes "averages are for average people".

I would take my sighting as carefully as possible and then stick to it, but never relying on it alone, if at all possible. E.g. if you are near land, you could use a pelorus to take a cross- bearing to verify your position.

... but the Cock Ring says we are here so that means we are really screwed...."

Now there's an instrument I haven't built yet... the Universal Cock Ring!

[Capt DaggezEber]

I would greatly appreciate it if you could show me a photo of your astrolabe. You could put it on the Gallery here at pyracy.com, for instance.

I posted them just now to the gallery

[Capn_Enigma]

Aye, and here be it, chesthair included :

Do ye know what it's made of? Pewter?

Judged from the style, it looks like a remake of an early arabic astrolabe to me.

[Fox]

Where period texts (Davis's Seaman's Secrets, Butler's Dialogues, Bourne's Regiment for the Sea etc) deal with the practicalities of navigation rather than the mathematics they tend to talk about using two instruments, one to check the other. Personally, if two readings do not agree I tend to try a third (in fact, the more people taking sightings the better).

One of the interesting things about period texts is the differences in preference for instruments. Butler, for example seems to have preferred an "astrolobie" - though I have a feeling that the section dealing with navigation in his dialogues was actually lifted verbatim from Mainwaring the pirate's works. Davis on the other hand, undoubtedly the greatest navigator of his age, says that the cross-staff is the only instrument worth using, though he does acknowledge the existence of others... Of course, he wrote the first edition of The Seaman's Secrets before he invented the back-staff.

Bourne too favoured the cross staff over the astrolabe, except when the sun was too high for the practical use of a cross staff. However, Bourne does make one very interesting point in his Regiment for the Sea. He suggests that when using an astrolabe (or "Sea-ring" as he terms it) the result should be checked for accuracy by taking another reading using the same instrument, but using the scale on the opposite side of the instrument.

[Capn_Enigma]

Are you positive on the cross- staff? Not a backstaff?

He suggests that when using an astrolabe (or "Sea-ring" as he terms it) the result should be checked for accuracy by taking another reading using the same instrument, but using the scale on the opposite side of the instrument.

This is a misinterpretation, since a normal Mariner's astrolabe has no reverse scale, just an obverse one.

What he means is to rotate the astrolabe 180 degrees and sight through the very same alidade, but the reading is done from a different sector on the same scale.

But this is done only as a precaution to countercheck a possibly faulty astrolabe.

[Captain Jacob Badger]

tsk!...is all I can say.......as I'm banned from Badgerin'............but god damn.........anybod' thats bin over the sea's knows what be the best fer them....in a nav way....some have tried this ....some have tried that....ifin it gets ye t' yer dest.........job done!.........there....I've finished.......no more Badgers.......

[Cap'n Pete Straw]

No, you can badger people... but only those named "Captain Jacob."

Wait, no -- then you'd be "Captain Jacob Badgerer."

[Fox]

Cap'n Enigma, that's exactly what I meant, I should have said "using the scale on the opposit side of the same face of the instrument".

Am I sure about what about the cross-staff? Yes. Bourne was writing before the invention of the back-staff, so he can't possibly have meant anything other than cross-staff. Davis wrote "the Sea Compass, Chart

and Cros-staff, are instruments sufficient for the Seamans use: the Astrolaby and Quadrant being Instruments very uncertain for Sea-Observations." Then later on "...with the help of your Cross-Staff, Quadrant, or Astrolaby, but the Cross-Staff is the only best Instrument for the Sea-mans use". Later still he describes the method of using a cross-staff:

First, Place the Cross-Staff to your Eye, in such good sort as that there may grow no error by the disorderly using thereof, for unless the Center of your Staff, and the Center of your Sight do joyn together in your observation, it will be erroneous what you conclude thereby: Your Staff so ordered, then move the Transversary upon your Staff to and fro as occasion requireth, until at one and the same instant you may set by the upper edge of your Transversary, halfe the body of the Sun, or Stars, or that the lower edge of or end thereof so likewise touch the Horizon, at that place where it seemeth that the skie and the Seas are joyned, having special regard in this your observation, as that you hold the Transversary as directly uprightly as possible you may

As you can see, that's the method of using a cross-staff, not a back staff.

There is some debate about the order of Davis's achievements during his period of semi-retirement in the 1590s, but I think it's pretty certain that he wrote The Seaman's Secrets before he developed the backstaff. Later editions of the book make mention of the backstaff, but the early editions do not.

[Capn_Enigma]

I see. Thanks for the clarification.

[Cap'n Pete Straw]

..But what about the Cock-Ring? Does he mention how a Cock-Ring is to be employed at sea?

[Fox]

Well, Bourne describes the use of a "sea-ring", I wonder if that's a dictaion error and he really meant "C-ring"... In which case it's very similar to a mariner's astrolabe, so similar in fact that they might even be described as the same.

.

[Capt DaggezEber]

Well, Bourne describes the use of a "sea-ring", I wonder if that's a dictaion error and he really meant "C-ring"... In which case it's very similar to a mariner's astrolabe, so similar in fact that they might even be described as the same.

.

My lord Foxe I was worried yee sense of humar be long departed! Good on yee!

As for mine, It be Pewter. Yee saw the calander on back? Aye I believe it Arab style. As to small amout of info I be find'n before yee educated me.

[Fox]

Takes a lot for me to lose my sense of humour - death followed by cremation I think will do it.

[Capt DaggezEber]

Aye, that tends bitter one the humor incline. :)