Astrolabes

[Fox]

Having built several astrolabes and used them at sea myself I can assure you that a: wooden astrolabes DO work at sea and on land, and that b: given that some astrolabes were 2ft or so across you REALLY want a wooden one from time to time. I have also come across wooden astrolabes with a sheet of lead around the bottom edge for weight. There's nothing either impractical or inauthentic about wooden astrolabes if made and used properly and sensibly.

You make a good point, but to say never (in bold no less!) is, in my experience, pretty wide of the mark I'm afraid :)

[Capn_Enigma]

given that some astrolabes were 2ft or so across you REALLY want a wooden one from time to time.

First off, I am talking mariner's astrolabes here.

Two feet astrolabes were never (Oops! I did it again ) used aboard a ship.

If you think differently, please kindly quote a mariner's astrolabe in Stimson, Gunther or another source.

Two- feet astrolabes were astronomical instruments with rete, changeable tablets etc., intended for static use on solid land. What's more, most of them were display pieces only, gifts to a sovereign for representation, never intended for real everyday use.

With all due respect, Sir, if you continue to insist that wooden astrolabes were used aboard a ship, you are not very familiar with nautical realities. I assure you, trying to aim at a 2nd magnitude star (Polaris) standing on a swaying deck is difficult enough without the astrolabe fluttering in the breeze.

If you have a deviation of only 1 degree in measuring Polaris, you are 60 nautical miles off in latitude and wham! there goes your navigation!

[Captain Jacob Badger]

Hmmmm....this be an interestin' thread.....cos' yer both right........ifin ye could afford a proper 'mariners astrolabe'....then ye were 'quids in' as it were.....cos' they did cost a packet then....as they do now!....

most seamen could naught spend 'that' much money on s'mat that only gave ye latitude......so most were made o' wood....wi' a lead weight on the end........not as accurate....but still worked.....so.......in theroy....yer both correct......aye

[Patrick Hand]

I'm not inciting a riot....... but this should be an interesting arguement.....

[Fox]

Bangs Head on table

I think it's pretty clear that Cap'n Enigma is attempting to engage me in a flame war (I've done this...you don't know what you're talking about...oops, I've done that thing you specifically asked me not to again...) and frankly I can't be bothered with that. This Foxe ain't biting today. Cap'n, if you'd like to discuss this like gentlemen then fine, but if you wanna flame then try someone else.

The first wooden astrolabe I made was a copy of a 19" 17th century mariner's astrolabe in Brixham museum (it's now been lent somewhere else, Falmouth I think), and I believe there are a couple in the NMM (I'm fairly sure that's where I came across the lead weighted one.)

I respect your experience Cap'n, please have the good grace to accept mine. I have used both wooden and metal astrolabes at sea and have enjoyed a similar degree of success with both. Some wooden ones gave me a spot on reading, others were wildly out. Same goes for metal ones. FWIW, I find that a quadrant or cross-staff is infinitely better suited to celestial navigation than an astrolabe anyway, and vice-versa for solar navigation.

The bottom line; whether wooden or metal, an astrolabe is a infinitely more authentic than a sextant for GAoP period living history. If you're interested in a wooden astrolabe (or other instrument) then drop me a line, if you're only interested in metal ones then happy hunting (and let me know if you find an affordable one). Since Cap'n Enigma has built several (presumably metal) astrolabes, perhaps he could offer us a quote or suggest an alternate supplier?

[Gentleman of Fortune]

does an astrolab have to be "tuned" to the century that it is in? Woul a 17th C astro work today?

It seem sot me that if the universe is expanding that things would not necessarily be in the same spot 300 years later.

Am I totally wrong or ?

GoF

[Fox]

All a mariner's astrolabe does, indeed all most period navigational instruments of that type (quadrants, Davis quadrants, octants, cross-staffs, even sextants) do is measure the angle between two points. Generally for navigational purposes this is the angle between either a particular star and the horizon, or the sun and the horizon. For inshore navigation the same instruments (except a quadrant) can be used horizontally to measure the angle between two features on the land for triangulation purposes.

So, what needs to be "tuned" are the tables one uses to work out what the angle read by the instrument means. A 17th century astrolabe will work fine in the 21st century, but 17th century declination tables will not.

[Capn_Enigma]

...Woul a 17th C astro work today?

It seem sot me that if the universe is expanding that things would not necessarily be in the same spot 300 years later.

Almost no 16th century astronomical astrolabe would work today, neither would most 17th and even some 18th century astrolabes.

This is due to the calendar reform carried out by Pope Gregor XIII beginning in 1582. He decreed that on October 4 of that year, the following day not be October 5 but October 15. This was due to inaccuracies of the Julian Calender used until that date.

While Spain carried out the reform on October, 4/15 1582, most other catholic countries followed suit only in the next years. Protestant countries took even more time to implement the calendar, Switzerland, for instance, took until 1655, Germany until 1700, while England waited until 1752.

Thus, the entire calendar scale of astronomical astrolabes manufactured before the reform in the respective countries is off by eleven days, thus rendering the astrolabe unusable.

[Cap'n Pete Straw]

Yikes! And then I found THIS topic, halved (wisely, I now see) off from the one in PLUNDER.

Not to carry this all the way back to the PLUNDER area again, but I would be very interested in seeing any plans, diagrams, or other schematics you may have, Foxe, as I am rather good in making things out of... well, other things.

[Capn_Enigma]

The following page allows you to download a template of a Mariner's Astrolabe, which is the only type of astrolabe used aboard a ship.

[Jolie Rouge]

Captain Enigma, yer bein a right twit. Ya just told someone that a period astrolabe would not work due to calender changes? If I may be so bold as ta quote the link you put up "To find the latitude of the ship at sea, the noon altitude of the Sun was measured during the day or the altitude of a star of known declination was measured when it was on the meridian (due north or south) at night. The Sun's or star's declination for the date was looked up in an almanac. The latitude is then 90° - measured altitude + declination"

The tool itself has nothin ta do with dates, ya use the astrolabe ta find a measurement that ya look up on a table that is not part of the astrolabe. Ya follow?

[Capn_Enigma]

'fraid the twit goes entirely on you, matey.

First, learn the diff'rence 'twixt a Mariner's Astrolabe and an astronomical astrolabe, then start searching for that brain in that dead - eye o' yars and then - perchance - ask 'gain.

[Jonathan Hawks]

Ere's a good link fer em if'n yer interested.

Astrolabs

[Dorian Lasseter]

The following page allows you to download a template of a Mariner's Astrolabe, which is the only type of astrolabe used aboard a ship.

Well now... I'll be tryin' my hand at makin one from that template... gonna make a wooden one first, use it as a template if it comes out good.

We have a small foundry where I work... I'll see ifI can make one out of brass, but we normally make stuff out of aluminium...

Hardest part will be all the degree markings...

[Duchess]

So, from an actual desire to know and not a desire to malign, impugn or otherwise incite...

Do you find that the expansion and contraction of a wooden astrolabe (from wetting and drying) affects the accuracy?

[Capn_Enigma]

Astrolabe accuracy is already small due to the small length of the alidade (usually about 8"), but the fact that wood expands / shrinks does not affect accuracy at all.

What really affects accuracy is the weight (or lack thereof) of a wooden astrolabe.

Imagine the following scenario:

It is in the middle of the night, you are standing on the quarterdeck of a heavy galleon. A stiff breeze is blowing, but stars can be seen through the clouds, for the first time since two weeks. Since then, you have been navigating by dead reckoning.

The ship is due to make landfall soon and you decide to shoot Polaris to get the latitude.

Now there are two possible ways to proceed:

1) You take your trusty brass astrolabe. It is about 7 1/2" in diameter, 1/2 " thick and weighs about 4 pounds.

You lift it to your eye and sight for Polaris. Although the wind is a coming in gusts now, the astrolabe hangs as steady as a rock.

You have no problems taking Polaris' angle and thus are able to determine your latitude.

And now for the alternative:

2) You take the wooden astrolabe that was sold to you by that slippery long- tongued merchant in the last harbor. It is about 10 " in diameter, 1/2" thick and weighs about 3 pounds.

You lift it to your eye and sight for Polaris. The wind is coming in gusts now, and due to the large surface and the smaller weight, the astrolabe starts to sway to and fro. You try to steady it, but to no avail.

Finally, after a struggle of several minutes, you manage to take an approximate altitude and thus latitude. What you don't know is that due to the swaying you had a deviation of 1.5 degrees in altitude and thus about 100 nautical miles error.

Unaware of that, you plot the presumed course and have the helmsman steer it. This course leads you directly into the maw of the most treacherous shoals around.

Well, which one of the two scenarios would you prefer?

[blackjohn]

Uhhh... the one I can afford. In this nautical cartographer's opinion, its kinda like the difference between a $50000 ECDIS system with gold bells and whistles or a $250 ECDIS system that will most likely put me where I should be.

[Fox]

LMAO, you just can't let it go can you?! I'm gonna assume that "slippery long- tongued merchant " doesn't refer to anyone we know.

On the one hand you could take your 8" brass astrolabe and try to read the tiny degree markings, or on the other hand you could take your 18" wooden astrolabe, properly weighted if you like, and well oiled to minimize the effect of the damp, and read the considerably larger and quite likely more accurate scale (or is Capn Enigma going to tell us that small instruments are always more accurate?).

I've used both wooden and metal astrolabes (and other instruments) on land and at sea, and as I've stated before have attained similar levels of accuracy and innaccuracy with both. Incidentally, whatever instrument one is using it's always best to use several others as well and compare the results. Only a suicidal fool would rely on the reading of one instrument.

BUT, to answer Duchess's query, wooden astrolabes, and other instruments need to be looked after, and the wood needs to be kept oiled. Properly looked after a wooden astrolabe will remain accurate throught its reasonable working life.

[MadMike]

Wooden astrolabe-

http://www.nmm.ac.uk/collections/explore/o....cfm?ID=AST0557

Why bother with an astrolabe when you can use a quadrant? Some examples in the site above.

Yours, &c.

Mike

[Capn_Enigma]

Wooden astrolabe-

http://www.nmm.ac.uk/collections/explore/o....cfm?ID=AST0557

Wooden astronomical / -logical astrolabe.

Again, not the type used on board a ship.

[Silver Steele]

I found one quick reference to a wooden astrolabe, assumed for maritime use, because of the user.

Source: Websters Instrument Makers Database - Letter F

Ruy Falerior, Spain, 1519, MIM

Made one wooden astrolabe and six wooden quadrants for Magellan.

HERE

[Capn_Enigma]

Source:  Websters Instrument Makers Database - Letter F

Ruy Falerior, Spain, 1519, MIM

Made one wooden astrolabe and six wooden quadrants for Magellan.

Excellent find, but this may also be an astronomical astrolabe to calculate sunrise /sunset, the transit of sundry stars in different latitudes or the position of planets. It does not necessarily indicate a Mariner's astrolabes.

And as the quadrants are concerned, they don't need to be heavy. A lightweight wooden quadrant gives the exact same readings as a metal one.

[Capn_Enigma]

To end the silly discussion whether or not wooden astrolabes were used aboard ships once and for all, allow me to quote from Dennis Fisher's excellent book Latitude Hooks and Azimuth Rings (p. 25 f):

"But taking accurate sights with an astrolabe, a relatively simple proposition in a flat, stable desert, was next to impossible aboard a ship at sea. Usually three men were required. One braced his back against the mainmast while holding the instrument aloft. Another sighted the star. The third would read the angular height from the degree scale. Lining up the dimly shining Polaris through pinhole sights on a pitching, rolling deck demanded more skill than most seamen had, and if a breeze caught the instrument, sight taking was nearly impossible.

To overcome the effects of wind and the ship's motion, marine astrolabes were often as small as 6 or 7 inches in diameter, and as heavy as 4 pounds. An astrolabe, however, is accurate in direct proportion to its size; a large instrument shows minute and second gradations more clearly than does a small one. As a result, errors of up to 5 degrees were common on smaller marine instruments.

The Portuguese explorer Vasco da Gama circumvented these problems by going ashore to take sights with a large, wooden astrolabe hung from a tree branch. But for mariners far out at sea, the astrolabe was of limited use. Columbus had an astrolabe on his first voyage, but apparently never got accurate readings from it."

Thus, if anyone tries to tell you that it is possible to sight stars with a wooden astrolabe aboard a ship, he's spinning a mighty gross yarn.

I rest my case.

[Captain Jacob Badger]

I'm still naught convinced!.......wooden was used....'tis writen in logs.......hmmmmm....we must continue!! :)

[Capn_Enigma]

we must continue!!

Very well, then.

'tis writen in logs

Source and quote, please?