Astrolabe 40S

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Make your own Model Astrolabe

Dominic Ford

October 2010

An astrolabe is an elaborate instrument which combines a mechanical model of the sky’s rotation through the night – similar to a modern planisphere – with anobserving instrument which allows the altitudes of objects in the night sky to bemeasured. Put together, these two components can be used the time of day, by

determining the altitude of a star, and also at what time of day the sky’s rotationbrings it to that height above the horizon.

Historically, the astrolabe was the most sophisticated astronomical instrument inwidespread use in the Middle Ages. In fact, it held this position for nearly twothousand years, from the time of Hipparchus (c. 190–120 BCE) until the turn of the seventeenth century, around the time that the telescope was invented in 1609.

Yet today this complex instrument is rarely seen outside of glass cases in museums,and those interested in learning about it may have some difficulty finding a specimento play with. Ornately carved brass reproductions are available from some telescopedealers, but with substantial price tags attached. These price tags are historicallyauthentic: mediaeval astrolabes were often made from high-cost materials and intri-

cately decorated, becoming expensive items of beauty as well as practical observinginstruments. But for the amateur astronomer who is looking for a toy with whichto muse over past observing practice, a simpler alternative may be preferable.

This document provides a cardboard cut-and-glue kit to make your own modelastrolabe, so that you can rediscover medieval observing practice for yourself. Theastrolabe presented here is design for use at a latitude of 40◦S . Alternative versions,prepared for other latitudes, are available from the author’s website:

http://in-the-sky.org/astrolabe/

Assembly Instructions

To build a model astrolabe tailored for a latitude of 40◦S , Figures 1, 2 and 3 shouldbe printed out onto paper, or more preferably onto thin card. Figure 4 should beprinted onto a sheet of transparent acetate. The two sides of the mother (Figures 1and 2) should be glued rigidly back-to-back, perhaps sandwiching a piece of rigidcard. The rete , printed onto transparent acetate2, should be placed over the climate ,which for simplicity is incorporated into the front of the mother in this document.

The rule and the alidade should be placed on either side of the astrolabe: the rule,marked out with a declination scale, should rotate over the front of the mother;

2Historically, the rete would have been made of the same material as the rest of the astrolabe

and marked with arrows showing the positions of prominent stars. As much of the material of the

rete as possible would then have been cut away to allow the climate below to be seen. We use

transparent plastic here because it is so much more practical than the traditional form of rete.

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MAKE YOUR OWN MODEL ASTROLABE 2

the alidade should rotate over the back of the mother. The two tabs on the side of the alidade should be folded out to form a sight used for measuring the altitudes of celestial and terrestrial objects. The whole construction may then finally be fastened

together by placing a split-pin paper fastener through the centre.

How to use an astrolabe

For more information about how to use your astrolabe, see the author’s website,http://in-the-sky.org/astrolabe/

or download a copy of the author’s paper about the astrolabe, which was publishedin the Journal of the British Astronomical Association, and can be downloaded here:

http://in-the-sky.org/astrolabe/astrolabe_jbaa.pdf

Customised Astrolabes

The astrolabe images presented here were produced using PyXPlot, an open-sourcevector graphics scripting language developed by the same author. PyXPlot has awebsite3 with extensive documentation, and is available as a standard package in anumber of Linux distributions including Ubuntu, Debian and Gentoo. Unfortunately,it is not available for Microsoft Windows at the present time.

The PyXPlot scripts used to generate the images in this document are included inthe accompanying file archive and may be modified to generate customised astro-labes. For example, to produce an astrolabe with your own choice of saints’ daysor birthdays on the back of the mother, the file RawData/SaintsDays.dat should

be modified. A python script, main.py, is included which rebuilds all of the imagefiles shipped in the astrolabe parts folder.

References

[1] Ford, D.C., J. Brit. astr. Ass., 131(1), 33 (2012).

[2] Chaucer, G., Treatise on the Astrolabe , in The Riverside Chaucer , ed. L.D.Benson (Boston, 1987)

[3] Eisner, S., J. Brit. astr. Ass., 86(1), 18-29 (1975)

[4] Eisner, S., J. Brit. astr. Ass., 86(2), 125-132 (1976a)

[5] Eisner, S., J. Brit. astr. Ass., 86(3), 219-227 (1976b)

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http://www.pyxplot.org.uk

c 2010,2014 Dominic Ford. Distributed under the GNU General Public License, version 3.

Document downloaded from http://in-the-sky.org/astrolabe/index.html

http://in-the-sky.org/astrolabe/http://in-the-sky.org/astrolabe/astrolabe_jbaa.pdfhttp://www.pyxplot.org.uk/http://in-the-sky.org/astrolabe/index.htmlhttp://in-the-sky.org/astrolabe/index.htmlhttp://www.pyxplot.org.uk/http://in-the-sky.org/astrolabe/astrolabe_jbaa.pdfhttp://in-the-sky.org/astrolabe/


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Figure 2: The front of the mother of the astrolabe, with combined climate. Shoulda climate for a different latitude be required, the accompanying file archive shouldbe downloaded. This includes alternative versions of this document for any latitudeon the Earth at 5◦ intervals.



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Figure 3: Left: The rule, which should be mounted on the front of the astrolabe.Right: The alidade, which should be mounted on the back of the astrolabe.

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Figure 4: The rete of the astrolabe, showing the stars of the night sky. Thisshould be printed onto a piece of transparent plastic; most stationers should be ableto provide acetate sheets for use on overhead projectors, which are ideal for thispurpose.


http://in-the-sky.org/astrolabe/index.htmlhttp://in-the-sky.org/astrolabe/index.htmlhttp://in-the-sky.org/astrolabe/index.htmlhttp://in-the-sky.org/astrolabe/index.htmlhttp://in-the-sky.org/astrolabe/index.html

Astrolabe 40S

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