Mrz 152010

For centuries the astrolabe was the ultimate astronomical tool. Although more precise instruments have long since replaced it, the brass disk still has a story to tell. A visit to one of the last astrolabe-makers reveals the secrets of this versatile and well travelled instrument.

“Is it blasphemy?” Martin Brunold asks himself. I’m slightly irritated by the question – didn’t I come to Abtwil in Switzerland to talk about science, astronomy and what is probably the most important instrument of the Middle Ages – the astrolabe? “Well, after all, this brass disk is an image of the heaven, of god’s creation,” Brunold continues, “and isn’t ‘thou shalt not make yourself an idol’ an important statement in more than one religion?”

Astrolabes truly were the computers of the Middle Ages

My confusion clears up after a quick look around Brunold’s study: He has obviously come to terms with the religious conflict the instrument presents – his desk and the walls are littered with astrolabes of various forms and sizes, some of them plain scientific instruments, but most beautifully ornamented works of art. Little wonder given the fact that Martin Brunold is one of only a handful remaining craftsmen worldwide who still practice the ancient art of astrolabe-making. “A pretty useless occupation by the way,” he adds, humbly referring to the fact that in the age of quartz watches and atomic clocks a Medieval timepiece with an accuracy of a few minutes at best seems kind of pointless. But the astrolabe is more than just a timepiece, much more than that.

A Sextant and a Star-Chart

Technically speaking, it’s nothing more than a combination of a simple, sextant-like targeting device and a rotatable star-chart. The base is a brass disk with scales on both sides, the so-called mater. The mater is suspended on a hinged ring to ensure it always points straight down; an important prerequisite since a double-rule with sights attached to the centre of the disk, the alidade, is used to measure the ‘altitude’ of an object, i.e. the angle between the horizon and the object.

When turned over, the astrolabe reveals its most impressive part, the rete. The rete is a rotatable open-work disk with an offset circle representing the Zodiac and many dagger-like pointers, which create an elaborate squiggled net. “It may not look like it, but this is a star-chart. Each pointer represents a prominent star like Aldebaran, Sirius, or Rigel and the Zodiac, of course, is the path of the planets and the sun across the sky,” explains Brunold.

Underneath the rete sits a plate engraved with lines and eccentric circles. The so-called tympanum looks a bit like a squashed globe. “Not that far fetched,” smiles the man from Switzerland, “legend has it that Ptolemy’s donkey stepped on one of his wire globes, thus creating the first astrolabe. It’s very doubtful Ptolemy knew the astrolabe but he did write a book about stereographic projection.” This is the method used by astrolabe makers to depict the three-dimensional celestial sphere on a two-dimensional disk: By assuming an imaginary viewpoint at the South Pole, the stars, the ecliptic, latitudes and longitudes are projected on a plane perpendicular to the celestial axis.

Although such a projection is, according to the fashion of the times, geocentric with the Earth as the centre of the universe, it’s nonetheless a stroke of genius: It elegantly combines two different coordinate systems – the rete uses the equatorial system of the stars with Earth’s North Pole pointing to the North Star while the tympanum depicts the horizontal system of the observer with the horizon as the equator. Since the latter depends on the observer’s latitude, astrolabes usually come with a set of different tympanum plates. With the correct plate inserted this clever combination of coordinate systems allows very complex spherical calculations to be solved with a simple twist of the rete – making the astrolabe nothing less than a Medieval computer.

Living in the Middle Ages

A computer designed with nothing more than a ruler, a pair of compasses, a protractor and lots and lots of patience – the same tools Brunold uses today. “I practically live in the Middle Ages,” the calm sixty-something smiles, “and cutting a complicated rete with the jigsaw is my kind of meditation.” His only concession to automation is a pillar drilling machine and etching instead of engraving. To cut all the circles, lines and letters by hand with a graver would be a meditation no-one is willing to pay for. “So I decided to etch the brass disk like a printed circuit board,” explains Brunold. A process which he had to outsource after his wife complained bitterly about the stains in the bathtub.

Astrolabe with lunar gearing.

The former village teacher came across his first astrolabe at an auction. Naturally, the ancient instrument was much too expensive for his budget and so he decided to build his own. “It was full of flaws,” Brunold admits, “but astrolabe-making became a pastime very dear to me. After all, it combines perfectly my life-long interests in astronomy and history.” About 15 years ago, during a period of unemployment, he decided to go full-time and earn his living with this Medieval art. The start wasn’t easy since Brunold specialised in copies of historic astrolabes and was soon accused of being a counterfeiter. But he always made a point of producing usable, correct instruments since quite a few of the originals are incomplete, flawed or due to their age simply out of date. Swiss precision ultimately saved his reputation and today he delivers astrolabes to museums and collectors worldwide. His large repertoire includes a Millennium astrolabe from around 1000 AD, a universal astrolabe for all latitudes, a heavy mariner’s astrolabe and even instruments with lunar gearing – meditatively cut gear-by-gear.

I pick one of the more basic looking instruments and ask for a hands-on lesson. How can such a simple mechanism made of nothing more than a few disks and a pointer be a clock, let alone a computer? “Oh, that’s easy,” says Brunold and the eyes of the former teacher are sparkling. “Imagine you target a star, say Sirius, with the alidade.” I’m glad it’s a lead-grey afternoon and I don’t embarrass myself by not knowing where to find Sirius. Struggling with the heavy brass disk I soon realise that for a quick look at this Medieval watch you need not only a sound knowledge of astronomy but also a very steady hand. “You then read the altitude from the scale and turn over the astrolabe,” continues Brunold. “Now you find the Sirius pointer on the rete and adjust it, so that it points on the circle on the tympanum representing the measured altitude. Now you have an exact image of the heaven above for this moment in time.” I’m lost instantly since my instrument is a replica of a Muslim astrolabe with the names and numbers in Arabic. But with a little help from Brunold I find the correct altitude line or almucantar and set Sirius to its current position. All that’s left now is to set a ruler to the current date on the rete’s Zodiac. Again, some basic astronomical knowledge comes in handy and helps to match the time of the year to the correct sign of the Zodiac. Finally, the true solar time can be read from the scale on the raised edge of the mater.

Taking Time to Read the Time

Clearly in the Middle Ages you needed a lot of time to read the time. But this is not the end of it, especially when you were a monk. Most people in the Middle Ages simply divided the time between dawn and dusk into twelve hours – regardless of the fact that, by doing so, hours during the day and hours at night had different lengths which varied with the season. Prayer times, among other events, were given as e.g. ‘one hour after sunset’. Unfortunately, these ‘temporal or unequal hours’ can deviate substantially from the equinoctial hours we find on today’s dials and the edge of the astrolabe. Luckily, other scholars thought of it and the monks could easily convert the hours with the help of special lines on the tympanum.

Martin Brunold collects and makes astrolabes of all shapes and sizes.

“And this is just one of the 1000 uses Umar al-Sufi mentioned in his treatise on the astrolabe in the 10th century,” notes Brunold, “but admittedly he was stretching it a little.” Over the centuries many have written about the astrolabe, but only a few have truly understood the instrument. Among the latter is a rather unusual candidate: Geoffrey Chaucer, the famous English Medieval poet. Besides his Canterbury Tales, he also wrote in 1391 an extensive description of the instrument for his eleven-year-old son Little Lewis. Among the uses Chaucer describes in detail are: finding a celestial’s body altitude or the sun’s longitude, determining daybreak, the end of evening twilight and the length of the day, obtaining the rising times of the signs of the Zodiac, locating the four cardinal directions or the point on the horizon where the sun rises, and so on. Other uses, probably more important to al-Sufi than Chaucer, include finding the Quibla or direction to Mecca and predicting the prayer times.

A Long Journey

The latter probably facilitated the introduction of the astrolabe in the Islamic world, counteracting the fact that it was regarded as an invention of nonbelievers and, therefore, blasphemy. The origin of the astrolabe is lost in time, but surviving descriptions and similar instruments put it around 300 AD somewhere in Greece. While Europe sank into the Dark Ages the knowledge of the astrolabe was conserved and greatly enhanced by Muslim mathematicians and astronomers. Around 1000 AD it was reintroduced in Europe where – what an irony of fate – it once again carried the stigma of being a nonbeliever’s tool and blasphemous. But the ‘Millennium-Pope’ Sylvester II, better known as Gerbert d’Aurillac, was a great admirer and reason ultimately won over prejudice. For centuries to come the astrolabe was a universal must-have tool for scholars: the laptop of the day, so to speak, a portable computer for all astronomical, astrological and spherical calculations. Not until the 16th century was it replaced by more precise instruments and mathematical tables. Moving from the scientist’s study to the monarch’s cabinets and the parlours of the elite, the astrolabe now slowly became nothing more than a richly ornamented curiosity to show off one’s education.

But somehow the astrolabe has survived until today, still enchanting astrolabe-lovers all over the word. It might be useless with all the clocks around us and all the freeware planetarium programs you find on the web. But then again, these disks hold a certain irresistible magic. Is it the golden glow of the brass? The craftsmanship? The mathematical genius behind it? Or simply the urge to hold the universe in our hands? Maybe Brunold is right, it’s blasphemy indeed.

Matthias Meier
Webseite des Astrolabienmachers Martin Brunold mit Hintergrundinformationen, vielen Astrolabien und Bestellmöglichkeit.
Der spannende und informative Vortrag von Tom Wujec auf der TED 2009 gab den Anstoß zu diesem Artikel.
Sehr umfangreiche Webseite zum Thema Astrolabium mit ausführlicher Linkliste.
Zwei Internetseiten, auf denen sich virtuelle Astrolabien zum Herunterladen finden.
Geoffrey Chaucers Abhandlung über das Astrolabium im Original und in zeitgenössischer Übertragung.
Umfangreiche Astrolabien-Sammlung des Museum of the History of Science in Oxford mit vielen hochauflösenden Bildern.
Mathematische Hintergründe zur stereographischen Projektion.



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