How did an Italian astrolabe end up on the New Zealand passport?

A few years ago I enjoyed a blog post in which National Maritime Museum curator Rebekah Higgitt wrote about the "navigation" theme of the new New Zealand passport design.  Higgitt discussed the use of John Harrison's H1 clock as an illustration in the passport.  She also noted that the description on the New Zealand government website mentioned an astrolabe, which piqued my interest.  I tried to find out more, but the website didn't include pictures, and none of my Kiwi friends had new-style passports.

So I forgot about it... - until last week when a friend of mine was talking about the designs that have been proposed to replace the current NZ flag.  I took the opportunity to ask to see his passport, and bingo! Here it is:

I was intrigued by the distinctive design and decided to see if I could find similar astrolabes in any published museum collections.  It didn't take me very long to track down this one:

Image courtesy of Museum of the History of Science, University of Oxford

This is astrolabe 50257 at the Museum of the History of Science in Oxford.  You don't have to be an expert to spot the similarities.  But the question is, are those similarities remarkable? How unusual is this design?

To answer that question, let's go back to basics for a moment.  The similarity you notice - the dark pattern of rings, heart shape and so on - is the rete of the astrolabe.  It was made by cutting holes in a sheet of brass, so that it resembles a net (that's what rete means in Latin) or, as Chaucer thought, a spider's web.  It sits inside the mater of the astrolabe, and rotates around that big pin in the middle.  Each of the pointers on the rete marks a star, making the rete a moving star map, able to simulate the daily (apparent) motion of the heavens around the earth.

This overall layout, with the rete turning above a plate engraved with a grid of coordinates and a horizon for a particular latitude, is by far the most common form of an astrolabe.  The basic concept goes back to Ptolemy in the 2nd century CE, and the instrument had this settled form, with the mater, interchangeable plates for various latitudes, the rete and rule (which also moves, as you can see from its differing position in the two pictures above), by the time John Philoponus wrote a description of it in the early 6th century.

But that still left a lot of flexibility for individual designers and craftsmen.  They could choose which stars to include on their rete, but more noticeably, the supporting brass "net" could be almost any shape, as long as it included that smallish eccentric (off-centre) circle which is the ecliptic, mapping the Sun's annual progress through the zodiac.  Craftsmen expressed their flair through ingenious designs - a favourite trick, as you can see here, was to make the rete symmetrical, even though the stars obviously weren't.

As far as I know, this particular design of rete is unique.  Luckily the astrolabe itself has the name of its designer on the back: Giovanni Domenico Fecioli of Trento, in the far north of Italy.  We also have the name of the man who commissioned it: Giulio Cesare Luchino, of Bologna.  The latitude of Bologna is 44° 30', which matches the astrolabe's single latitude plate.  It's dated 1558, just when the popularity of astrolabes in Europe was at its peak.

Image courtesy Museum of the History of Science, Oxford

In technical terms there's nothing particularly remarkable about this astrolabe, but the design is an attractive one.  The illusion of interlocking circles at the bottom of the rete was a popular motif in that period - compare, for example, this one by the prolific Arsenius workshop in Louvain.  Arsenius was famous for his tulip-shaped designs (can you see the tulip?), and Fecioli's rete doesn't quite match the delicacy of Arsenius's intricate brasswork, but it's still beautiful, and I'm sure it was much prized (and proudly displayed) by Luchino.

I have no idea why the New Zealand government chose a Fecioli astrolabe as an illustration for their passports.  But it's worth saying a few words about the place of this instrument in the overall design scheme.

Here's what their website says about it:

The passport’s new design evolved from the concept of navigation and our evolution from a place of discovery, to a place of destination and follows the journeys of the earliest explorers of New Zealand through to the journeys made by Kiwis today. Themes of arrival and departure, navigation and time are represented figuratively and metaphorically throughout the passport.

And about pages 20-21 specifically:

The astrolabe and chart – the astrolabe is an astronomical instrument used by astronomers, navigators, and astrologers. Its many uses included: locating and predicting the positions of the sun, moon, planets and stars; determining local time using local longitude and vice-versa; surveying and triangulation. 

Of course the theme of navigation is perfect for a passport.  But the astrolabe's place in that theme is less certain.  It was a multifunctional compendium of astronomical and astrological functions, not particularly suited to use at sea.  It's true that the mariner's astrolabe was a popular navigational device (I made and tested one for a previous blog post), but that was a quite different instrument.  It's certainly highly unlikely that Fecioli's beautiful piece ever went offshore.

Even if it had, it could never have been used to determine longitude (or to predict the positions of the Moon or planets).  The relationship between longitude and time was well known in the age of the astrolabe, and astrolabes could certainly be used to find local time.  But longitude is a relative measurement (these days we measure it east or west of Greenwich); if you want to go from local time to local longitude, you also need to know the time at the reference point (e.g. Greenwich) from which you're measuring longitude.  Local time was easy to find, but when Fecioli made his astrolabe, a reliable, robust method of keeping reference time - a clock that could withstand ocean passages - was still 200 years away.

But local time itself is unattainable with the astrolabe pictured in the passport, because the designers have cut off the suspension ring and throne that attaches it to the mater.  The ring is crucial for sightings of the Sun or a star, because the astrolabe has to hang vertically.  Without it, the astrolabe is no longer an observational instrument.

That may seem like nitpicking, but it tells us something about the way astrolabes are (and were) seen.  They have always had symbolic value, representing astronomical knowledge as well as artistry and the owner's wealth and status.  Today in many museums they are presented as art objects, out of context and, often, supported from below rather then suspended by the ring (admittedly for sound preservation purposes).  They now seem to symbolise ancient, arcane knowledge, and this symbolic value trumps accuracy, much like the "save" icon in most computer programs is a 3.5" floppy disk, though you're unlikely to be storing your work on those any more.

Of course I don't mind that - I'm happy people find astrolabes evocative and attractive.  And, as I say, they've always had symbolic value.  But once in a while it's worth reminding ourselves of the specific contexts where, for a thousand years, they were also complex scientific objects with practical purposes.

Update, 25/08/2015: I wrote to the New Zealand Passport Office to see if I could find out more, and received a very friendly response.  The officer wrote: "I have spoken directly with some of those involved in the design process of the latest version of the New Zealand passport, which was produced in 2009 and the astrolabe design appears because it is a symbol for travel and a relatively complex shape. [...]
The specific astrolabe was not chosen for any particular reason except that it fit with the overall theme.  I was unable to ascertain whether the astrolabe featured in the book is the specific 16th century Italian you have identified but your analysis would suggest that it is the case.  Unfortunately I am not able to be more specific than that.  
Thank you very much for writing to us and I have noted with pleasure your compliment for our 'beautiful' passport."

Precision and accuracy in medieval astronomy

What is the difference between precision and accuracy?

In modern English they are used almost interchangeably.  But there is a difference, of course.  I wonder what time it is now, when you are reading this.  Is it about eleven o' clock?  Or is it 09:34?  Of course, I have no way of knowing which of those guesses is more accurate.  But the second is obviously more precise.

Which of these two timepieces
is more accurate? Well,
they've both stopped...

That distinction may be more or less clear to us.  But that wasn't always the case for medieval astronomers.  What if I were to refine my guess, and say you're reading this at 09:34:27?  Is that any better? It's obviously more precise.  But when is it preferable to be more precise?  The answer to that might be more complicated than it appears.  In general, we might say that precision is only preferable when it increases accuracy.  But medieval scholars didn't always see it the same way.

I study astronomical tables.  Take a look at this amazing digitised version for an example.  That link points to a table of the daily precession of the stars and planetary apogees.  It's a lot more exciting than it sounds!

(Here's a brief astronomical explanation: skip it if you want...  Precession is the phenomenon that means that the stars appear to move very gradually around the sky, so that they're not in exactly the same place from year to year.  I don't mean the obvious daily rotation around the North Star that's caused by the Earth spinning on its axis - I mean a much slower change, caused by a "wobble" in the tilt of the Earth's axis.  The stars are moving 1° every 72 years - pretty hard to spot, but it explains why, right now, the Sun is still just about "in" the constellation Sagittarius (i.e. in front of those stars) even though it ought to be passing from Capricorn into Aquarius.  To be clear: the astrologers haven't got that wrong, because when they say it's the cusp of Aquarius, they mean the Sun has gone 120° around the sky (in modern terms, we've completed a third of our orbit) since the last equinox.  It's just that the background of stars has moved since the Ancient Greeks assigned them to their positions between equinoxes and solstices.)

So what?  The point is, precession is a VERY slow motion.  It's obviously almost impossible to observe with the naked eye.  It's impressive enough that ancient astronomers had even noticed it, so we shouldn't be surprised that their estimate of the rate was a bit different from ours.  That's why the table I linked above represents a precession of 1° every 136 years (their theory of precession included a separate, non-linear component that made up most of the difference).

But I said above that that table is a table of DAILY precession.  What's the point of tabulating daily values for something that changes one degree every 136 years?!

Good question! Here's another one: What's the point of tabulating those daily values to a precision of billionths of billionths of degrees?!  I don't even know what a billionth of a billionth of a degree is called, but that is the precision represented by the daily value of 0;0,0,4,20,41,17,12,26,37.  (That's a sexagesimal number: 0°, 0 minutes, 0 seconds, 4 thirds... In decimal terms it's 0.0000201148235466718.)  The 37 in the final column of the table is 3.67 x 10^-15.  To put that in context, that's one 98,000,000,000,000,000th part of a complete circle. It would take approximately 750 billion years for these daily 37s to accumulate to even a degree’s difference.

That level of precision in the tables clearly didn't arise from naked-eye observation of the stars.  No, it's a result of the way the tables were computed.  And astronomers clearly realised that - they understood that such precision was unobservable.  Yet they maintained it when they copied and recomputed the tables.  Why?  Because, I suppose, they reckoned that more precision is better than less.  To put it another way: you say why keep those 37s?  They would say, what makes you so sure you can get rid of them?

Isn't that silly?  Hold on a moment - you may not be much better.  A friend of mine recently posted this on Facebook:

I know how these things work: authors of recipe books work out their recipes in their own ways.  Delia Smith was obviously used to using pounds and ounces.  She used 2 oz of sugar.  2 oz is about 56.75g, but no editor will let that go into the published cookbook.  So it gets rounded down to 50g.  Then when Delia calls for 6 oz (about 170.25g), it gets rounded up to 175g.

Here's the weird bit: I know this is what's happening - I even have a magnetic converter on my fridge door that tells me that 2 oz is 50g and 6 oz is 175g.  But that doesn't stop me measuring out the quantities with exaggerated care, paying attention to the slightest fluctuation on the scales.  And what about the eggs?  I'm precise to the last gram of sugar even in recipes that use eggs, when I'm well aware that the size of eggs can vary widely.  If I can sustain this kind of cognitive dissonance, perhaps I shouldn't be too critical of the medieval astronomers.

Ships, Clocks & Stars

I recently visited, and very much enjoyed, the exhibition at the National Maritime Museum about the quest for longitude.  It's on until 4th January, and I highly recommend it.  I liked it so much I wrote a review of it for the Science Museum journal!

Stars Without Streetlights

I spend a lot of time trying to imagine what it must have been like for a medieval astronomer to use an astrolabe.  It's tricky.  Apart from all the things we might not know, like precisely who used them and why, there's another major problem: light pollution.  Even in a small city like Cambridge the street lights blot out all but the brightest stars.

Which is why I was blown away when I saw this picture:

New York 40° 42’ 16’’ N 2010-10-09 lst 3:40

It is by French artist Thierry Cohen.  Here's what his website says about his work:

He photographs the world’s major cities, seeking out views that resonate for him and noting the precise time, angle, and latitude and longitude of his exposure.  As the world rotates around its axis the stars that would have been visible above a particular city move to deserts, plains, and other places free of light pollution.  By noting the precise latitude and angle of his cityscape, Cohen is able to track the earth’s rotation to places of atmospheric clarity like the Mojave, the Sahara, and the Atacama desert.  There he sets up his camera to record what is lost to modern urban dwellers.

Compositing the two images, Cohen creates a single new image full of resonance and nuance.  The work is both political and spiritual questioning not only what we are doing to the planet but drawing unexpected connections between disparate locations. Equally importantly it asks: what do we miss by obscuring the visibility of stars?  As the world’s population becomes increasingly urban, there is a disjunction with the natural world which both Cohen and science posit causes both physical and psychological harm.  Cities that never sleep are made up of millions of individuals breaking natural cycles of work and repose. Cohen’s photographs attempt to restore our vision.

You can see more of his beautiful and thought-provoking images of starlit cityscapes on the Danziger Gallery website.

Paris 48° 50’ 55’’ N 2012-08-13 lst 22:15