Tag: Astrolabe

Astrolabes and S•Town

On Thursday, February 16, at 5:36 PM I was standing in a faculty meeting when my phone vibrated. I fished it out of my pocket and looked at the screen. I had just received a voicemail and a text from the same number, a number I didn’t recognize. The text asked, simply: “Is this the phone of Darin Hayton?”

I stepped outside and listened to the voicemail. The person identified himself as a researcher for This American Life, asked if he had reached Darin Hayton, and wanted to ask about astrolabes. His message sounded urgent. I was intrigued. Why would anybody feel a pressing need to learn about astrolabes, at 5:30 on a Thursday evening? And why would that person not just turn to Wikipedia or some other on-line resource?[1] So I decided to respond.

As I was still, at least physically, in a meeting, I texted rather than phoned and offered to call later that evening or the next morning. He asked that I call him as soon as I was free.

Texting with the guy who turned out to be a researcher for This American Life.
When I phoned he immediately started asking about astrolabes. He had clearly done some research on them but wanted to confirm what he had learned—e.g., Hipparchus had developed the mathematics but not an instrument; early instruments dated from the late 9th century; you could use it to tell time. He was particularly interested in developments introduced by 10th-century Islamic scholars. He asked about different innovations we might attribute to them and wanted to know how they improved the astrolabe. Most of the innovations he mentioned cannot easily or definitively be traced back to early Islamic instrument makers. We chatted for 10–15 minutes. As our conversation wound down, I tried to find out why he was so interested in astrolabes. He offered few details, saying only that he was doing research for an up-coming This American Life show on a man from Alabama who had studied astrolabes and had even built his own. He wouldn’t tell me the man’s name, but did mention that he had recently died.[2]

After we hung up I tweeted about my brush with fame. I am clearly a nerd since I think having This American Life phone me constitutes fame.

I tweeted about my brush with fame—clearly I’m a nerd because I think talking to This American Life constitutes fame.
Fifteen minutes or so later as I stood in my bathroom brushing my teeth, my phone rang again. Same guy confirming a couple points and asking if his formulation was correct. Something to the effect: the Greek astronomer Hipparchus developed the mathematics behind the astrolabe and 10th-century Islamic scholars refined it to time their daily prayers. Yes, I said, that’s fine.

Because I am always late to the party, I didn’t hear about S•Town until late April, a month or so after it was released and became an instant hit.[3] Finally, when a friend suggested I listen to it because they “talk about astrolabes,” I downloaded it and listened while I repaired my washing machine. Sure enough, about 15 minutes in John B. McLemore (the main character) mentions astrolabes:

Because kids are talking about getting girls, or deer hunting, or football. Whereas I was interested in the astrolabe, sundials, projective geometry, new age music, climate change, and how to solve Rubik’s cube.


But he doesn’t say much more. Then, 30 minutes later, the astrolabe suddenly returns in the context of telling time.[4] Brian Reed, the host, reflects on various methods for tracking time, then describes the astrolabe:

BRIAN REED: The astrolabe looks kind of like a clock crossed with a compass. It’s a flat dial with a map of the night sky laid over it, and a pointer, or I guess a sight, attached on top of that. You pick a star in the sky, and aim the sight at it, twist the sky map until it aligns with the sight in a certain way. And then the dial shows you your direction, as well as the month, day, and time.

It’s a beautiful, complex device. And as a kid, John longed to figure it out, to put himself inside the brains of the people who puzzled through the earliest versions—the Greek astronomer Hipparchus, who devised the mathematics behind it, or the 10th century Islamic scholars, who refined the invention to help them time their daily prayers.

John wanted to go through what they had to go through to create an astrolabe. Which is why he made his own, designed specifically for the coordinates of this house. It hangs on the wall of his mother’s bedroom. That’s what he’s showing me, his astrolabe, when Skyler Goodson happens to walk in the front door.

When I heard this, I immediately recalled the man who had phoned six weeks earlier asking about astrolabes. There, in Brian Reed’s brief description, was the final version of what the man on the phone had crafted. It turns out that the man on the phone had been doing research for S•Town.

Thanks to S•Town and John B. McLemore, astrolabes are enjoying their 15 minutes of fame. Reddit pages on S•Town have astrolabe discussions. Websites promise to show you “How to Build an Astrolabe Like John B. McLemore From ‘S- Town’.” S•Town fans are turning up in museums asking to see astrolabes.

Visitors to Harvard’s museum are S•Town fans and want to see astrolabes.
Hey This American Life, perhaps you would like to do a whole show on astrolabes. While not as eccentric as John B. McLemore, I have built my own astrolabe, I know its history better than most, and I’m available. Your researcher/fact checker has my number. Have him give me a call.


  1. In talking to him, it seems he started with on-line resources, including my An Introduction to the Astrolabe.  ↩
  2. He probably used a euphemism, but somehow I think John B. McLemore would have preferred “died,” and I prefer it.  ↩
  3. And because I can’t just be late to the party, I find out late that I am late to the party, I learned about S•Town while listening to an old podcast of Wait Wait…Don’t Tell Me featuring Sarah Koenig[3a] that I had downloaded and then didn’t listen to for nearly a month. And even then I was in no hurry to listen to S•Town.  ↩

    3a. I should probably point out that the name Sarah Koenig meant nothing to me because I am one of perhaps only a handful of people, including John B. McLemore, who has never listened to Serial and only vaguely knows what it is.  ↩

  4. To be exact, Brian Reed’s description of the astrolabe comes at 44:05 into chapter 1. Astrolabes are mentioned in two other places: the first time is about 16 minutes into chapter 1; the last time is 2:35 into chapter 7. I don’t think I would say, as my friend did, that they “talk about astrolabes” in S•Town, but any popular culture reference is better than none.  ↩

Nicephorus Gregoras’s “περὶ κατασκευῆς … ἀστρολάβου”

An illustration of a rete from a 15th-century copy of Nicephorus Gregoras’s “περὶ κατασκευῆς καὶ γενέσεως ἀστρολάβου,” BN suppl. graec. 0652, fol. 285v.

A nice drawing of the rete from Nicephorus Gregoras’s “περὶ κατασκευῆς καὶ γενέσεως ἀστρολάβου” (“On the Mathematical Origin and Construction of the Astrolabe”). This rete, like other diagrams in copies of Gregoras’s text, lacks stars and finer details. In the few copies I’ve seen that include the stars, the rete closely resembles the one surviving Byzantine astrolabe, especially the number and style of the star pointers.

The only Byzantine astrolabe known to survive dates from the 11th century and reflects the style of other early, Islamic astrolabes. Today this astrolabes is in Brescia.

Interesting, at least to me, is the fact that the illustrations in different copies of Gregoras’s text often label the diagrams in different ways. In this copy diagrams are labeled counterclockwise starting at the 9 o’clock position, e.g., the diagrams on fol. 284v of BN suppl. graec. 0652. In other copies, diagrams are labeled counterclockwise or clockwise from various starting points (the 9 o’clock position is the most common starting point). In one manuscript, diagrams on consecutive folia are labeled in opposite ways. The absence of illustration in many copies of Gregoras’s work makes it challenging to follow the text. In most such cases, the copyist left no room for illustrations, suggesting he was working from an unillustrated copy. In some manuscripts, the copyist left large spaces for the illustrations.

A Spherical Astrolabe

The only spherical astrolabe that survives is this one made in A.H. 885 (1480 CE), most likely somewhere in present-day Syria. Today it is in the Museum of the History of Science, item # 49687.

The most common type of astrolabe is the planispheric astrolabe, which works by projecting the sphere of the heavens onto a plane in a way that preserves angular distances and allows the user to carry out a wide variety of calculations.[1] The rarest form type of astrolabe is the spherical astrolabe. Although Latin and Arabic descriptions of such instruments exist, and other sources indicate that some mathematicians might have owned spherical astrolabes, only this single example survives. This astrolabe is about the size of a baseball, .[2]

The rete on this astrolabe is a cage-like structure that would rotate around the globe inside it.[3] As with the rete on a planispheric astrolabe, this one includes a number of bright stars, 20 to be precise, a zodiac, and a meridian line. Unlike a planispheric astrolabe, which has different plates engraved for each latitude to depict the portion of the sky above the horizon at that latitude, this spherical astrolabe as a single “plate”—the globe on which the heavens are depicted. To adjust it for each latitude there were a different holes in the globe. A pin was inserted through a hole the cage-like rete and into the hole in the globe, both fixing the portion of the sky visible from the latitude and providing a point around which the heavens would rotate.

The maker signed and dated the globe, “The work of Mūsa. Year 885.” Unfortunately, we know nothing more about the maker, whose name is rather common. Based on the decoration and calligraphic style (a Kufic script, if you care), it seems likely to have been produced in the Eastern Mediterranean somewhere, probably Damascus or Cairo.

If you want to see more, it is Museum of the History of Science, item #49687.


  1. The mariner’s astrolabe is not, as far as I care, an astrolabe. The mariner’s astrolabe is an instrument for observing the altitude of a celestial object, e.g., the sun, and from that determining latitude. For me, the key difference is the fact that mariner’s astrolabes cannot be used to carry out any of the various calculations that define the planispheric astrolabes. I realize that some people want to call the mariner’s astrolabe a type of astrolabe. So be it. I, however, will not.  ↩

  2. For those more familiar with cricket, a cricket ball and baseball are nearly the same size: men’s cricket ball is 224–229mm in circumference; an official, professional baseball is 228–234mm in circumference. Field hockey your sport? A field hockey ball is about the same circumference, 224–235mm. Don’t like sports? Neither do I really.  ↩

  3. The rete is the map of bright stars that rotated about the pole. For more information about astrolabes, see my Guide to the Astrolabe.  ↩

Astrolabes or Mariner’s Astrolabe—A Primer

Celebrations are afoot in Ontario celebrating 400 years of Francophone presence in the region. An important part of those celebrations is Samuel de Champlain’s exploration of Ontario and his early encounter with First Nations cultures. Simcoe.com has a short post on an exhibit that includes one of Champlain’s navigational instruments: “Historic astrolabe on display in Midland believed to have been Champlain’s.” Unfortunately, there’s a bit of confusion about this instrument, which is not in fact an astrolabe.

The Simcoe article faithfully reports information from the Sainte-Marie among the Hurons site. Unfortunately for the Sainte-Marie among the Hurons site, the Canadian Museum of History, which owns the instrument, contributed to the confusion. The museum lists the instrument, artifact #989.56.1, as an astrolabe (the museum also identifies its two other similar instruments—artifact #988.58.1 and artifact #LH994.142.1.2 as astrolabes). Buried toward the end of the museum’s description is a passing comment that identifies the instrument as a “mariner’s astrolabe.”

This passing comment is the only place that Champlain’s instrument in correctly identified as a “mariner’s astrolabe.” Although the two instruments share one possible function—determining the altitude of star (usually the sun or the pole star)—that’s it. The astrolabe combined observations and calculations, allowing the user to perform hundreds of operations. It was both a complex, technical device and a status symbol. The astrolabe has been compared to iPhones and more recently to a complex Rolex watch.

Astrolabes—pre-modern Rolex or iPhone, you decide.
Astrolabes—pre-modern Rolex or iPhone, you decide.

These comparisons capture the astrolabe’s status and superabundance of operations its operations. My pamphlet offers a handy introduction to the history, fabrication, and use of astrolabes. Hundreds of astrolabes survive.

The mariner’s astrolabe, by contrast, was utilitarian and singular in function. It allowed the user to determine the height of the polar star or the sun and, thus, the observer’s latitude. The instrument’s design reflects its utilitarian function. Mariner’s astrolabes are typically heavy, made from a thick brass ring (only the limb of astrolabe) to limit them from swinging too much as the ship’s deck swayed and rocked at sea. Some had a ring at the bottom of the instrument from which to hang a weight for added stability. The body of the instrument was often cut away to reduce, scholars claim, the effects of wind blowing on the body of the instrument.

On the left is a typical mariner’s astrolabe from ca. 1600, from the Museum of the History of Science, inventory #54253, found here. On the right is a planispheric astrolabe, usually called simply an astrolabe. This is an early 16th-century astrolabe from the Museum of the History of Science, inventory #52528, found here.
On the left is a typical mariner’s astrolabe from ca. 1600, from the Museum of the History of Science, inventory #54253, found here. On the right is a planispheric astrolabe, usually called simply an astrolabe. This is an early 16th-century astrolabe from the Museum of the History of Science, inventory #52528, found here.

The limb was typically graduated from 0°–90° in the upper quadrants, once again reflecting its use as a basic observational instrument. A simple alidade with rather crude sighting vanes was attached to the front of the instrument. At night the navigator would look through the holes in the alidade to align them with the pole star. Then he could read the altitude of the star from the graduation on the limb, which altitude was, roughly, his latitude. If he wanted to know his latitude during the day, at noon he rotated the alidade until the sun shown down through the holes in the vanes (he would not look at the sun for obvious reasons). He read the sun’s altitude from the scale on the limb, added or subtracted the earth’s tilt based on the day of the year, and subtracted the result from 90° to obtain his latitude.

The mariner’s astrolabe was a nautical/navigational tool. Although an astrolabe could have been used at sea as a navigational tool, it is unclear that they were. The instrument’s many functions and finely graduated limb would have made it unnecessarily complicated and difficult to use on the deck of moving ship. Moreover, the astrolabe’s cost and status make it seem unlikely that a mariner would have owned one when there were other, more specialized and less expensive instruments that did the same thing. There are a few illustrations of astrolabes being used on ships, but whether these are idealized or meant to reflect contemporary practice is unclear. The various instruction manuals that include canons on how to use astrolabes at sea, e.g., Johannes Stöffler’s Elucidatio fabricae ususque astrolabii (1513), do not demonstrate that they were so used. Authors of such manuals sought to distinguish themselves and demonstrate their expertise by cataloging as many possible uses for astrolabes as they could imagine, regardless of whether or not anybody actually used astrolabes in those ways.

Surely few people used an astrolabe to make the many observations Cosimo Bartoli cataloged in his Del modo di misurare (1564). Google has scanned it here.
Surely few people used an astrolabe to make the many observations Cosimo Bartoli cataloged in his Del modo di misurare (1564). Google has scanned it here.

Whereas traditional astrolabes were expensive, status symbols and were, therefore, collected and displayed, mariner‘s astrolabes were working tools. They were not, as a rule, collected or displayed. Consequently, much rarer today—only about 100 survive and most of those were recovered from shipwrecks (the Museum of the History of Science has a nice audio guide to the mariner’s astrolabe here).

Champlain’s instrument was graduated from 0°–90° in each quadrant. The body has largely been cut away. And on the front is a large alidade for sighting.

From this photo it is clear that Champlain’s instrument was a mariner’s astrolabe. From the Canadian Museum of History description—direct link to photo.
From this photo it is clear that Champlain’s instrument was a mariner’s astrolabe. From the Canadian Museum of History description—direct link to photo.

It is plausible that he brought a mariner’s astrolabe with him as he explored Canada. But the story of Champlain losing his instrument by a lake, it having lain there in the forest for 250 years before a 14-year-old boy found it, and its subsequent sale to different collectors, seems almost too good to be true. And the instrument’s remarkable shape, having spent more than two centuries in the dirt, is equally surprising.[1] Whether or not it was ever owned by Champlain, his instrument is clearly a mariner’s astrolabe.


  1. In its description of the instrument’s provenance, Canadian Museum of History expresses some but not much skepticism about the story (my emphasis):

    In May 1613, Samuel de Champlain, the French explorer-cartographer, travelled up the Ottawa River. To avoid the rapids, he chose a course through a number of small lakes near Cobden, Ontario. Champlain and his men were forced to portage and to climb over and under fallen logs at one particularly difficult point by Green Lake, now also known as Astrolabe Lake. It was here, according to several nineteenth-century authors, that Champlain lost his astrolabe. If this is correct, the astrolabe remained where it had fallen for 254 years. Eventually a 14-year-old farm boy named Edward Lee found it in 1867 while helping his father clear trees by Green Lake. Captain Cowley, who ran a steamboat on nearby Muskrat Lake, offered Lee ten dollars for the astrolabe. Lee never received the money nor saw the astrolabe again. Cowley sold the astrolabe to his employer, R.W. Cassels of Toronto, President of the Ottawa Forwarding Company. He in turn sold it to a New York collector, Samuel Hoffman. The astrolabe was willed to the New York Historical Society in 1942 where it remained until June 1989, when it was acquired by the Department of Communications for the Canadian Museum of Civilization. This astrolabe is unique. It is the smallest of 35 mariner’s astrolabes surviving from the early part of the seventeenth century and the only one from France. It is in excellent condition, except for one missing piece, a small ring on the bottom edge of the disk, to which a weight was likely attached to help keep the instrument plumb. The ring was probably broken off sometime in the late nineteenth century, since it appears in an 1879 photograph of the astrolabe.

    For a more thorough analysis of the connection between Champlain and this instrument, see “The Mystery of Champlain’s Astrolabe” or, more recent, “19th century manuscript sheds new light on ‘Champlain’s Astrolobe [sic]’ thought lost by French explorer.”  ↩

ePamphlet Guide to the Astrolabe

I recently posted An Introductory History to Astrolabes over at PACHS. There seemed to be considerable interest in that post, so I expanded it a bit and converted it to an ePamphlet. Much of the material comes out of my introduction to the history of science course. The goal was to produce a convenient introduction that educators at colleges and universities as well as at museums could use. It would be wonderful if general readers would also find it useful.

I am making it available for free in PDF and iBooks formats.
PDF: An Introduction to the Astrolabe
iBooks: An Introduction to the Astrolabe