A 16th century astronomical instrument

Suppose you need to know the time but have no clock. You can get the approximate time with a sundial, an astronomical instrument that has been around at least since pharaohs ran Egypt – assuming it is oriented right. Now imagine you’re in the same clock-less fix, only it’s night. What do you do?

I came upon an ingenious answer a week ago when my family and I visited the Art Institute of Chicago, one of my favorite places on Earth. In a modest case were two small devices that were so beautifully made that they deserved their place in the art museum. They were astronomical instruments from the Renaissance: a celestial globe showing star positions, and a compendium. (The small case also held a terrestrial globe model, but that’s not an astronomical instrument.)

“The celestial globe had its origins in antiquity and presented the stars and constellations with their mythological personifications,” reads the museum’s description.

The compendium was most interesting to me. Small enough to hold in one’s palm when folded up, its three bright brass sections were octagonal, shining with gilt, silver and glass. Each section looked about three inches across. It was made by the workshop of Christoph Schissler Sr. in Ausburg, Germany, around 1557. A loan of the nearby Adler Planetarium and Astronomy Museum, the device is incredibly intricate. Its delicate etchings and small precision dials are beautiful.

[The Art Institute of Chicago display of Christoph Schissler Sr.’s compendium, a photo I took on July 31.]

One of the faces shows a map with a device to calculate longitude. Another has a sundial with a tiny brass gnomon whose shadow will tell the time when the compendium is oriented correctly. The map and a compass that is built in to the compendium would allow one to get the sundial into the right position for telling time.

The third major part of the compendium is the nocturnal. A web site maintained by the Museum of the History of Science in Oxford, England, explains how it worked:

“The nocturnal is made up of a series of superimposed disks of different diameters,” wrote the Museum of the History of Science’s Mara Miniati.

“The largest disk bears the days of the month, the median one bears the months and the signs of the zodiac, and the smallest one bears a series of divisions. The polar star can be sighted through the central hole, after the disks have been arranged according to the day and month of the observation. An index arm is then oriented in such a way that it can follow the movement of a certain star, close to the chariot of Ursa Major (The Great Bear). Other nocturnals were used for calculations of the phases of the moon and allow the duration of the phases themselves to be checked.”

Knowing the date and location, with a nocturnal a person could tell the time by checking the progression of the stars across the night sky. Of course, it wasn’t much good on a rainy night.

Telling time by our local star was easy. Telling time by other stars took learning and skill.

That science, precision machining and art are closely linked, as with the compendium, is no surprise to anyone who uses a fine telescope. That such a sophisticated astronomical instrument was used more than 450 years ago, half a century before Galileo focused his ‘scope on the moon, I found startling.

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