{"id":5145,"date":"2025-09-25T17:50:08","date_gmt":"2025-09-25T22:50:08","guid":{"rendered":"https:\/\/jonvoisey.net\/blog\/?p=5145"},"modified":"2025-09-25T17:50:08","modified_gmt":"2025-09-25T22:50:08","slug":"astronomicum-caesareum-facsimile-restoration-project-the-constellation-volvelle","status":"publish","type":"post","link":"https:\/\/jonvoisey.net\/blog\/2025\/09\/astronomicum-caesareum-facsimile-restoration-project-the-constellation-volvelle\/","title":{"rendered":"Astronomicum Caesareum Facsimile Restoration Project \u2013 The Constellation Volvelle"},"content":{"rendered":"

The first volvelle in\u00a0Astronomicum Caesareum<\/em> is the constellation volvelle. It depicts a view of the northern constellations to a declination of approximately -20\u00ba. The depiction is based on a 1515 star map by Albrecht Durer.\u00a0<\/span><\/p>\n

Although this volvelle (as well as all of the others) are illustrated to be reminiscent of an astrolabe, this one is not capable of depicting the sky at a specific time as an astrolabe can<\/span>1<\/sup><\/a><\/span><\/span>. Rather, the purpose of this volvelle is related to the precession of the equinoxes – a slow change of the position of the vernal equinox in the zodiac over a 26,000 year cycle. Determining this is important because it carries the line of apsides and the nodes of the planets along with it.<\/span><\/p>\n

The understanding that the equinox precessed is generally attributed to Hipparchus around the 2nd Century BCE. However, most of Hipparchus\u2019 writings have been lost and what we know of them generally comes to us through Ptolemy’s writings in the 2nd Century CE. In the <\/span>Almagest<\/span><\/i>, Ptolemy tells us that Hipparchus believed the equinox to move 1\u00ba per century. Ptolemy checks this number and eventually concludes the same<\/span>2<\/sup><\/a><\/span><\/span>. The value was revised over subsequent centuries, but was eventually complimented by the concept of trepidation sometime in the 9th century. This hypothesis proposes that the position of the equinox has a back and forth motion in addition to the linear precession<\/span>3<\/sup><\/a><\/span><\/span> of Ptolemy. While incorrect, this hypothesis was widely accepted as part of medieval cosmology although there were disagreements in how large the back and forth motion was and its period. Thus, the use of this instrument first requires two parts: Adjusting the volvelle for the linear precession and an adjustment for the trepidation.\u00a0<\/span><\/p>\n

\"\"<\/a>The first of these is done by first finding a hidden scale beneath the rotating disc along the right side (shown at right). The horizontal line which divides the years before Christ (ANNI ANTE NATIVITATEM CHRISTI) from the years after (ANNI POST NOTIVITATEM CHRISTI) is marked with a cross (\u2720). A corresponding tab on the rim of the rotating disc is also marked with the same symbol and should be turned to this position, which sets the dial for the year 1 CE. Then, the number of years before or after this is determined and the tab on the disc rotated to the corresponding\u00a0 year<\/span>4<\/sup><\/a><\/span><\/span>. Obviously, the scale is far too small to turn to the exact year, as each division represents a full century.<\/span><\/p>\n

 <\/p>\n

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\"\"<\/a>Next, the disc must be adjusted to account for the trepidation. This is done by<\/span>\u00a0locating a small, oval shaped scale immediately next to the tab with the cross (shown right). Again, this oval is marked with the years before and after Christ. To make use of it, a thread from the center of the volvelle is pulled to the\u00a0correct year, held in place, and then the disc rotated so that the flat side of the tab marked \u201cAux Y\u201d is lined up with the thread. Once this is done, the disc is set properly.<\/span><\/p>\n

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Apian tells us that we can learn several things once the disc is aligned. The first is the ecliptic coordinates of the stars depicted. The ecliptic longitude can be determined by pulling the thread through the star in question and finding where it falls along the outer zodiac ring on the base. For the latitude, Apian instructs the user to again draw the thread through the star, and then use the feet of a compass to measure the distance between the star and the ecliptic (which is depicted as a solid black line). This distance can then be compared to a scale near the tail of Sagittarius.<\/span><\/p>\n

But, more importantly, the dial can now indicate the positions of apogee for each of the five naked eye planets. This is done via a series of tabs placed around the border. This information will be important in later calculations for the planets.<\/span><\/p>\n

Issues & Restoration<\/strong><\/span><\/p>\n

Gingerich identifies two issues with this volvelle. The first is that the central thread is missing. Second, some of the tabs around the circumference of the disc were improperly trimmed off. In my copy, I also found that the disc was not properly centered. Lastly, there may have been a small seed pearl on the missing thread that could be slid back and forth to be used as an indicator as Apian\u2019s instructions appear to mention a pearl (<\/span>margaritam<\/span><\/i>).<\/span><\/p>\n

Reviewing the digital copies listed above, there does not appear to be a standard color for the threads used. Red and green are the most common colors. None show any evidence of a pearl.<\/span><\/p>\n

\"\"<\/a>To restore the volvelle, I first removed the disc to investigate how it was constructed. Removing the disc resulted in some tearing to the disc itself (seen in the image at right), although the paper was thick enough that it did not pull all the way through to the top surface. The image at right shows the top of the volvelle removed making the misalignment obvious. In the image you can see the true center (blue), which is clearly offset from the center of rotation (red). Although slight, this difference could create an error of 1-2\u00ba in the results from using this volvelle. Given that the positions of the apogees of the planets are used later, it is important that this volvelle be as accurate as possible lest the error propagate. As such, I decided to remove this assemblage and recreate it more carefully.<\/span><\/p>\n

\"\"<\/a>Removing the remaining layers that allowed the disc to rotate was quite difficult as the glue holding down the larger circle with the hole was still in quite good condition<\/span>5<\/sup><\/a><\/span><\/span>\u00a0resulting in the paper, both of the assemblage and the base page tearing as can be seen (below) where the page tore towards the top and the circle tore near the bottom. Fortunately, the paper on the page was sufficiently thick that this did not tear through to the reverse side of the page.\u00a0<\/span><\/p>\n

Examining the pieces, I found this volvelle was attached to a circular shaft of thick card stock affixed to another larger circle beneath it, also on card stock. This was placed on the base of the page, but not affixed.<\/span><\/p>\n

To hold it in place while it rotated, another larger circle of similarly heavy card stock with a hole cut out for the central shaft was placed over it with another ring of thinner card stock around the edge. In this way, both the central shaft and its base were constrained. A cross section of the layers is shown below.<\/span><\/p>\n

\"\"<\/a><\/p>\n

I then took measurements of all of the pieces\u00a0<\/span><\/p>\n