Apologies for being so quiet lately. There’s a few things that have been on my plate recently. One of the big ones has been some research for a new video for the YouTube channel. What I thought was going to be reading 2 books and a handful of paper has turned into 6 books an undergraduate thesis, and a handful of papers. Needless to say, it’s taken much longer than expected.
However, while doing that research I encountered this image.
This is from the Universitätsbibliothek Heidelberg, cod. Pal. germ. 832, fol 16r (1491) and it’s a stunningly beautiful volvelle. I knew I wanted to look into it further and thought I bookmarked it, but, when I came back a few days later, I couldn’t find it. I did remember some key words, and was able to find some similar volvelles.
Egerton MS 848
Cambridge, Gonville, & Caius College, MS 336/725
NLW MS 3026C
Ashmole 370
BL Sloane 4100
BL Sloane MS 405
Trinity College MS R.1.86
When I found these, I realized these were rather interesting instruments and decided I wanted to make my own.
Despite not being nearly as pretty, these three are actually far more sophisticated.
The outer ring on each is the date. The pointer on the lower disc can be moved to the current date and then, from the next ring in on the base, the position of the sun in the zodiac can be determined.
It should be noted that the ring of the calendar and the zodiac are both concentric which means that it won’t represent the true position of the sun – only the mean.
Once the lower (solar) pointer is set, then the upper pointer can be turned. You’ll notice that the edge of the lower disc with the sun pointer has numbers going $1$ to $29$. Or, more accurately, $29 \frac{1}{2}$. This is roughly the number of days in a full cycle of lunar phases, known as a lunation. Thus, if you know the number of days since the last new moon, you can set the top (lunar) dial using this.
But, how many of us pay attention to such things? If you were the sort to own one of these in periods, you probably were. But in case you forgot, there’s a second way to set the lunar pointer. On the three just above, there is a small window which will illustrate the lunar phase. At first, I thought this would just be an off center dark circle on top of a bright background, but when I tested that out, it didn’t work well at all for first and third quarter.
However, I found another manuscript which reveals the actual shape underneath:
Egerton MSS 2572
This one is missing the top (lunar) dial, revealing the shape underneath. Here, you can clearly see it’s not simply an off-center circle. Rather, it’s a cardioid shape.
This is also confirmed in another manuscript:
Lambeth Palace Library MS 454 fol 26r
Although we can’t see the figure directly, this particular manuscript has been subject to a reconstruction in which a conservator carefully lifted the sections of the discs and it was photographed from numerous angles. The individual layers were then digitally reconstructed and printed for assembly.
Interestingly, in this one, we find that the shape is decidedly asymmetrical.
I sketched this out briefly and tried it, but it just didn’t look right at first and third quarter. As such, for my version, I opted for the ring of moon phases around the solar dial. Which was fine, as I really liked how it looked.
The basic geometry and divisions I worked up in some vector image software as I learned from experience when making my Instrumentum Primi Mobilis, that making minute divisions on arcs is a painfully tedious task. However, I left off all of the lettering and coloring (except for the divisions) so I could do that by hand.
The lines on the top disc are related to the astrological aspects – angles of the various luminaries can be with respect to one another that were believed to have additional effects.
Overall, I’m quite happy with how mine turned out.
However, after completing it, I discovered that another SCAdian, Richard Wymark also studied this type of volvelle several years ago. Better yet, he figured out what some of the circles on the base represented. Some relate to the number of hours of day/night while some are the maximum altitude of the sun at noon throughout the year. These two figures are latitude dependent, although it appears that all of the ones that include these pieces of information have them the same indicating that they were all meant to be used at a similar latitude or their creators copied them without fully understanding them.
I’ll probably add these circles to my version at some point, and might also test out the cardioid lunar window, but that will have to wait awhile.