Aztec Calendar

      I got the idea for this when I was at the library reading a book on Aztec history and culture.  I saw a drawing of two interlocking wheels that the Aztec people used to represent the days in their sacred calendar.  I immediately wanted to build one and I photocopied several pages of the book to use as a reference.

      I built the entire project out of cabinet-grade plywood that I had left over from other projects.  The base is made from " plywood and the gears themselves are " plywood.  The base measures 12" from front to back, 21" tall and 24" wide.

      The gears are mounted on #10 screws that are smooth for the first inch below the screw head.  That allows the gears to spin on the smooth metal.

      I stained the base using a polyurethane stain in the color of Red Oak.  I have used this in other projects, including the CD Dividers, and it has always turned out well.  I hand-painted the symbols on the fronts of the two gears using acrylic paints and a very fine brush.  The 20 Aztec symbols on the large gear were hand-drawn first, based on illustrations of the symbols that I found on the Internet.  I painted the edges and backs of the gears in black so that the symbols on the front seem to stand out more.  The front of each gear was finished with one coat of clear polyurethane and one coat of varnish.

           The booklet:

      The sloped front of the display base has a laminated booklet explaining the history and significance of the Aztec calendar.  The text, which I got from, follows:

The tonalpohualli and Aztec cosmology
      The tonalpohualli, or day-count, has been called a sacred calendar because its main purpose is that of a divinatory tool.  It divides the days and rituals between the gods.  For the Aztec mind this is extremely important.  Without it the world would soon come to an end.  According to Aztec cosmology, the universe is in a very delicate equilibrium.  Opposing divine forces are competing for power.  This equilibrium is in constant danger of being disrupted by shifting power of the gods, of the elemental forces that influence our lives.  This struggle cannot be won by any god.  The notion that everything ultimately consists of two opposing forces is essential to the Aztec worldview.  The world is always on the brink of going under in a spiritual war, a war of gods competing for supreme power.  To prevent this from happening, the gods have been given their own space, their own time, their own social groups, et cetera, to rule over.  The tonalpohualli tells us how time is divided among the gods.
The system of the tonalpohualli
      The system of the tonalpohualli can be best understood by imagining two wheels that are connected to each other.  One wheel has the numbers "one" to "thirteen" written on it.  The second wheel has twenty symbols on it.  In the initial situation, number "one" combines with the first symbol.  This is the first day of the tonalpohualli.  Now the wheels start moving and number "two" combines with the second glyph.  This is the second day.  After fourteen days, an Aztec week (trecenas in Spanish) of thirteen days has passed.  The wheel with the numbers shows number "one" again.  The other wheel now shows the fourteenth symbol.  After 260 days, the two wheels have returned to their initial position.  The tonalpohualli starts all over again.
Dividing time among gods
      A day (tonalli) in the tonalpohualli consists of a number and a symbol or daysign.  Each daysign is ruled by a god.  The nature of a day is also influenced by its number.  More important, each trecenas has a god that rules over that very 13-day period.

           Making the gears:

      I decided to cut two gears to represent the wheels.  One of the wheels needed to have 13 teeth (and 13 slots) and the other needed 20 teeth (and 20 slots).  The teeth and slots on each gear needed to be the same size if they were going to fit together.  I decided to use 1" increments for the teeth and slots.  Eventually, the teeth were angled so that they were actually " at the outer ends and 1 " at the base.  Conversely, the slots are 1 " at the edge of the gear and " near the center.

      So the circumference of the two gears (still circles at this point) would need to be 26" and 40".  The diameter of each circle is just the circumference divided by Pi (approx 3.14).  Thus, the diameters of my two circles were roughly 8.25" and 12.75".  I cut the two circles using my circle-cutting jig and my bandsaw.

      To cut the teeth and slots I needed to first mark one-inch increments along the circumference of each circle.  This was easy on the larger wheel since 360 degrees divides easily by 40 (points).  Each mark was therefore 9 degrees apart.  But, dividing 360 degrees by 26 (points) gives you 13.85 degrees!  That was a little tough to do with just a protractor.  Fortunately, my aunt, Pam Balthazar, just happens to be a high school math teacher and she has a computer program that helped me quite a bit.  She had the computer draw a circle with 26 evenly spaced points around its edge.  She sent that drawing to me and I used it to mark the points on the smaller circle.  Thanks Pam - I couldn't have done it without you!

      Finally, I cut the gear teeth with a bandsaw.  Once they were cut, I mounted the gears onto a scrap piece of wood with a nail through the center hole of each.  I was thrilled, and slightly surprised, to find that the gears meshed together and turned perfectly.  I even took a picture of them spinning just because I was so proud that they turned out so well.

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