Drafting the Astrolabe 9: The Calendar Ring

Well… That was a bit more intense than I expected.

I was expecting that drawing the calendar ring on the back of my astrolabe would be rather involved, but actually doing it took more patience than I expected. This was definitely the hardest part so far.

Drawing the offset ring was straight-forward. As described previously computing the direction and distance of the offset is fairly simple, once you have the equations, and drawing the lines presents no problem. Where the difficulty lies is dividing the calendar ring. As described earlier, I need to mark off a section equal to the equivalent 5.25 days and then divide the remainder of the circle into 360 parts. Because I’m no longer working with a full circle, I can’t just quarter and then trisect like i did with the zodiac ring. Here I have to bisect each angle, and fudge “trisection” with the compass. Fudging the trisection by adjusting the compass to mark off 1/3 of the arc by eye is full of potential for small errors. Moreover, the first bisection that has to be made is to divide a huge arc – It was harder to do that accurately than I expected. I had to invent techniques to make it easier to divide accurately. After much cussing and erasing, I have a result that is fairly accurate, I need to ink and label it and then I will post a scan of the result.

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