The next big chore will be creating the major scales for the back of the astrolabe. This will involve drawing one set of rings representing the Zodiac and another set representing the Calendar.
Drafting the zodiac is straight-forward, if tedious. The circle needs to divided into 360 one-degree ticks. The process is the same one I used to create my protractor (link).
The next step will be -um- interesting. The calendar scale sits inside the zodiac ring and is oriented such that when placed on a date, the adidade pointer will also point to the position of the sun on the zodiac for that date. Unfortunately, where 360 divides up well, 365 does not. in fact, the only divisors other the 1 and itself are 5 and 73. Now I can divide the circle into 5 equal parts (quintrants?)by cheating with the compass, but after that 73 is prime and not divisible equally. In addition the year is not 365 days…. it is 365.25.
So I looked at my sources. Stoeffler has this to say:
“Make the subdivisions of the days and of the months in this way. Set the rule on the center E and on the 20th degree of Capricorn and draw a line segment crossing all the eccentric circles. It will point out the beginning of January and be marked with a G. Starting from it, in the opposite direction to the succession of the Signs, count around 5 degrees 20 minutes. Set the rule between this point and the center E. Draw a line from the first eccentric circle to the second, which will be H. The remaining arc must now be divided (this small arc excepted) into 360 equal parts…”(Stoeffler:67)
So I am to remove the angle equivalent of 5.25 days from the circle, and divide the remaining arc into 360 ticks. Much easier. Or rather somewhat easier; as the 360-day section of the circle is NOT 360 degrees, I will not be able to quarter it and trisect etc. like I will the zodiac. I will need to bisect repeatedly and do some fudging with the compass when I need to divide an angle other than in half.
Question: Am I dividing into 365 or 365.25?
It occurs to me that at the finished diameter of my astrolabe, the .25 day tick will be approximately the same as the width of the line used to mark it. If I remove the .25 and use 365 ticks, each tick will be thrown off a bit: By approximately .0007 of the width of the space. This is not going to be measurable with this instrument.
An examination of the backs of several period astrolabes shows that leaving out the .25 is a regular practice. In fact, going back to Stoeffler: “The GH arc will be divided into 5 parts and 1/4, if it is held to precision” (Stoeffler:67). So for this project I’ll go with 365 days.