## Drafting the Astrolabe 10: The Shadow Squares

- on 01.16.12
- Astrolabe Project, Drafting The Astrolabe
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Getting the zodiac and calendar rings done was probably the worse, most finicky part of this project; the rest of the tasks remaining might require careful measurement, and attention to detail, but orders of magnitude less work than the last section.

The next task is to finish off the rear of the astrolabe. Two scales remain to be drawn, the Unequal hours and the shadow squares.

Drafting the shadow squares is straight-forward: Two square boxes in the bottom half of the astrolabe back, each side divided into a set number of sections. The only trick bit will be finding the correct size for the boxes. I want the shadow squares to be as big as possible, as larger means more accurate to draw and to use, but they also need to fit into the remaining space inside the calendar ring. Because the calendar ring is offset, I have to take that offset into account

The largest dimension of the shadow square is its 45 degree diagonal, so if I draw a line at 45 degrees from the center to the right bottom side of the astrolabe, stopping at the inside edge of the calendar ring, I can use that point to erect the rest of the square.

Step 1: Draw the 45 degree diagonal from the center down and right to where it touches the inside of the calendar ring on the bottom right side of the astrolabe back. Mentally label this point “A” (See Figure A).

Step 2: Erect a vertical line from point A to the horizontal center line of the astrolabe (see Figure B).

Step 3: Erect a line parallel to the horizontal center line of the astrolabe, running from point A to the left side of the astrolabe (see Figure C).

Step 4: Draw the 45 degree diagonal from the center down and left to where it touches the line drawn in the previous step. Mentally label this point “B” (see Figure C).

Step 5: Erect a vertical line from point B to the horizontal center line of the astrolabe as you did in step 2 (see Figure D).

Step 6: Draw a scale line inside the box you just drew (see Figure E).

Step 7: Divide and label the Shadow Squares.