The Astrolabe Project

The first step in drafting the astrolabe climate plate is to lay out the Tropics and the Equator, along with the meridian. As these markings will be the same for all the plates, it makes sense to draw each line on all the plates before moving on to the next line. This way, I can set the compass once for each line, and I only have to draw construction lines on a single sheet.

Draw a horizontal line to become the true east-west line; and erect a perpendicular line at its center point to become the meridian. Repeat this on all the climate sheets. Next, use the compass to draw a circle the desired diameter of the plate (remember to get this measurement from the finished astrolabe front drawing, the plate will need to fit in the mater hollow). Without changing the setting of the compass, repeat for all plates. Finally, set the compass to a slightly smaller diameter and draw the circle for the Tropic of Capricorn. Repeat on the other plates.

Note: The space being left at the edge of the plate allows room for the outer edge of the rete. Some astrolabes have this space some do not.

When done with this set all the plates should look like A in the figure to the right.

The current obliquity (angle to the equator) of the Ecliptic is approximately 23 degrees 26 minutes [Morrison]. The next step is to project the circle for the Equator, so at an angle of 23.5 degrees (rounding off to the nearest half degree as my tools are no more accurate than that) I draw a light construction line from the center of the plate to the circle representing the Tropic of Capricorn (#1 on the right).

As I already have the Tropic of Capricorn defined, the first circle to project will be the Equator. To do this, place one end of the straightedge on the place where the true east-west line (the horizontal line) crosses the Tropic of Capricorn, and the other end on the point where the construction line intersects the same circle. Draw a second construction line (#2).  The point where this construction line crosses the meridian (the vertical line), marks the radius of the Equator circle. Place the point of the compass at the center and draw a circle at this point. Repeat this circle on the other plates.

Repeat this process to project the circle of the Tropic of Cancer: This time using the intersections with the Equator circle (See B on the figure to the right).

After the construction lines are removed, all five plates should look like C in the figure to the right.

The next step will be more complicated: Drawing the almucantars.