Drawing Astrolabes: Dividing the Ecliptic

15 Jan 2020 - Ray

Stereographic projections of circles are circles. The ecliptic is a circle that runs between the extremes of the tropics of Cancer and Capricorn. The projection of the ecliptic has a radius that is the average of the tropic radii. Its center is half their difference.

The procedure for dividing the ecliptic is (Figure 6-7 with the following steps numbered):

1. Locate the ecliptic pole on the meridian at $R_{eq} \tan(\epsilon / 2)$ from the center.
2. Divide the equator into equal segments of longitude: 12 divisions of 30 for the entry into each zodiac sign: more divisions depending on the resolution desired.
3. Draw a line from each equator division to the ecliptic pole. The corresponding longitude point on the ecliptic is where this line intersects the ecliptic circle.
4. A tic mark on the ecliptic is drawn toward the center of the instrument.