Night SkyAnalemma Society  

Build an Analemma Curve


1. Draw a perpendicular to a line proportional to the gnomon.
2. Determine length of equinoctial shadow.
3. Mark off this length on gnomon. (For "Observatory Park" latitude the equinoctial shadow for a 20' gnomon is 15' 6".)
4. From the point of intersection of gnomon and line, make a mark to the right on the line and on the gnomon the length of the equinoctial shadow.
5. Connect, with straight line, top of gnomon to mark on line; this is equinoctial shadow of gnomon.
6. Construct a circle from top of gnomon using length of gnomon as radius; this is the meridian.
7. Construct a straight line through circle parallel to line using length of gnomon as distance; this is known as the horizon.
8. Calculate I/15 of circumference.
9. Using this length as radius, make two marks on circle on intersection of equinoctial shadow line and circle.
10. Draw lines from top of gnomon through two points of intersection of the two circles. Line on right is winter solstice sun-ray; line on left is summer solstice sun-ray.
11. From these two points of intersection, draw two lines to opposite side of circle parallel to the equinoctial shadow line; these are called winter and summer diameters.
12. Divide these two lines in half.
13. Construct two semicircles using these points on the outside of the lines. Semicircle on right represents summer hours; semicircle on left represents winter hours.
14. Extend summer and winter solstice shadow lines to opposite side of circle.
15. Construct straight line through circle connecting centers of the two semicircles; this is the axis.
16. From the intersections of summer and winter diameters, construct line parallel to axis to semicircles.
17. Construct parallel line connecting the points of intersection of the two circles.
18. Using intersection of this line with the equinoctial line as center, describe a circle that passes through intersections of the meridian circle with the summer and winter solstice lines.


Analemma - 19th Century