The Analemma

The analemma shows a relationship between the sun's declination and the difference
between sun time and mean time. This relationship is of importance in designing a
sundial that allows reading of mean sun time or clock time.

A computer program may be used to calculate the analemma. Below are URLs to a C language
program to calculate the analemma using two different methods and to the output of the
program using the more exact method of computation. The data was calculated at approximately
2 1/2 hour intervals throughout the year.

Following that is a URL to a C language program that calculates the analemma for each
hour and projects the result onto a user defined plane surface.

Following that are URLs to gif files showing the output of the above program for various
configurations and finally a photo of the analemma and some information on spherical trigonometry.

Analemma Calculation
Analemma Data
Analemma (projected onto a sphere as viewed from the outside)
Analemma (with arrows showing direction of yearly sun motion)

Analemma Projection

Notation is .del.dec where del is the offset of the users meridian from the noon meridian
and dec is the users declination.

horizontal.0.0
horizontal.5.0
horizontal.0.15
horizontal.0.35
horizontal.0.-35
horizontal.0.66
horizontal.0.80
horizontal.0.35
equatorial.0.35 ie. plane facing the equator
vertical.south.facing.0.35
vertical.20.deg.east.of.south.0.35

Photo of Analemma (as seen from inside the sphere and taken over a year period)
Courtesy of the photographer, Dennis di Cicco and "Sky & Telescope" magazine www.skypub.com