WHAT IS AN ANALEMMA? An analemma is a natural pattern traced out annually in the sky by the Sun. Shaped like a bowling pin or a figure of “8”, it can be detected when the Sun’s position in the sky is frequently recorded over a period of one year. The recordings must be made from the same location each day, using the same moment of standardized time.The distinctive pathway results from the combined effects of Earth moving around the Sun in an elliptical orbit, and its axis of rotation being tilted nearly 23.5° to the plane of its orbit. When the patterns for each effect are summarized together they make a predictable model known as the equation of time (at this link, go to summation effect), also referred to as the “correction to the sundial”. [Also see Wikipedia: Equation of Time and
Analemma.]
The analemma provides an interesting way to follow Earth’s progress in its orbit around the Sun. It also provides a graphic illustration for why each of the four seasons begins at a specific moment in time. For example, the official beginning of autumn in 2007 was September 23 at 06:51 ADT, 09:51 UT. The defining moment was marked just as the apparent motion of the Sun crossed the celestial equator at declination 0° and right ascension 12 hours. Declination (Dec) and right ascension (RA) are the co-ordinates of celestial latitude and longitude. (the diagram on the left is linked to a larger, similar copy).
CREATING AN ANALEMMA An analemma can be created by any method that records the Sun’s position the same moment each sunny day, from the same location. A few patient, faithful people have done this with photography. Here are 3 examples: (Ex 1), (Ex 2), (Ex 3). The analemma at the upper left of this page and on the page it is linked to, was created using a computer program called Starry Night. It was set to plot the Sun’s position every 7 days for a full year (each green dot). The star background is for the date shown at the top of the diagram. View Today’s Sun
The ANALEMMA
Sun’s position on the analemma for November 23, 2007, at Mean Solar Noon (12:16:54 AST) at Horton Bluff, N.S.. The analemma is projected against the sky background straddling the meridian of Horton Bluff (45.1° N, 64.2° W) at the above date and time. Click the diagram for a larger version with more details about the Sun and the analemma.
The white dot marks the Sun’s position on the celestial sphere and analemma for the date and time given. The thin red line is the Celestial Equator (the star background that is overhead at the Earth’s equator); the thin green line is the ecliptic (Sun’s apparent path on the celestial sphere). The straight line extending vertically from the south point (S) on the horizon is the Meridian (imaginary line dividing east sky from west, where Sun is highest above south point at mid-day). Note: the diagram is linked to a larger copy having further details.
( This link to a neat demo of Celestial Sphere, works best if you have highspeed internet and quicktime).
DIAGRAM SYMBOLS AND TERMS
At one time the analemma pattern was commonly seen on classroom globes. It included dates indicating the Sun‘s position (overhead, in the zenith, at noon) for that date, relative to the equator. [analemma2]
Celestial
Equator
Horton Bluff horizon
