Analemma, the sun path

Sunday, December 23, 2007


"The difference in time between what your watch reads and the position of the sun (clock time vs. sun time) is called the Equation-of-Time. If you are in the northern hemisphere and the sun’s position is to the east of where your watch indicates it would be, the Equation-of-Time is negative. If the sun is to the west, the Equation-of-Time is positive.

There is an easier way to see this effect. Find a place where the sun shines on the ground at noon all year long - winter, spring, summer, and fall. Place a rod about 3 feet tall into the ground, being very careful not to bump the end of it during the year. If you were constructing a sundial, this rod would be called a gnomon.

On the first day of each month, at the same time every day, (subtracting one hour if you are observing daylight saving time) place a mark with another shorter rod (you will need 12 of these) where the sun makes a shadow with the tip of the longer original rod. At the end of 12 months, you will see that the short rods make a figure-8 pattern on the ground.

Why does the sun take this strange path? There are two reasons and they are completely independent from each other.

1. The Earth is tilted on its axis 23.5° in relation to the plane of its orbit around the sun.

2. The Earth does not orbit the sun in a circle, but in an ellipse.

It is simply the sum of these two effects that causes the analemma.

It is hoped that with the aid of a few diagrams and animations, the analemma can be readily understood. "


Anthony Ayiomamitis of Greece, has the most beautiful images of the Analemma.

"Strange as it may seem, only seven times has someone ever managed to successfully image the solar analemma as a multi-exposure on a single piece of film. For those not familiar with the term, an analemma is the figure "8" loop that results when one observes the position of the sun at the same time during the day over the course of a year. Due to the earth's tilt about its axis (23.45°) and its elliptical orbit about the sun, the location of the sun is not constant from day to day when observed at the same time on each day over the course of a full year. Furthermore, this loop will be inclined at different angles depending on one's geographical latitude.

Since the analemma is considered one of the most difficult and demanding astronomical phenomenon to image, I immediately set out on such a marathon during the summer of 2001 by pursuing a complete set of analemmas from 08:00:00 to 17:00:00 UT+2 (hourly intervals) as well as the special case of the perfectly vertical analemma on the meridian (12:28:16 UT+2)."

Posted by Kaks at 6:42 AM  


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