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Analemma

Page history last edited by Johanne Morisseau 13 years, 10 months ago

 Johanne and Barry

 

 

This figure-8 path that the sun makes in the sky is called the analemma. On some days you might notice that the sun is not in the sky where, according to the time on your watch, you would expect it to be.

 

The difference in time between what your watch reads and the position of the sun  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 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.

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.

 

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