Analemma

I was talking with a friend a few days ago, and he asked me about this "infinity" or "figure-eight" symbol that is found on globes. "What does it mean?" he asked me. I honestly didn't know, and hadn't really ever paid any attention to this particular symbol on globes, but since he asked the question, I began to notice these things, and started to wonder what the answer was to his question. So, I did some research, and this is what I found:

This figure-eight on a globe is called an analemma. It is simply this: if you could record the position of the sun in the sky at the same time every day, let’s say sometime around noon and subtracting one hour if you are observing daylight saving time, you would notice that the sun takes a rather strange path. You might notice that at certain times throughout the year the sun's position not only varies higher and lower (North and South) as you would expect with the change of the seasons, but also slightly east and west. 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 (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.