It's really satisfying, after coming up with a question, to make a stab in the dark with your Web browser and find the answer immediately. Sky & Telescope subscriber Dan Kalikow of Natick, Massachusetts, had this satisfaction when his daughter announced that it was about time she learned about the figure-eight shape on his recently purchased globe. She had long noticed the same figure eight made of Suns in the sky in a photograph on his wall.
So to help his daughter, he performed a simple Web search that turned up the obvious URL of "www.analemma.com." In a few seconds he was looking at the well-conceived site of Robert Urschel of Valparaiso, Indiana, who created a multimedia tutorial about what an analemma is and why it looks the way it does. Through the use of QuickTime animations, Urschel illustrates the effects of Earth's noncircular orbit and tilted axis on where the Sun appears in the sky if you look at the same clock time every day.
The site had all the information Kalikow and his daughter needed. "What a masterful job!" he writes. Not only does Urschel provide a graphical overview of what causes the analemma, but he includes all the supporting mathematics for those wanting a truly thorough explanation.
The most fascinating aspect of the site is the interactive analemma generator, which allows you to input any axial tilt and orbital eccentricity and see how Earth's analemma changes. Throw caution to the winds and see just how bizarre the analemma would be if the Earth had a life-challenging orbital eccentricity of 0.5 and the poles were inclined 85[degree sign].
Urschel has an open e-mailbox for suggestions. I have a couple that would make the site a more complete resource. A question often asked of the Sky & Telescope editors this time of year is why the year's earliest sunrise and latest sunset come many days before and after, respectively, the longest day at the June solstice. (We get a similar swell of questions around the December solstice.) The least time- consuming way to explain this disparity is to have a picture of the analemma handy, as described in the December 1998 issue of this magazine (page 124). The answer has to do with how the analemma is tipped with respect to the horizon as seen from your latitude.
My other suggestion is to expand the analemma examples to include those of the other planets. As explored in the March 1982 issue (page 237), these extraterrestrial analemmas aren't as weird as the ones you can create with Urschel's simulator, but they will surely be of interest to the Mars colonist in us all.
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|Author:||Goldman, Stuart J.|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 1999|
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