Newsletter of the Desert Foothills Astronomy Club
Issue #10: February 3rd, 2007
About Quid Novi
State of DFAC
Quote of the Month
Contact the Editor: Dan Heim, phone: 623.465.7307 or email:
|Sep 27||7:00 pm - 9:00 pm||DFAC Lecture Meeting #1||Boulder Creek High School, 40404 North Gavilan Peak Parkway, Anthem, AZ 85086|
|Oct 25||7:00 pm - 9:00 pm||DFAC Lecture Meeting #2||Boulder Creek High School, 40404 North Gavilan Peak Parkway, Anthem, AZ 85086|
|Oct 30||5:30 pm - 8:00 pm||Ladies Guild Astronomy Night||6609 E. El Sendero Drive, Cave Creek, AZ 85331 (contact Dan Heim for gate code)|
|Nov 28||5:30 pm - 7:30 pm||Student Astronomy Night||Foothills Academy College Prep, 7191 E. Ashler Hills Drive, Scottsdale, AZ 85262|
|Nov 29||7:00 pm - 9:00 pm||DFAC Lecture Meeting #3||Boulder Creek High School, 40404 North Gavilan Peak Parkway, Anthem, AZ 85086|
|Jan 18||5:30 pm - 8:00 pm||Student Astronomy Night||New River Elementary School, 48827 N. Black Canyon HWY (Exit 232 east to frontage north)|
|Jan 31||7:00 pm - 9:00 pm||DFAC Lecture Meeting #4||Boulder Creek High School, 40404 North Gavilan Peak Parkway, Anthem, AZ 85086|
|Feb 28||7:00 pm - 9:00 pm||DFAC Lecture Meeting #5||Boulder Creek High School, 40404 North Gavilan Peak Parkway, Anthem, AZ 85086|
|Mar 10||6:00 pm - ?||DFAC Observing Session||Heimhenge|
|Mar 21||6:30 pm - 8:30 pm||Student Astronomy Night||Eastside Explorers HomeSchool Group, address TBA|
|Apr 25||7:00 pm - 9:00 pm||DFAC Lecture Meeting #6||Boulder Creek High School, 40404 North Gavilan Peak Parkway, Anthem, AZ 85086|
|May 30||7:00 pm - 9:00 pm||DFAC Business Meeting||Boulder Creek High School, 40404 North Gavilan Peak Parkway, Anthem, AZ 85086|
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|State of DFAC: By Dan Heim, President|
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|Last Meeting: Wednesday, January 31st, 2007|
|Dan Heim spoke to our group on the
topic of "Amazing Astronomy Multimedia Online,"
including audio, video, and still image content. We heard
Ray Bradbury speaking to us from the surface of Mars,
took a 3D fly-through of the Orion Nebula, and puzzled
over a simulation of the Mars analemma. This puzzle, by
the way, has since been resolved (see Space Debris
below). Dan also shared his catch from the recent Mercury
transit, including stills and a time-lapse sequence of
3rd-4th contact. Some of these images are now posted on
the Astrophotos page of this website. Check them out for
a higher contrast view than we had at BCHS.
Speaking of bad contrast, the doors opened late, due to a communications glitch, so we were didn't have the time to disable those unswitched lights that pollute the projection screen. Trust that we are working to resolve this problem. As a last resort, we can manually disconnect the fluorescent bulbs. It's just a matter of climbing up on a chair on top of a table ... but there's gotta be an easier way.
It was a good turnout despite lousy weather. We had 9 members in attendance, all of whom stayed for the entire program. We thank these hardy souls for venturing out that rainy night. Feedback indicates the drive was worth it, as all enjoyed the show and participated in some lively discussion.
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|Next Meeting: Wednesday, February 28th, 2007|
|Our February speaker will be Dr. John
Fountain, recently retired of the UofA Lunar and
Planetary Laboratory. He will enluighten us on the topic
of Archaeoastronomy (not Astroarchaeology). Here's a
short excerpt from his bio:
John Fountain began his study of the moon and planets while an undergraduate at the University of Arizona, helping to analyze the first spacecraft pictures of the moon returned by the Ranger Project. He co-authored the Consolidated Lunar Atlas which was used by the Apollo astronauts. Earthbased imaging of the planets in support of NASA missions occupied much of his career. He is the codiscoverer of two satellites of Saturn, and he deduced the thickness and distribution of material in the Saturn rings from earthbased observations. He supervised production of the images from the first missions to Jupiter and Saturn by the Pioneer 10 and I I Spacecraft. He was also a participant in the Mariner and Voyager missions.
Some 15 years ago he began a serious investigation of astronomical references in the ancient rock art of North American Indians. His work shows that pre-Columbian Native Americans placed rock art in such a way that beams of sunlight interact with the rock images on important seasonal days such as equinoxes and solstices. Such markers often have remarkably high precision. In addition to his own studies of numerous rock art sites, he has established a database of rock art solar markers which demonstrates they were surprisingly common and widespread. He has also done research on geoglyphs and Oriental archaeoastronomy. He has led archaeoastronomy tours for Crow Canyon Archaeological Center and was featured in the KUAT-TV production of The Desert Speaks - Shadows of the Ancients. He is co-editor of the book Current Studies in Archaeoastronomy: Conversations Across Time and Space, and has published 35 scientific papers.
This should be a truly fascinating lecture. We hope to see you all there.
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|Quote of the Month:|
|[On the topic of alien intelligences
as portrayed in science fiction] "So far as I
know, every such story has alien intelligences which
treat humans as approximate equals, either as friends or
foes. It is assumed that A-I will either be friends,
anxious to communicate and trade, or enemies who will
fight and kill, or possibly enslave, the human race.
There is another and more humiliating possibility
alien intelligences so superior to us and so indifferent
to us as to be almost unaware of us. They do not even
covet the surface of the planet where we live
they live in the stratosphere. We do not know whether
they evolved here or elsewhere will never
know. Our mightiest engineering formations they regard as
coral formations, i.e., seldom noticed and considered of
no importance. We aren't even nuisances to them. And they
are no threat to us, except that their engineering might
occasionally disturb our habitat, as the grading done for
a highway disturbs gopher holes. Some few of them might
study us casually or might not."
Robert A. Heinlein, "Grumbles from the Grave"
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|Are you ready for an astronomy lesson?
Good, because we'll have a test on this at our next
meeting. :) We belong to DFAC in part to learn, so here
At our January 31 meeting, we discussed the unusual shape of a simulated Mars analemma. I expressed my puzzlement regarding where the equinoxes fit in. It was suggested that perhaps we were only seeing half the total analemma, even though the image was labeled "Martian analemma" and not "half the Martian analemma." I pleased to report I have solved the mystery, and in the process learned something valuable about the Earth analemma as well.
First, the image in question, downloaded from the Astronomy Picture of the Day website. See: http://antwrp.gsfc.nasa.gov/apod/ap061230.html.
What puzzled us (or perhaps I should say "What puzzled me") was where the equinoxes were on this curve. On the familiar Earth analemma the equinoxes are at the intersection of the "figure-8" curve. Or so I had thought ... turns out they are close to that intersection point, but not exactly on it. Here's what I learned by doing a little research. For any planet, the analemma is a superposition of two sine waves, much like the Lissajous figures you can get on a dual trace oscilloscope. [See: http://www.ngsir.netfirms.com/englishhtm/Lissajous.htm for a java demo of this effect.] For an analemma, one sine wave is due to the planet's equatorial inclination; the other is due to the orbital eccentricity. Either alone would cause the Sun to oscillate sinusoidally along a single axis (vertical for inclination, and horizontal for eccentricity). What complicates things is, these two sine waves have different lengths, amplitudes, and phase. The result is an irregular Lissajous figure. See: http://en.wikipedia.org/wiki/Equation_of_time.
The equinox occurs when the planet's polar axis lies in a plane tangent to its orbit, causing the Sun to appear directly overhead at the equator. On Earth, that's March 20 and Sep 23 (±1 day). On the analemma, those points would by definition be located exactly halfway between the top (dec +23.5º)and bottom (dec -23.5º) of the curve. In other words, the Sun would have a declination of exactly zero. No problem there. That's fairly easy to visualize.
The real epiphany occurred when I looked at a labeled higher resolution image of the Earth analemma (image below). It became immediately obvious that my assumption was wrong. When you only look at pretty astrophotos of the analemma, or the crude one often stamped on Earth globes, and don't really think about it critically, you can end up with bad science. And that's exactly what I had done. The equinoxes are most definitely not at the intersection point. The fact that they're even close to the intersection point is a geometric coincidence. They must be located where the declination is zero, and since the analemma is asymmetric, the intersection point can't possibly be the equinoxes. So for all these years, I had incorrectly assumed the equinoxes had to be at the intersection point. I'm embarrassed to say I even taught that "fact" for 22 years. Mea culpa.
The east-west extent of the analemma is a function of the orbital eccentricity. As the planet approaches perihelion (increasing in speed) the Sun gains on clock time. Approaching aphelion, the opposite occurs. Note how the four points at which Sun time agrees with clock time are asymmetrically located. Surprisingly (at first), the points at which clock time agrees with Sun time do not occur at perihelion and aphelion. The fact that perihelion and winter solstice don't occur on the same date skews the analemma clockwise slightly. Note on the Earth analemma how the zero point is reached on June 15 (17 days before the July 3 aphelion) and Dec 25 (8 days before the Jan 2 perihelion). The intersection point (Apr 13 & Aug 30) is also not a zero point. This skewing can be explained as the result of the phase difference in these two sinusoidal cycles.
It's also important to note that the precise shape of the analemma depends critically on both latitude and the time of day the Sun is imaged. The original Mars analemma image was simulated as visible from Sagan Memorial Station, home of the Sojourner Rover, for an unspecified time in the "late afternoon." Below is a graphic comparison of Earth and Mars analemmas, compared side by side, both for 45º latitude at noon.
I'll close this with one last interesting point. Not surprisingly, the analemma looks different on every planet. Here's a tabulation of the shape geometry for our Solar System:
straight east-west line
So now you know all about the analemma. Probably more than you ever wanted to. If you still have a thirst for more, and want to understand the math behind all this, download the 46-page PDF titled "The Analemma for Latitudinally Challenged People" by Helmer Aslaksan and Shin Yeow Teo by clicking here (1.7Mb).
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