Hi Tom, That's a neat idea. Maybe someday someone will invent a reverse engineering computer program for sundials where you simply enter the dial's hour angles and the declination and it calulates the best latitude match. I seem to remember an article Fred Sawyer wrote about a dial he reverse engineered to determine its design location.
John John L. Carmichael Jr. 925 E. Foothills Dr. Tucson Arizona, USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Sundial Sculptures Website: http://www.sundialsculptures.com Stained Glass Sundials Website: http://advanceassociates.com/Sundials/Stained_Glass ----- Original Message ----- From: Tom Egan To: [email protected] Cc: John Carmichael ; Geoffrey Lane Sent: Tuesday, February 10, 2004 11:53 PM Subject: Re: Reverse Engineering Perhaps the work done for NASA to draw hour and date lines on images from the Mars rovers' sundials could be used to reverse engineer photos of earthbound dials. Woody Sullivan posted some background info on the MarsDial a few weeks ago (see below). One of the URLs he recommended (http://redrovergoestomars.org/marsdial/images.html) mentioned "image analysis software": "Student Astronaut Abby and Student Astronaut Shih-Han used image analysis software to determine that the center of the sphere's shadow corresponds to a time on Mars at the Gusev Crater site of 11:01 AM on sol number 11 of the mission." Given the playful, but strict, nature of nature's laws, the reversing engineer will not be able to extract enough info from a 2-dimensional photo to completely solve the 3-dimensional problem posed by Geoffry Lane. It helps greatly if some of the variables are known, for example -- as John Carmichael notes -- the declination of the wall on which the dial is mounted. Perhaps the MarsDial image analysis software can be tortured into a form that will at least take the drudgery out of cut-and-try approximations by the reversing engineer? Tom Subject: MarsDial - Two Worlds One Sun Date: Fri, 16 Jan 2004 08:10:59 -0800 From: Woody Sullivan <[EMAIL PROTECTED]> Reply-To: [email protected] To: [email protected] Greetings: The first extraterrestrial sundial now sits on a planet about 150 million km away. When it was fabricated here at the U. of Washington in 1999, I held it in the sunlight falling on our planet, and now it's being bathed by light from the same Sun, but at a distance about 50% farther, and therefore an intensity about ~25% as much. (Note also that the angular size of the Sun is ~20' as seen from Mars, which makes the penumbrae of all shadows smaller, i.e., sharper-edged.) We're still working out some of the bugs in the software to accurately project a grid of hour lines and date lines on to the image of the MarsDial as taken by the main camera on the mission (Pancam). For instance, if you look at the posted images, the 12-hr line does not appear to pass through the center of the base of the gnomon, which it should for a level surface. The Rover was, however, tilted by a few degrees while sitting on the lander (and of course just yesterday it moved on to the Martian soil - hurrah!), in which case the 12-hr line does *not* pass through the center of the base. Our software does take care of any arbitrary tilt, but I'm still not 100% confident we've got the superposition right. Note also that there are really two main cameras (for stereo imaging), and for some reason one camera is giving different results than the other! Stay tuned. Cheers, Woody Sullivan Here are some URL's of interest: http://redrovergoestomars.org/marsdial/ - home page for MarsDial project, with basic explanations (this text is now being improved and expanded) http://redrovergoestomars.org/marsdial/images.html - actual images from Mars of the MarsDial with hourlines and datelines http://www.planetary.org/mars/tpr_marsdial.html - detailed article about the MarsDial and its development; originally published in "The Planetary Report" (serious gnomonicists should start with this article) http://planetary.org/news/2004/mars-dial.html - press release on MarsDial (8 Jan. 2004) http://pubs.giss.nasa.gov/abstracts/1997/Allison.html = 1997 technical article (PDF) by M. Allison; for those who want **all** the details and equations for time on Mars ****************************************************************** Prof. Woodruff T. Sullivan, III Center for Astrobiology & Early Evolution Dept. of Astronomy Box 351580 Univ. of Washington tel. 206-543-7773 Seattle, WA 98195 USA fax 206-685-0403 - Dave Bell wrote: One significant problem we have in reverse engineering many of these dials is that we cannot be sure the original photo was taken straight-on, normal to the dial face. Many of the photos submitted to John have been shot from the ground or other accessible vantage point. In orthogonlizing the images, errors are introduced, because the gnomon and dial face are not in the same plane. I don't think these can easily be corrected. -
