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Sunday, July 3, 2005
Your Sky Orbital Element Upgrade
With last week's transition of Your Sky to stateless operation having gone smoothly (at least no problems reported or evident in the log so far), I went ahead and installed a long-planned upgrade to the parser for the orbital elements of asteroids and comets these programs permit you to track and display in sky maps. Almost everybody expresses these orbital elements in the same terms (although there are some differences, for example some specifying the semi-major axis distance for elliptical orbits while others prefer the perihelion distance, valid for parabolic and hyperbolic orbits as well, these quantities are easily mathematically transformed into one another), there are about as many ways of writing them on a page, or in a table, or as an item in a database, or other means of communication among people and computers as there are people who have ever found the need to do so. The IAU Minor Planet Center's Ephemeris Service, for example, provides orbital elements in a total of eighteen different formats, one of which I will confess as having brought into existence in a moment of youthful indiscretion. In addition to these more or less well-defined computer-oriented ways of expressing orbital elements, a variety of sloppier formats used in the days when astronomical discoveries were reported by telegraph and teleprinter remain in use. My goal when implementing Your Sky was that, to the extent possible, one should be able to cut and paste orbital elements from E-mail announcements, newsgroup postings, Web sites, etc., and have them work with little or no fiddling. This is why the Your Sky orbital element documentation lists five different formats for orbital elements and, in fact, supports additional variants of those listed and permits sloppiness in column alignment and other fine points of formatting not discussed in the document. These standards, if you can call them that, are, like everything else in the solar system, a moving target. In particular, the Minor Planet Center has revised its own syntax for cometary orbital elements, and the extremely useful (and freely available without a subscription) DASTCOM Small-Body Orbital Elements database from the Solar System Dynamics group at JPL has its own single-line elements format for asteroids and comets. This update of Your Sky now parses all of these additional orbital element formats. To celebrate the imminent collision of the Deep Impact spacecraft with Comet 9P/Tempel 1, the following URLs will display the current position of the comet in the sky with the Your Sky virtual telescope, with orbital elements specified in: I haven't yet added documentation and examples of the new formats to the Your Sky asteroid and comet tracking help page; I'll do that in a few days when I'm confident the new code is behaving well, at which time I'll integrate it into Solar System Live as well.Posted at
21:30
