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data are shown in Guide. Choose among hours/degrees, decimal minutes, or decimal seconds for RA/dec; decide if leading zeroes should be shown in positions; and if declinations should begin with "+/-" or "N/S". Also, you select the epoch in which positions are to be shown. In this "Format" dialog, one also controls how latitude/longitude values are shown, and switches between metric and "traditional" units (inches, miles, etc.) There's also an option to reset the time/date format, so that you can have dates displayed year-first, or with two-digit years, or in Julian Day format, and so on. The choices you make here will be used throughout Guide. The "TLE=" option lets you select a new file of orbital elements for artificial satellites. Artificial satellite elements are almost always provided in files called "Two-Line Elements", or "TLEs"; these files usually have a .TLE extension. You will need to get updated files quite often. Satellite motion is not entirely predictable; satellites maneuver (using thruster rockets), new satellites are launched, and the way in which atmospheric drag works can't be predicted very far in advance. The model used for satellite motion was provided by the US Government, and is mediocre; the really good motion models are classified. In fact, the .TLEs provided with Guide will already be somewhat out of date by the time you get this DVD. You might be able to get, say, an approximate time at which ISS will pass overhead. But getting current data is a good idea. When you click on the "TLE=" option, you'll see some information about sites where you can get satellite elements, with links provided. In most cases, you can simply click on an option to download data for all satellites, or for a specific group of satellites. The Projections submenu offers a long list of chart projections: stereographic (the default), orthographic, gnomonic, Mercator, and more. Each serves a different purpose. Stereographic projection preserves the shapes of objects quite well, no matter how large an area is being shown. At Level 1 (180 degree field of view), the other projections will show very distorted constellations near the edge of the chart. Stereographic projection results in exaggerated sizes of objects near the edge, but their shapes are okay. Therefore stereographic projection is used for "all sky" charts in the center of many astronomy magazines. Orthographic projection is best suited for terrestrial maps, not celestial charts. In terrestrial maps, one gets an "Earth-from-space" sort of view. You may want to use it when displaying eclipse/occultation paths on the Earth. Gnomonic projection involves horrible distortion at large fields of view. However, it shows great-circle routes as straight lines.
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