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Next, you'll get a menu allowing you to adjust and save all the elements labeled above. Because orbital data tends to confuse the uninitiated (and, at times, the initiated), there is a paragraph or two provided on the screen to clue you in to what the variable in question means. After entering all the elements, you can save the results, and Guide will add that comet or asteroid to its list. Keep in mind that the new object obeys the same rules as any comet: if you have comets turned off in the Data Shown dialog, or if the comet is not brighter than the limiting magnitude for comets, it will not show up on the chart. Be aware that a particular set of elements is good for a limited time only (usually a few months around the epoch). Over time, gravitational effects of other planets will change the elements and, therefore, the object position. After a few months, if the object doesn't come too close to a planet, errors of a few arcseconds will accumulate; over a few years, the errors will grow to a few arcminutes. APPENDIX G: ASTRONOMICAL MAGNITUDES The creator of the first star catalog, Hipparchos, was also the creator of the system used to measure the brightness of stars. He assigned a magnitude to each star: the brightest were magnitude 1 stars, those slightly fainter magnitude 2, and so on, down to magnitude 6. This was based strictly on looking at the star and guessing how bright it was. The invention of the telescope revealed objects fainter than mag 6, so the scale was extended to higher numbers. It was eventually decided that five magnitudes should represent a change in brightness of a hundred-fold; that is, a mag 2 star should be 100 times brighter than a mag 7 star. This also means that each magnitude represents a 2.512 fold change in brightness. This is a somewhat unwieldy number, but we're stuck with it now. This system means that one can measure fractions of a magnitude (Polaris, for example, is a mag 2.02 star) and that really bright objects can have negative magnitudes (Sirius is mag -1.58, Venus can be as bright as mag -4.4, the Sun is magnitude -26). It is still true, however, that most humans cannot see an object fainter than mag 6, though this limit varies among humans. APPENDIX H: COPYRIGHT AND LIABILITY NOTICES This program and data are copyrighted (except when otherwise specified) @1993-2011 by Project Pluto. They are distributed under the GNU General Public License. See the file 'license.txt' in the root folder of the DVD for details. Note that not all source code is available yet, though it is hoped that this will gradually be fixed. Please let us know if particular parts would be useful to you.
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