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(Non-solar system objects are not affected. The parallax effect is ignored for them.) The Moon can be shifted by up to 2 degrees if the wrong lat/lon is used. Asteroids and comets passing very near to the Earth can be still more seriously affected. Planets are usually not affected by more than a few arcseconds. TIME. If the time is 13 days off from its expected value, make sure you are using the Gregorian calendar. Check the Time Zone setting. Remember that Guide uses a 24-hour clock; 8:13 PM becomes 20:13. (This is one reason it's a good idea to have the time and time zone displayed in the legend area, as they are by default.) Finally, it's worth considering the precision of the source used for the "expected" value. Perhaps the most common source of error is the assumption that some software has generated a "precise" value, and that any difference must be due to errors in Guide. It may be flatly stated that, properly used, Guide will indeed generate data to the precisions listed in the next section. APPENDIX E: NOTES ON ACCURACY The subject of the accuracy of any astronomy software is a very complex one. The accuracy of positions shown by Guide varies from about a thousandth of an arcsecond (for stars in the Hipparcos catalog) to being as poor as several degrees (for some asteroids whose orbits have not been thoroughly studied, or artificial satellites that have maneuvered or haven't been updated recently). For moving objects, the accuracy is also a function of time; that of positions of planets within a century of the year 2000 is of the order of milliarcseconds, but probably no better than arcminutes for very distant dates. There is a very thorough description of accuracy at These describe what level of accuracy can be expected for various data in Guide, how that level was determined, and what factors can affect that level of accuracy. If your sole need is to be able to find objects visually or telescopically, then the accuracy given by Guide will always be far more than sufficient. APPENDIX F: ADDING NEW COMETS Guide is shipped with a fairly complete list of comets and with a set of over 158,000 asteroids. New comets and asteroids are always being discovered, and in some cases, new observations allow the precision of existing orbits to be improved. Guide will be fairly up to date on comets when you receive it; as new ones are discovered, they are added to COMETS.DAT. Guide will also show some future
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