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matter much to them. Most people, though, will instead stick with the built-in asteroid data in Guide. And when dealing with dates a year or so into the past or future, the fact that the built-in data accounts for perturbations will usually mean that it is more accurate than MPCORB anyway. The MPC (Minor Planet Center) has very kindly provided orbital elements for comets, suitable for use in Guide, on its Web site. If you click on "Add MPC Comets/Asteroids", Guide will offer you an option to download comet data. Click on this, and Guide will download the MPC file (it's about 35 KBytes, and should be fast even over dialup) and merge in the newly-found objects, and update some comets already known to it. You'll also see options to get asteroid data in this manner, but it's strongly recommended that you use only the 'comet' option. For asteroids, there is a better solution: use of the MPCORB database. This totally replaces Guide's asteroid data, instead of just attempting to update it. In the "Edit Comet data" option you may add new comets and asteroids by entering their orbital elements (detailed instructions on page 73). This is useful if you want to enter an "imaginary" object, but if you're trying to find a real object, it's highly recommended that you use the MPCORB and 'Add MPC Comets' options described above. By default, Guide shows a "trail length" for asteroids of zero days, that is to say, no trail at all. If you set this to, for example, three days, then each asteroid will have a little "tail" behind it showing its motion over the preceding three days. Fast-moving objects will have long tails; slow-moving ones will have short tails. This can cause objects with unusual orbits (close to us, highly eccentric, or steeply inclined to our own orbit) to stand out from the crowd of "normal", main-belt asteroids. The "Line of Variation" function is a very specialized option, added for use by people trying to recover comets and asteroids with poorly-determined orbits. In most such cases, the object won't be recovered exactly at the predicted position; instead, it will be found on a line passing through the predicted position, called the line of variation (LOV). To show this, click on the Line of Variation checkbox and set the length of the line in days; a starting value of one day is usually a good idea. (This corresponds to a guess that the object may be one day "ahead of prediction" or "behind prediction".) Guide will display a one-day LOV for all asteroids and comets on the screen, indicating the set of points where they would most probably be found. Under most circumstances, you'll probably want Guide to show all the asteroids it happens to know about. However, it's possible to
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