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comets such as Halley in 2061 and P/Swift-Tuttle's next return in 2126, because these comets were observed carefully when last visible and their future orbits computed. However, new comets will be discovered after you receive Guide (usually, a few are found each month), and you may want to display them in Guide. If you have access to the Internet, the easiest way to get up-to-date orbital elements for comets is to download them from the Minor Planet Center (MPC) Web site. You can use the Extras... Asteroid Options dialog, then click on "Add MPC Comets/Asteroids", and then use "Click to download updated comet data and add it to Guide". This method guarantees good, current data, and removes the need to understand orbital elements. The amount of data downloaded only about 35 KBytes, so it's quite fast, even over dialup. You can replace Guide's built-in set of asteroids using the MPCORB dataset. This is described on page 60. The only problem is that the download is quite large (over 20 MBytes), but this is hard to avoid; there are simply a lot of asteroids out there. If you lack Internet access, or if you want to add elements not available through the MPC (say, from an article or elements for a theoretical object), then you will have to enter the elements by hand, using the "Edit Comet Data" function. Here's how to do this. First, some background on how an orbit is defined: Usually, five or six figures, called orbital elements, are needed. When you "click for more info" on a comet or asteroid in Guide, these elements are among the information listed. Orbital elements can be expressed in a variety of ways. Usually, a comet's elements consist of a time of perihelion (the time it comes closest to the Sun), represented by a capital letter T; the distance from the comet to the Sun at the time of perihelion, or "q"; the orbit's eccentricity (a measure of how "stretched out" the orbit is; a value of 1 or greater means the comet won't return), or "e"; the longitude of the ascending node, represented by an uppercase Omega (looks like a horseshoe); the inclination, or "i"; and the argument of perihelion, represented by a lowercase omega (looks like a curly w). These last three are angles that define how the orbit is oriented in space. Asteroid elements usually replace the time of perihelion with an epoch time and a "mean anomaly", an angle defining the object's position along the orbit at the epoch time. Also, the semimajor axis ("a") is sometimes used in place of the perihelion distance. The "Edit Comet Data" function in the Asteroid Options dialog box will provide a list of recent comets, plus "(new comet)" and "(new asteroid)" entries. You can select an existing object to alter it (useful for element updates), or either class of new object.
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