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clusters, the Zwicky clusters, or both. All three are possible, and a check mark is placed in the menu beside the current selection. The Asteroids submenu also shows some extra options controlling the labelling of asteroids: asteroids labelled by number; asteroids labelled by number if available, provisional designation otherwise; asteroids labelled by name, provisional designation otherwise. By default, asteroids are labelled with their number. (Unnumbered asteroids are therefore also unlabelled asteroids, and are simply shown as cross-marks.) For the Planets there is a panel with two switches: "Full precision" and "Label by Name". The first toggles between normal precision (the default) and full precision for positions. Planet positions are calculated using either the VSOP ("Variations Seculaires des Orbites Planetaires") theory or the PS1996 theory, both compiled at the Bureau des Longitudes in Paris. In each theory planetary positions are calculated as the sum of a long series of trigonometric terms. When full precision is used, all terms in the optimal theory are used, and planetary positions are precise to roughly .01 arcsecond. Computing them can be slow on older computers. (If you have a more modern computer, you will probably not even notice the slowdown.) Note that for certain time spans and objects, a still better method may be used: JPL ephemerides. These are the current "standard" for planetary ephemerides; VSOP and PS1996 are based on the JPL ephemerides. (The method used for a given planet or satellite will be shown in the "more info" section for that object.) "Normal" precision provides a precision of about an arcsecond over the period 0 AD to 4000 AD. It is based on VSOP, but omits many of the smaller terms. It is the truncated version used in Jean Meeus' _Astronomical Algorithms_ (Willmann-Bell, 1991). For normal use arcsecond precision is more than sufficient, but there are cases where the additional level of accuracy in "full precision" is truly necessary. It also allows Guide to claim higher accuracy than other software. Those wanting specific details of Guide's accuracy should look here: 7c: Planet display When Guide shows a planet or natural satellite, and the field of view is so large that the object would appear as only a few dozen pixels, that object is shown as a small cross-mark labelled with a symbol. You can change its color from the Data Shown dialog, and you can click the "Label planets by name" option in that dialog to cause Guide to use names in place of symbols. The real capabilities in how Guide shows planets only become apparent when you zoom in far enough for the object to show a disk. For many objects, a "bitmapped surface" is provided. Zoom in on, for example,
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