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At any time, you can save your present view as a mark file, and recall it at any future time. You do this via the "Save a Mark" and "Go to Mark" options in the File menu. When you ran Setup, it copied a few sample mark files to your hard drive. Click on "Go to Mark" and take a look at them. The sample marks point to "interesting" astronomical events and times. "Gervase occ. in 1170" shows Mars crossing in front of Jupiter. This sort of planet-crossing-a-planet event occurs about once a century; the next time one will occur is in 2065. The most recent was in 1818. The one shown here was recorded by a monk known as Gervase of Canterbury in 1170, and also by the Chinese Imperial Observatory. Another mark file, "Earth transit from Mars", shows the Earth silhouetted against the Sun as seen from Mars. This sort of event is called a transit; from time to time, we on Earth see Venus and Mercury cross the Sun's disk. Such events do not occur frequently; for example, transits of Venus as seen from Earth occurred in 1874 and 1882, and the next transits will occur in 2004, 2012, and 2117. The particular event shown here was calculated by Jean Meeus (see the acknowledgments) and used in a short story, "Transit of Earth", by Arthur C. Clarke. Had you been on Mars with a filtered telescope, you would have seen a small black disk crossing the sun, with a disk a quarter that size (the Moon) trailing behind. It does illustrate how big the Sun really is. The "Jul '91 Mexico eclipse" shows the total solar eclipse of July 11, 1991, as seen from Mexico City. The "May '94 annular eclipse" shows the less dramatic annular (doughnut-shaped) solar eclipse that was visible from Project Pluto's corporate headquarters. In this case, the moon wasn't large enough to block out the sun, and a ring of sun was visible around the moon. The "Initial position" mark file is used when the program starts up, and ensures that you resume where you were when you last ran Guide, with whatever settings you had at the time. If you happen to get your settings thoroughly confused, you can select this mark file to restore things to the way they were when you started the program. If this does not restore things properly, you can use the "Factory default settings" mark, which puts everything back the way it was when Guide was installed. You can save a mark by hitting "Save a Mark". You will be asked to type a mark name, which must be less than 20 letters. You can also delete an existing mark file with the "Delete Mark" option. This option will bring up a list of mark files; select one, and you'll be asked to confirm that you wish to delete that mark file. 20: USER-ADDED (.TDF) DATASETS As delivered, Guide displays an extremely wide variety of datasets
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