"the three minute rule." Simply put, the human eye may perceive an object to be on the CM for up to 3 min. In making your transit timing, if you note the minute at which the feature first appears to be on the CM, and then note the time that the feature is obviously no longer on the CM, then an average of these two times will usually prove to be the best estimate of the transit. I have used this "three minute rule" extensively and it has always produced reliable results. The goal is to note the transit to the nearest minute. Therefore, a timepiece or some other means of noting time to the nearest 30 s, such as the time signals on a short-wave radio, is required. When using my watch, I always set it first to the time signals on the short-wave.
Of course, the timing just made must be recorded, and the forms used for making sketches can also provide a place for this. The A.L.P.O. forms have a place to record this data. Once again, Universal Time should be used to record the time and date of the transit. The feature's longitude, either in System I or System II as appropriate, should also be recorded. As noted previously in this book, visually Jupiter has two systems of rotation, System I and System II.
A clear, straightforward description of the feature observed should also be recorded. Phillip Budine developed a system of nomenclature that is remarkable in its accuracy and simplicity. This system of nomenclature was discussed in Chap. 2 of this book. This method of notation, when properly used, will provide an abbreviated, yet accurate description of the feature, including its location among the belts and zones of the planet (Figs. 2.2 and 2.3). For example, the center of a dark condensation observed to transit the CM on the northern edge of the North Equatorial Belt would be described as: Dc, sm cond., n edge NEB. Similarly, the center of a bright oval observed in the center of the South Temperate Belt would be described as: Wc, oval, center STB. And therefore, the descriptions used would include D for dark, W for white, p for preceding end, c for center, and f for following end. Along with these notations would be recorded the System I or II longitude of the feature and the Universal Time of the transit. The System I or II longitude can be determined and entered onto the observing form at a later time, when the ephemeris is consulted. An ephemeris can be obtained from any number of sources today, including a web site maintained by the Jet Propulsion Laboratory. Also, there are computer programs available that can yield the CM longitude when the date and time in Universal Time is entered.
I have spent many a wonderful night behind a telescope watching Jupiter rotate, recording transit timings for hour upon hour. I find it very satisfying to collect data that I know is valuable to the study of Jupiter. And there is the sense of the hunt, waiting for the next feature to appear from around the following limb of the planet. There is always the anticipation of what may appear next!
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