Drift Charts

-o j? o After recording a series of central meridian transit timings, it will be useful to plot kiec these positions on a chart so that the drift rates of features can be determined. A

a R drift chart also helps us see how features behave in relation to each other and it

Z O helps us predict their future behavior and position. Thus, the "drift chart" becomes a valuable part of the scientific record for an apparition. A drift chart is also referred to as a "drift line graph."

To complete a drift chart by hand, the longitudes of features are entered on the chart against the dates of observation. We normally find the chart to be most easily interpreted when the date is represented by the vertical axis and longitude is represented on the horizontal axis. Almost any graph paper purchased at an office supply store will suffice. The graph paper should have enough graduations so that it is relatively easy to plot longitude along the top margin of the paper to the nearest degree, even when interpolating between lines. Running vertically, down the left margin of the paper, a distance of about two inches per month normally provides enough room to accurately represent the days of the month. With a little practice, setting up the graph becomes an easy task. Since the apparition will run for several months, continuation pages will be necessary. You can also use software programs to make measurements of images. Jupos is a piece of software with amazing capabilities that is easily located by searching the Internet.

A typically plotted drift chart is illustrated (Fig. 9.8). Here we see that features drift in decreasing longitude with time, giving the plot the appearance of a slanted line. Sometimes features can drift at inconsistent rates or may pause altogether for a short period of time, or they may speed up. It is therefore desirable to have enough observations so we can have confidence in the chart being produced. A chart with too few observations may be suspect, especially if the results seem to indicate a varying drift rate. With too few observations you cannot be certain if the feature is really speeding up or slowing down, or if there is simply error in the CM timings themselves. A large number of observations will tend to cancel out random errors in timings. As illustrated, I like to plot the observations in the form

Fig. 9.8. A drift chart created by the author to reduce transit timing data. This one depicts a conjunction of the GRS and South Temperate Ovals BE and FA. Note how the drift rates of the ovals changed as they approached and then passed the GRS. (Credit: John W. McAnally).

of a scatter diagram, then using a least squares solution, construct a line through the plots to interpret the drift rate of the feature. I feel it is important to plot all of the timings since this adds to the credibility of the graph by allowing the reader to see the raw data that was collected and how it was interpreted. Presenting the data in this visual way is simple and straightforward, allowing anyone reading the graph to judge the deviation in the observations and to access the adequacy of the number of plots recorded. In the past, some observers have been criticized for not allowing all the plots to be seen, simply showing the interpreted drift line by itself, inadvertently precluding any scrutiny of the work. I believe it is more desirable and credible to plot all the observations, as shown. The illustration (Fig. 9.8) shows oval BA, a bright oval, overtaking and passing the GRS, a dark feature.

Since Jupiter is divided into System I and System II rotation rates, it is helpful to have several graphs to present like features together. For example, one chart might present the plots for features in the latitude from the middle of the South Equatorial Belt to the South -South Temperate Belt (SSTB) (System II), or for the region of the southern edge of the North Equatorial Belt to the northern edge of the South Equatorial Belt (System I), and so on. I normally subdivide the plot of features even more than this, to avoid overcrowding on the charts. One of the most fascinating behaviors to observe and measure is that of the GRS, south temperate oval BA, and the smaller ovals of the SSTB. In recent years, the GRS has been relatively stationary and plots of oval BA will show oval BA overtaking and passing the GRS. The drift rate of oval BA is affected by the GRS when it passes and this is easily seen when plotted on the drift chart. Likewise, the behavior of the SSTB ovals is also fascinating. But, more importantly, the data being collected is very important. I have previously related the story of the South Temperate Dark Spot of 1998. Transit timings plotted on a drift chart alerted us to what turned out to be a new, significant feature.

I also like to prepare a table displaying the dates of the transit timings, the longitudes, and the names of the observers to be published along with the drift chart. When research is published it is accepted procedure to disclose how the observations were made, the instruments used, and the data that was collected in arriving at the findings. Again, I believe publishing the data in this way helps provide complete disclosure to the astronomy community.

Once the transits have been plotted on the drift chart, it is possible to calculate the rotation rates of the feature located at that latitude, thus allowing the speeds of currents and jet streams to be determined for the apparition. This data, gathered from apparition to apparition, allows patterns of behavior to be determined. With this information, scientists can create computer models to analyze atmospheric conditions on Jupiter.

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