Central Meridian Transit Timings

Of all the visual observing programs, the central meridian transit timing is the most valuable observation the amateur can make. The data produced by central meridian (CM) transit timings is extremely important to the professional community because, above all other amateur endeavors, it contributes to the understanding of Jovian wind currents, jet streams, and weather. Here, the amateur truly makes a meaningful contribution.

When properly performed, CM transit timings result in data that provides the longitudinal position of features in Jupiter's cloud tops. By recording the positions of features over time, the speed of wind currents and jet streams at various latitudes can be determined. All that is needed is a steady telescope, an accurate timepiece, patience, and dedication. The transit timing is easily accomplished but requires an understanding of some basic principles.

The Central Meridian is an imaginary line that runs from Jupiter's north pole to its south pole, evenly dividing the planet (Fig. 2.1). Like the Earth, Jupiter is divided into 360° of east-west longitude. By referring to an ephemeris, it is possible to determine, for any given minute, the longitude that happens to be on Jupiter's central meridian at a given time. For any feature observed on the CM, if we determine the longitude of the CM, we also determine the longitude of the feature itself at that date and time. Simple enough!

Some advanced amateurs may use a filar micrometer that is available to them to mark the central meridian. However this expensive piece of equipment is really not necessary. For years I have made this measurement with nothing more than my eyes. I simply make a visual estimation of the CM noting the crossing of features over my imaginary line. With practice, it is possible to become quite good at this, and it is always satisfying to see your timings in agreement with other observers around the world!

Of course, noting the crossing of a feature across the CM is also a bit subjective. An observer might be uncertain and delay the marking of the transit for several minutes, in error. What then may be done to minimize this problem? Phillip Budine, past A.L.P.O. Jupiter Section Coordinator, recommends what I refer to as

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