International Occultation Timing Association
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Large North Shift Expected for Sunday night’s (879) Ricarda Occ’n, n. Europe-n.e.&s.cen. USA, & n. Mexico

The path of this occultation, predictions for which are given at http://www.asteroidoccultation.com/2018_09/0924_879_58830.htm , is now expected to pass 3.0 path-widths north of the nominal prediction (which was generated on August 12), based on observations of another occultation by Ricarda that took place a month earlier in Maryland. So rather than passing over Waco, Texas, the narrow (19 km wide) path is now expected to pass over the southern and eastern suburbs of Dallas, at 3:06 UT Sept. 24 (that’s 10:06pm CDT Sunday, Sept. 23; it takes about 20 min. for the shadow to pass from Belarus to southern Baja; see the prediction link for details). We believe that the new prediction is more accurate than +/-0.9 path-widths, maybe even half that; observations are sought to verify the new prediction, and to better characterize (879) Ricarda. The new path, expanded to include the +/-0.9 path-width uncertainty, is shown between the two dark gray lines in the three maps below, showing the path across the south-central USA, the northeastern USA, and Europe. You can duplicate these maps, and zoom in on them in more detail, or select other paths of the path (which also crosses southern Baja, northern Mexico, and the Canadian Maritime Provinces), by using Derek Breit’s interactive Google map for the occultation at http://www.poyntsource.com/New/Google/20180924_58830.HTM and specifying +40 and +74 in the two boxes above the map. Although we are rather confident in this calculation, those at fixed observatories in the path (two are declared in Occult Watcher) are still encouraged to observe, and from other nearby locations, especially between the old and new paths.

Background

The occultation on August 24th was recorded by David and Joan Dunham. They set up three stations, one at their home in Greenbelt, MD; one farther south in Westphalia, MD; and the other a little north of the predicted central line at Cheltenham. An occultation with close to the predicted central duration was recorded 10s early at Westphalia; the other two stations, and two other observers farther from the path, all recorded misses, rather tightly constraining where the shadow went. The sky plane plot below, generated by Brad Timerson and David Dunham, is below.

The positions of the occulted stars can now be predicted to less than 1 milli-arc-second (0.001”), thanks to Gaia Data Release 2 last April. So the observed error on Aug. 24th is due entirely to errors in the ephemeris of Ricarda. Experience with some past events shows that the shift of the center of the asteroid from its predicted position in the sky plane (the predicted position is marked with the number “1” in the plot) remains about the same during a given apparition of the asteroid. The corrections measured from the plot are -19 km in RA and +56 km in Dec. The motion was strongly from south to north as Ricarda had recently passed its stationary point in RA, starting its retrograde loop. On Sept. 24, well into the retrograde loop, the motion, in PA 249 deg., is almost perpendicular to that on Aug. 24. Consequently, the large time shift seen on Aug. 24 causes a large north shift of the path on Sept. 24, while on Sept. 24, there should be less than a second error in the time predicted by Steve Preston, and Occult Watcher.

David Dunham
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