An Occultation by (690) Wratislavia

An Occultation by (690) Wratislavia

By Andrew Lowe

(This article was published in the February 1983 issue of Starseeker, the newsletter of the Calgary Centre of the Royal Astronomical Society of Canada).

During the early morning of November 14, 1982, the asteroid (690) Wratislavia was predicted to occult a faint star in the constellation of Taurus for observers somewhere in North America. The nominal prediction, issued late in 1981, showed the path running east-to-west across the central United States.

My interest in this particular occultation came as a result of some updates to the asteroid's and star's positions in August, showing that the predicted path would be considerably further north. The revised prediction, which also appeared in the November Sky and Telescope, showed the path roughly straddling the Canada-U.S. border. I began to prepare to observe from Montana.

The plan changed quickly, however, when David Dunham of the International Occultation Timing Association phoned me late on November 10. He described how plates taken at Lick Observatory two nights earlier showed an even greater north shift placing the path about 100 miles north of Edmonton. It seemed like an ominous prediction; an occultation by (52) Europa two months earlier in the same area had been clouded out.

Nevertheless, I decided to try for it. Until recently, I had lived just east of Edmonton on an acreage. My observatory containing a 12.5" reflector was located there. I originally had planned to travel to the centre line and observe with my 4" Schmidt-Cassegrain, but the star to be occulted was just too faint to be seen with the small scope. I decided to observe with the 12.5", even though I was a full path diameter away from the centre line. I informed Dr. Doug Hube, at the University of Alberta, of the event and he promised to observe from the Devon Astrophysical Observatory, which is southwest of Edmonton.

Well, for once the skies were clear and it wasn't too cold considering the time of year. I checked the star field with the 12.5" and the star was easy to find. As well, the asteroid itself could be seen clearly as an extra "star". With 4.5 hours to go, it had quite a way still to travel. I checked the shortwave radio and the tape recorder and everything worked O.K.

Occasionally during the next few hours, I watched Wratislavia gradually approach the star. The greatest danger in missing an occultation is to observe the wrong star, but I was confident I had the right one as the two images gradually merged about 10 minutes before the predicted occultation time. Unfortunately, I had no power for the drive and so I had to keep the magnification quite low.

As WWV announced 9:36 UTC, I started the tape recorder and began monitoring the star continuously. The predicted occultation mid-time of 9:41.0 0.1 approached. As WWV started to announce the 41st minute, I was starting to get very nervous. No only that, but I had the radio so loud that it was quite distracting. I blinked, and when my eyes opened, the star had gone. I shouted at the tape recorder. The faint asteroid was still visible. I was ready for the star's reappearance and it soon popped back into view. I shouted into the recorder once more, and the two important times were on the tape. I observed for another five minutes, then packed up the scope and replayed the tape. My reaction times for the disappearance and reappearance were estimated and subtracted from the times on the tape. The results:

disappearance 9h40m54.6s  0.3s UTC
reappearance  9h41m07.8s  0.2s 
duration      13.2  0.3 seconds

The next morning, I called Dr. Hube. He told me that the drive on the 20" died on him shortly before the event. The star's disappearance could not be timed accurately, but the reappearance was at 9h41m11.5s 0.5s UTC.

Unfortunately, we were only about 10 kilometers apart in the direction of the shadow's motion, so that an accurate determination of the size or shape of the asteroid was not possible. My duration, however, does give a minimum diameter for Wratislavia of 156 4 km (the expected diameter from indirect measurements is 175 km).

In retrospect, we were exceptionally lucky in seeing the event. In fact, if we had travelled to the predicted centre line, we would have missed the show.

For every person who sees an asteroid occultation, there are many others who are not lucky enough to be in the shadow path. In Calgary, probably the closest observations outside the path were made. Steve Morris and Geoff Kennedy obtained a photoelectric record at the Priddis Observatory, while Neil Laffra and Don Hladiuk observed at the Calgary Centre's observatory at Strathcona-Tweesmuir. Bob Loblaw also obtained a negative observation in north Calgary.

For me, it was a great feeling to have finally observed an asteroid occultation, although it was frustrating not to have more observations from the Edmonton area. Perhaps we'll have better luck next year.

Predicted occultation path across Canada, courtesy of David Herald's winOCCULT software. My observing site at Ardrossan, Alberta is indicated with a + (partially hidden by the number 41).