By Dave Scherping & Erik Hubl
It was Saturday, December 9, 1995. Being only a few of days past full moon, there were no serious plans for observing. That was until Erik reminded me about a premier occultation scheduled for early that evening. 11th magnitude asteroid 85 IO was predicted to occult 8th magnitude star SAO111235 in Taurus at approximately 6:42 p.m. CST.
The path from which the occultation was visible was published in the February 1995 and December 1995 issues of Sky & Telescope. This path was originally predicted to be approximately 200 km wide, extending through southern Kansas. As the time of the event drew near however, the path was redrawn 250 km farther north and posted on IOTA’s web page (International Occultation Timing Association). This meant that the occultation should become visible about 80 miles south of Lincoln. Saturday afternoon, we decided to venture south to observe this event.
With a Kansas map, we found a potential observing site 10 miles south of Marysville, Kansas, or 1.5 hours from Lincoln. If we hurried, we would have just enough time to get there. We used MegaStar to print out several detailed star maps to assist in locating the correct star and one chart showing magnitudes of surrounding stars to allow us to estimate magnitude drop. We planned to observe through my 20″ Dobsonian and Erik’s 6″ Newtonian. After loading up our equipment, we stopped by Hyde Observatory and picked up a WWV radio that could receive timing signals. We also brought along a portable tape recorder. Soon we were on the road.
It was an extremely clear afternoon as the sun set and stars began to appear. We closed in on our observing location, but time was running short. We found the site ok but there were trees all around and it was right next to Hwy 77, so we headed east and found an access road leading onto farmland. There were no houses around so we decided this was it. The clock told us it was our only hope. It was after 6:20 p.m.
We began setting up in record time. There was snow on the ground and it was mighty cold (lower single digits). By about 6:35 we were set up and collimated. Now we had to find the star. This really put our star-hopping skills to the test, but within 3 minutes we found it and confirmed it with the charts. The asteroid was not visible; it must have been too close to the star. Then there was the inevitable problem of warm optics. The stars were much larger than usual and would oscillate. But it would have to do, so we turned on the radio and the tape recorder and began observing. 6: 42 p.m. came and went, as did 6:43, 6:44, & 6:45. Then just before 6:46 p.m., Erik thought he saw the star flicker, then we both saw the star began to dim and remained that way for nearly 20 seconds. We wondered if we actually saw the occultation or if it was an atmospheric phenomenon.
It was then that we realized we were cold. I hadn’t even put on my boots or my winter pants yet! Climbing into the car, we turned on the radio and they reported the temperature at a mere 3 degrees Fahrenheit. After warming up with coffee, we continued observing. By now the scopes had cooled and the stars were much more pinpoint in appearance. At 7:16, a dim “star” appeared to the lower left of SAO111235. It was the asteroid! We observed it for nearly an hour and it appeared to brighten as it moved away from the star. This left no doubt that we had been watching the correct star. We only wished we had been there early enough to see the asteroid before the occultation, but all-in-all, it was a great experience. It was incredible to comprehend an asteroid 150 km across occulting a star much larger and farther away, leaving a narrow path cutting across the midwest. It was coincidental that we drove nearly 150 km to see the event (136 to be exact)!
The following Monday, we submitted our results. Both of us were surprised to find that we had co-observed with approximately 30 other astronomers from Sweden, Newfoundland, Ontario, Missouri, Kansas, Arizona and California. The Telescopes used ranged from a 4″ to a 30″. One pair of 11 x 80 binoculars was also used. IOTA reported that “more chords were observed for this event than any asteroid occultation event since 1991”. After interpreting the results, it turns out the asteroids shadow path was really an additional 100 miles south and the event just missed our site. However our results help to define a northerly extent for a diameter measurement. Had we been in the asteroids path, we would have seen the 8th magnitude star completely blink out – for 18 seconds.
Martin Gaskell was quite interested in our observation and attempted to help us explain what we might have seen and why is was about 3 minutes late. Was it the unsteady optics, or perhaps is there a secondary star to SAO111235? Could it be that the asteroid IO 85 has a companion? What Erik and I do know is that this event was fun to do, and we intend to observe stellar occultation’s in the future. The next time we won’t be so rushed and we will let our optics adjust to the temperature. With email and World Wide Web sites, reports can be shared easily and results known very quickly. It was a chance to participate in some real science.
Timing occultation’s are new to both of us but we’re looking forward to many more. We encourage others to give it a try as well. If you would like more information, look up http://www.anomalies.com You’ll see our report as well as others. Information about IOTA can be obtained from Terri & Craig McManus, 2760 SW Jewell Ave., Topeka, KS 66611 (913) 232-3693. They also have an article in both the January and February 1996 Sky & Telescope Magazine.