Minor Planet Occultation - 14th October 1999

At the October meeting on the 13th October, Danie Overbeek showed that there was a good possibilty of the occultation of an 8th magnitude star by the minor planet 48 Doris being visible from the Gauteng area. Danie had received last-minute predictions that put the shadow track of the 190 km wide asteroid right over the Johannesburg area. He was so confident of the path that he decided to go down to the Vaal river at Villiers to get a better spread of observers.

His enthusiasm worked, sort of, because about 10 or 11 observers attempted the observation. Not all of them, however, obtained successful results.

For one observer the star moved behind his neighbours tree and so could not be observed from his fixed telescope. Another observer bumped his telescope just at the wrong time and so missed seeing the event. Then, because the event occurred about 5 minutes later than the original prediction and not everyone knew about this, one observer stopped looking about 2 minutes before the event and so missed it. Three people were looking at the wrong star or were a bit late and did not find the star. In the end five observers saw the event, two from the same location. Still, a very good result, which, but for the above sad tales, might have been the all-time best-observed occultation from the Southern hemisphere!!

It was a very good learning exercise for all. Some of those who missed will get good observations next time. We have realised that the procedure that we have been using to get a time signal is not totally reliable and are probably going to switch to a different method in future. Don’t think that the Telkom time service and/or the SABC 6 "pips" are accurate!! They may not be. The CSIR have a PC-based time service, but this is not very helpful out in the country. They have advised that they are looking at a voice linked time service, which would solve all our problems.

The attached diagram shows the cords obtained by the four stations, as well as a possible outline of the shadow as it passed over our part of the Earth. This profile can be juggled around and altered in a number of ways and the suggested figure shown may differ from the real shadow somewhat. But it does show how you can work out the shape and size of a piece of rock that was 495,000,000 kms away at the time, with just a small telescope in your backyard. One observer saw a "blink" that certainly suggests that Doris has a small moon. And that is why we do these observations. Its all in the pursuit of serious science. But fun too.

Brian Fraser