Bill Robertson’s report on the 4 September 1973 occultation of Ceres, courtesy David W Dunham

Here is a belated observing report about an observation that was made back in 1973! In an important observation Sydney Observatory astronomer Bill Robertson was one of five observers in Australia who observed an occultation or eclipse of the asteroid (now minor planet) Ceres on 4 September 1973. It was important as this was the first time that such an occultation of an asteroid by the Moon had been observed.

Government Astronomer Harley Wood using the astrograph at Sydney Observatory in 1968. He is looking through the top tube which was the guide telescope later used by Bill Robertson for the 1973 observation. Image by David Mist, courtesy Powerhouse Museum

Why are occultations valuable? They are valuable as they provide a way of measuring the size and shape of small distant objects that cannot be seen directly. Bill Robertson timed how long it took for the dark edge of the crescent Moon to completely hide the asteroid. Knowing the motions of Ceres and the Moon, this gave one chord across the unknown shape and size of the lump of space rock.

At the University of Texas David W Dunham used Bill’s observations together with that of the four other observers at different locations in Australia to estimate the diameter or width of Ceres. He found it to be 1200 +- 250 km. Within the errors this fits in with the current value of 950 km.

With thanks to a stellar triplet of Daves:
*Dave Gault who brought this observation to my attention
*Dave Herald (one of the five observers) and who asked for the old report to be searched for
*David W Dunham, who received the report at UNSO in 1973, wrote the paper on the observation, giving measurements of Ceres, stored the report all this time and actually found the report.