Grazing Occultations for Fun and Profit

By: Brian Fraser

If you want to do a little serious astronomy and get some enjoyment out of it at the same time, then there can hardly be anything more worthwhile doing than observing a grazing occultation of a star by the moon.

On September 2nd last year a group of us went out to the Benoni/Boksburg area to observe an occultation of a magnitude 5.9 star by the moon at about a quarter to midnight.

Tony Hilton and Melvyn Hannibal went to Bill Lockhart’s house, which happened to be conveniently located in the path of the graze track. Bill has a lovely Meade LX200 telescope, computer controlled and housed in a very nice roll-off roof observatory which he built last year.

Danie Overbeek assisted by his son Andy, and Tim Cooper found a nice safe site in some school grounds not too far away.

Brian and Val Fraser met Frans de Jager at a workmate’s house in Boksburg and positioned themselves at suitable points in the area. Frans brought some friends all the way from Vanderbijlpark to observe the graze.

Although the star was quite bright and the weather conditions almost perfect, it turned out to be quite a difficult graze to see as the star skimmed over some sunlit peaks near the moon’s terminator. At times it was impossible to tell the difference between the star and the sunlit peaks!

Fortunately, 5 observers obtained good timings and these are plotted on the accompanying chart. The dotted line is the moon’s predicted position and profile. ( Over the page…apologies…the Editors. )

From the observers timings it can be seen that the predictions are in error by about 1/3 of a second. That is why we do these grazes. The results are sent to IOTA (International Occultation Timing Association) and to Tokyo, and are used to refine the position of the star, the moons profile and the lunar orbit. It is all jolly serious science. And fun too.