An Invitation to Participate in a Rare Event  

INTRODUCTION

During the evening of February 14, 1998 the bright, easily located star Menkalinan will be occulted by the minor planet 1116 Catriona.  A prediction by International Occultation Timing Association member Edwin Goffin shows that the event will be visible along a line stretching from Portugal to the Cape.

Using only their naked eyes or binoculars and a simple timing procedure, individuals who have never made astronomical observations in their lives, can provide data which will improve our knowledge of the orbit, size and shape of Catriona. This is all the more remarkable, considering the fact that Catriona will have the angular diameter of an orange as seen from a distance
of some 400 kilometres.

MENKALINAN, CATRIONA AND ITS SHADOW.

Menkalinan (Beta Aurigae) is a magnitude 1.9 eclipsing binary. The eclipses are shallow and will not affect our observations. At the time of the occultation, it will be 11 degrees above the northern horizon for observers at the Cape, 19 deg at Gauteng and 27 deg at Harare.  It is 7 deg east of the zero magnitude, extremely prominent star, Capella.

Catriona is about 45 kilometres across.  Its visual magnitude at the time of the occultation will be 13.7 and it will be quite invisible next to the very bright Menkalinan.  We do not need to be able to see Catriona in order to do the observation. Catriona's shadow as cast on the surface of the earth will extend roughly 50 km in an east west direction and rather further in a north south direction. The shadow will traverse Africa in eleven minutes.

THE NEED FOR A SHOTGUN APPROACH

Because Catriona's orbit and Menkalinan's position are not known with great precision, we cannot predict the exact path of the shadow.  That is why we hope to mobilise large numbers of observers scattered across Southern Africa.  Many will be outside the path of Catriona's shadow but if there are enough observers, a number will be almost sure to succeed.

Readers of this article are urged therefore, to try to involve their families, friends and affinity groups such as schools and clubs. For the best results, observing stations should be spread out as widely as possible in a roughly east west direction. ASSA members living away from the main centres are specially requested to mobilise teams of observers. Such organisers should contact me if at all possible.

ACTUALLY DOING IT

All that an observer needs to do is to locate Menkalinan near the northern horizon and watch it carefully with the naked eye or (preferably) binoculars from SAST 2020 to 2038 (8.20 pm to 8.38 pm). If he or she sees the star disappear suddenly, he or she should shout GONE and when it reappears some six seconds or less later, shout BACK into a tape recorder.Each station should have a voice tape recorder or camcorder running and a radio within earshot, tuned to the new radio station called SAFM.  This station used to be known as "the English programme" and will be found between 104 and 107 megahertz on the FM dial.  More than one observer can share the radio and tape recorder.  Observers who cannot receive SAFM should contact me for advice.

IF AN OCCULTATION IS SEEN

Observers lucky enough to witness the event should send their tapes to me together with a description of their observing sites so that I can reduce the data. I use a high speed strip recorder which can make a graphic record of the comments against the SAFM background so that the exact times can be deduced.  The times and observers' positions can then be used to generate a profile of Catriona.

IF AN OCCULTATION IS NOT SEEN

Negative observations will also be most valuable.  With details of your observing site, a definite observation of a “miss” could be most useful

FOR HI TECH ENTHUSIASTS

Some observers may want to experiment with  camcorder telescope combinations.  Video records of the occultation plus the SAFM signal will be most valuable.  Such occultation observations have been made in the past by South African amateurs Tony Hilton and Frans de Jager.  Space does not permit a description of the method but readers who are interested should contact me for more information.

SOME PRACTICAL TIPS

Binoculars seem to get heavier and to develop wills of their own after a few minutes.  Make sure  that you are sitting comfortably and that the binoculars are firmly supported during the 18 minute period.  The star will travel slowly from right to left, remaining at nearly the same altitude.

Try to be vigilant but relaxed during the watch period.  I find that tension is brought under control if I munch something and even exchange small talk with a companion.  Discussing the mortgage or the children’s school fees will be counterproductive. Telescopes at low power can be used, if you are confident that the dog or a toddler will not upset the tripod at the critical moment. (Yes, this has happened.  One observer even fell into his swimming pool during a planetary occultation observation).

If you have young eyes and a reasonably clear, dark view to the north, your naked eye will be quite OK.  A station could have members working in all three visual modes.  But although it is more fun when observers are clustered together, it makes scientific sense to have as many stations as there are tape recorders, widely dispersed.

The importance of practice sessions  cannot be overstated. Rehearsals  will give you confidence that you are looking at the right star and will also help you to set the radio's volume to a suitable level, so that the programme content is clearly recorded without drowning  out the commentary.  I used the word "shout" advisedly:  Many observations have been spoilt because the person analysing the tape was unable to make sense of the observer's unintelligible muttering.

Radio tape recorder combinations known as "ghetto blasters" usually cannot record the radio programme simultaneously with a microphone input and so two separate instruments will probably be needed for each station.  A car radio is useful for providing the radio signal. Readers of this article should warn participating non astronomers that, although an occultation WILL occur, there is no guarantee that THEY will witness it.  This should temper their disappointment with observational astronomy. They should be told that even negative results will have some scientific value. ALL observations should be reported, whether or not an occultation occurs at your site.

CONCLUSION

I have been involved with planetary occultations intermittently for 45 years and have never before encountered such a golden opportunity for the broad membership of the Society and the general public to make a real contribution to our knowledge of the Solar System.

I hope that readers will make the most of this rare event.
Please contact me if you require more details and do contact me afterwards, whether you have been successful or not.

Danie Overbeek
Director,  Occultation Sections.
Box 212
Edenvale 1610
E-mail: danieo@global.co.za
Tele:  (011) 453 6918