A RAPID APPROXIMATE METHOD OF CALCULATINC; 

 THE OCCULTATION OF STARS BY THE MOON (FOR 

 THE CENTRAL TRANSVAAL). 



By James Moir, M.A., D.Sc. 



{With one text figure.) 



(Read July lo, igi8.) 



OccuLTATiONS are, I imagine, amongst the amateur astroncmer's 

 most favourite observations. The easily visible movement of 

 the moon towards the " doomed " star and its wink-like dis- 

 appearance are phenomena not only unique in the heavens, but 

 accessible to the weakest of instruments, even to lield-glasses. 

 and — in the case of very bright stars — to the naked eye. Seme 

 of you will recollect the remarkable daylight occultaticn of Verus 

 here two years ago, v hen it was easy to see where cur enemy 

 the Turk got his national S3 mbol — although the progress of Turkish 

 heraldic science has since led to the st^r being depicted in the 

 ridiculous position of being inside the horns of the mocn, just 

 as our British heraldic experts have converted the rhinoceros 

 into the unicorn. 



Now the only fault of occulta tions is that they are very 



difficult to predict. The Nautical Almanac gives a list of those 



visible at Greenwich, but this is quite useless for the Southern 



Hemisphere : in fact, I remember long ago that certain of those 



predicted for Greenwich did not ccme off in Aberdeen, which 



is only 500 miles away. Here in Johannesburg, the apparent 



path of the moon among the stars is displaced (as ccmpared with 



Greenwich) to the north by an amount which is more than two 



diameters of the moon, so that the Greenwich cccultation stars 



all lie a long way above the moon's path. The Almanac, however, 



also gives a hst of occulta ble stars for the w hole earth, in connection 



with which limits of latitude are laid down in each case, whereby 



an observer can at least see whether his latitude lies cutside these 



limits, in which case the occultation will noL be visible in his locality. 



It does not follow, however, that, even if his latitude lies within 



the limits, the occultation will be visible : such an occultation 



may be visible only on the other side of the Southern Hemisphere 



when the moon is not above our horizon, or the moon, even if 



visible, may be so displaced by parallax, being ccmparatively 



near the earth, that it misses the star if it is near its rising or setting. 



The official and accurate method of calculating occultaticns 



is a ven/ lengthy and cumbrous affair, quite unsuited for the use 



of amateurs. If the amateur has no local tables giving the value 



of the constants in the formulae for his locahty, the calculation 



takes the best part of an hour, and is also liable to error due to 



the trickiness of working with Icgaritfms of minus quantities. 



Now all that the amateur wants is (i) to know w hether such-and-such 



an occultation will take place and be conveniently visible from 



