CALCULATJNC. THE OCCULTATION OF STARS BY THE MOON. 781 



his locality, and (2) when (within five minutes) to expect the 

 disappearance and re-appear?nce of the star. 



If one could be certain that any particular occultation wiii 

 come off, there is fortunately an easy way of roughlj? predicting 

 the times of immersion and emersion, viz., the use of DGv\nes' 

 Tables. These give the displacement of conjunction* due to 

 parallax when the moon's rate of motion east {x' in the Almanac) 

 and its local hour-angle H (H in the Almanac plus the longitude 

 of the locality) are given. The displacement of conjunction is 

 generally called by the Greek letter t, and may be roughly calculated 

 from the formula : 



(minutes) 



54 



sir. 



(f) 



where h is in minutes and the bracketted quantity is to be treated 

 as degrees. This is an operation which can be done in ten seconds 

 on the slide-rule. 



For the benefit of those who wish a more exact value I append 

 the following table : 



Table for t for the Southern Transvaal. 



Use x' from Almanac and (H + longitude) = h in foregoing 

 formula. 



*Conj unction is when the star, the moon's centre and the celestial 

 pole are in Une. The Greenwich conjunction is given in the Almanac. 



