CALCULATING THE OCCULTATION OF STARS BY THE MOON. 'j'^J 



A note may be added on the " limits of latitude " given in 

 the Almanac, and the possibiht}^ of using them as a rough-and-ready 

 guide to the quantity a on which almost everything in an occulta- 

 tion depends. Now I am not acquainted with the exact formula 

 by which these limits are calculated, but I find that their average 

 corresponds closely with the approximate formula : L = 60 Y + \ D, 

 in which L is the algebraic average or " central latitude." and 

 D the declination. Probably the factor 60 is the moon's distance 

 with the earth's radius as unity. The upper and lower limits 

 are obtained from L by adding or subtracting a range which 

 depends on the moon's distance from the equator, being the same 

 whether this distance is + or - . This range is given approxi- 

 mately by R2 = 432- 1-450% 



Now in the formula for a (page 784) it will be seen that nothing 

 can be done with the term y' t, so I have attempted to express 

 (a — y' t) in terms of the latitudes [i.e., in terms of L) and the 

 declination only, but find that this does not give reasonably correct 

 results unless the hour-angle is of medium size. An approximate 

 formula which is generally right within 0-05 on a is a = y' 



-f •4+;^ + — (H+4). in which H is the Greenwich hour-angle 

 60 300 ^ 



rounded off to the nearest half-hour. Much the same results are 



given by a = y' v-l-ynu (4«+/ + D (H -f 4) ), in which u and / are 



the upper and lower limits from the Almanac with / always negative, 



and li positive unless south. These formula- will probably be 



of some use for quick selection of probable occultations. 



I wish to express my best thanks to Mr. H. E. Wood, M.Sc, 



for providing me with numerical and calculative data about 



observed occultations, without which this paper could not have 



been written. 



TRANSACTIONS OF SOCIETIES. 



South African Society of Civil Engineers. — Wednesday, November 

 13th: A. D. Tudhope, M.I.C.E., Vice-President, in the chair.— Presidential 

 address: M. R. Robinson. The address dealt with some of the prob- 

 lems of railwaj' construction in South Africa, and, in particular, with the 

 subjects of standard of construction, branch lines, the departmental system 

 of construction, and main line improvements. 



Wednesday, March 12th : W. Crai.ar, M.I.C.E., President, in the cliair-— 

 Presidential address : Vv^ Craig. A discussion of some of the problems 

 to be dealt with in the future in South Africa by the engineer. Attention 

 was specially directed to railways, harbours, irrigation, roads, bridges, 

 water supply, and sewerage, land reclamation, and street construction 

 and maintenance. 



Wednesday, April gth : W. Craig, M.I.C.E., President, in the cliair.— 

 "Some road and bridge work in the East African Campaign" : Capt. H. M. 

 Ladell. An account of the operations of the ist Road Corps in East 

 Africa. The corps comprised nine officers, 61 European non-commissioned 

 officers and men, and 1.240 natives. The subject of labour and rationing 

 was briefly dealt with, details of organisation and equipment were given, 

 and a detailed description communicated, together with plans and sketches 

 of between 500 and 600 miles of road constructed and maintained. 



Wednesday, May 14th : W. Craig, M.I.C.E., President, in the cliair. — 

 " Kalk Bav Harbour ll'orks": C. le S. Furlong. Details of the con- 



