( 316 ) 



Tn the meeting of our section of last March a provisional account 

 of these measures by de Sitter was communicated bv Messrs J. C. 

 Kapteyn and E. F. van de Sande Bakhuyzen ^). 



Our computations were then already too far ad\'anced to keep 

 them back altogether; but we hope that by the side of these mea- 

 sures they still may have their use, for this reason that conjunctions 

 and mutual occultations of the satellites may well be observed at 

 several observatories which are not equipped for taking photographs. 



From the preceding numbers we find for the position of the fixed 

 plane relative to the ecliptic (for 1908,0). 



Ascending Node 336°48'23" =: i2, 



Inclination 2 717 =1, 



Now, if Rqr, Tj^ and ^ represent the radius vector, the longitude 



and the latitude of Jupiter; R., L^ the radius vector and the longi- 



o o 



tude of the Earth, (those given in the N. Almanac after correction 

 for aberration), the condition that the fixed plane must pass through 

 the Earth is expressed by : 



R^. cos 8 si?i (Lr,r — i2) — i?«, sin ^ cot J ^= R+ sin {L — i2) , 



-T -r 'T o o 



which is satisfied July 8, 1908 at 19^38'n-3. For at that moment 

 log R^ = 0-728527 % R^ = 0-007179 



L,r = 141^23' 9"-0 L^ = 286°40' 3"-5 



r o 



[3 =-\- 52 26 -73 i^ =: 336 48 52 



so that our equation becomes 



1-423706 — 2-204190 = — 0780484 



Similarly we find for the instant at which the same plane passes 

 through the centre of the sun : 



25 April 1908 at 18"-5 M. T. Grw. 



On both sides of this latter epoch there exists the possibility of 

 an eclipse of one satellite by another, at the time of the heliocentric 

 conjunctions. We hope to treat this subject in the second part of this 

 communication. 



1) This provisional account may be considered as a sel^uel to the thesis of 

 Mr. DE Srn'Eu. This thesis, mainlained by him al Groningen on 17 May 1901, bears the 

 title: Discussion of Heliometer-observalions of Jupüefs satellites made hysir David 

 Gill K.C.B. and W. H. Finlay 3Ï.A. Further particulars will be given in the 

 Annals of the Roycd Observatory at the Cape of Good Hope. 



