( 305 ) 



geocentric conjunclion. Eastern elongation, denoted by e . e, has an 

 amplitude of S', western elongation, iv.e, one of 9^ . 



Not to interrupt the text unnecessarily, all particulars have found a 

 place at. the end of the paper. 



FIRST PART. OCCULT ATIONS. 



In the numbers 3846 and 3857 of the Astronomische N^achrichten 

 we find two communications relative to observations of the occul- 

 tation of one satellite of Jupiter by another. Tlie first (1) is b}^ 

 Mr. Ph. Faüth at Landstuhl, dated 8 December 1902, with post- 

 scripts of 29 December 1902 and 14 January 1903. The other (2) 

 by. Mr. A. A. Nijland at Utrecht, dated 27 February 1903. 



Fauth notes in addition that Houzeau, in his' Vademecum, p. 666 

 mentions a couple of similar observations (3), and further that Stanley 

 Williams, on the 271'^ March 1885 at 12'' 20'", saw the third satellite 

 pass the first in such a way that the two satellites combined had a 

 pear-shaped appearance. (4) 



The satellites of Jupitei' move in orbits but little inclined to the 

 plane of Jupiter's equator. Laplace assumed a fixed plane for each 

 satellite; the plane of the satellite's orbit has a constant inclination 

 on this fixed plane, whereas the line of intersection, the line of 

 the nodes, has a slow retrograde motion. The inclinations of the fixed 

 planes on the plane of Jupiter's equator amount only to a few 

 minutes; their intersection with the plane of Jupiter's orbit is identical 

 with the line of the nodes of the equator. The value generally 

 adopted for the inclination of the latter plane on the orbit of Jupiter 

 is 3^4', whereas the longitude of the ascending node, which therefore 

 is also that of the fixed planes, is at present about 315^°. 



In order to be able to assign the time at which, as seen from the 

 Earth, an occultation of one satellite by another is possible, it is necessary 

 to know the longitude of the ascending node and the inclination of 

 the mean fixed plane on the orbit of the Earth. At the time that 

 the mean fixed plane, prolonged, passes through the Earth, occultations 

 of one satellite by another may be observed. As Jupiter completes 

 a revolution around the sun in nearly 12 years, these times will 

 succeed each other after periods of six years. Jupiter will pass 

 alternately through the ascending and the descending node of the plane 

 which passes through the centre of the sun parallel to the mean 

 fixed plane. 



It follows that, as occultations of one satellite by another haA-e 

 been observed in 1902, we must expect that these phenomena will 

 be again visible in 1908 (5). 



