OBSERVATIONS FOR TIME AND LONGITUDE. 
203 
making the immersion or emersion of one of his satellites, and if Jupiter’s 
altitude be taken at the instant of observing the immersion or emersion, 
the use of a watch will be unnecessary, as the " Nautical Almanac ’ will 
furnish the Greenwich date required; this, of course, can only be done 
when there are two observers. As a rule, the first satellite is to be 
preferred, as its motion is more rapid than that of the other three. The 
explanations given in the " Nautical Almanac ’ are so clear that they leave 
nothing to be added. 
The Observation. —Having estimated the local time of the phenomenon 
with the assumed longitude, and the time given in the "Nautical 
Almanac,’be r^ady Some time before the eclipse will take place, with a 
telescope having a magnifying power of not less than 40, and note the 
instant of the disappearance or re-appearance of the satellite. It must be 
remembered that either of these events (being caused by the shadow of 
the planet) may take place when the satellite is at a considerable distance 
from Jupiter. The difference between mean time at place when the 
observation was taken, and the mean time at Greenwich given in the 
"Nautical Almanac/ is the longitude as shown in the following 
example 
January 6th, 1899, observed the immersion of the 1st satellite of 
Jupiter at 7h. 20m. 30secs., watch 22m. SOsecs. slow of local mean time. 
H. M. S. 
Time by Watch .. .. 7 20 jo 
Error of Watch . + 22 30 
7 4 ? co 
M. T. at Greenwich (* Nautical Almanac ’) 4 7 2 9 
_o / // 
Longitude in Time . 3 35 31 = 5\ 52 45 E. 
