RamBaut—Rotation- Period of “ Garnet” Spot on Jupiter. 398 
We must, therefore, in the first instance reduce the time of 
transit to what it would have been if seen from some particular 
direction. For this purpose it is convenient to select the longi- 
tude of the Sun at opposition, represented in fig. 2 by the line 
J,L,S, or the line JH’ which is parallel to it. We thus see that 
previous to opposition the spot will transit with regard to this 
line before the apparent transit takes place, and after opposi- 
tion the apparent will take place before the real central transit. 
- That is to say, that before opposition we must diminish the time 
of observation by the interval which the spot takes to move 
through the angle H’ JH in the figure, whereas after opposition 
the correction will be a positive one. 
To calculate its amount, we remark that the angle 
ESE = SJE - J Sd. 
But if Z and L, denote the longitude of Jupiter at J and Ji, 
and if / represent the longitude of the earth at H, and if R and D 
denote the distances of the Sun and Jupiter expressed in units of 
the mean distance of the Earth from the Sun (all of which may be 
obtained from the Nautical Almanac), we see that 
sin SJE = sin JSEH. = 
or 
; ae 
SJE = sin? (5 sin J} sz) =a, say. 
Also, 
JSJ=L,-L, and JSE=L-1, 
so that the correction which is to be added is the time of describing 
the angle 
SPE = sin 5 sin (¢— Di) ' (Zn z L) 
Knowing approximately the time of rotation (P), we have, finally, 
the correction for parallax 
iP 
z [sin Ip sin( 2 2) +(Z.-2)| x gee (A) 
(2). The correction for the velocity of light.—In consequence of 
the finite velocity of light, any phenomenon taking place on 
Jupiter will not be seen by us, even at opposition, for thirty-five 
2F 2 
