DEFINITION OF JOVIAN LONGITUDES 

The International Astronomical Union has accepted three longitudinal coordinate 
systems, System I, II, and III.  System I and II are useful as observing frames 
of reference.  Cloud features at +/- 10 deg latitude and the high speed jet at 
20-25 deg N drift slowly in System I coordinates, while mid-latitudes drift at 
rates similar to System II.  The third system, System III has been derived from 
the drift rate of radio features and is considered to be the rotation rate of 
the core and a reference for understanding atmospheric circulation and energy 
transport relative to the core.  These systems are defined as follows. 

Longitude = Initial Long + Drift Rate * Elapsed Time 

Sys        Period of rotation          rate   
                                    deg/hour     deg/day 
I      9h 50m 30.003s  9.84167h       36.579     877.900
II     9  55  40.632   9.92795        36.261     870.270
III    9  55  29.711   9.92492        36.272     870.536

The easiest way to deal with the initial longitudes is to 
use the tables in Section E of The Astronomical Almanac of the Naval 
Observatory.  Here longitudes are tabulated for 
0 hours UT and INCLUDE LIGHT TRAVEL TIMES.  The following 
table has been extracted from the 1996 edition.

Date          I         II         III
Jan 1       293.31    261.06     113.91
Jan 15      341.11    202.03      58.61
Feb 1       142.44    233.65      94.75
Feb 15      191.01    175.39      40.22
Mar 1        37.80    267.72     136.54
Mar 15       87.29    210.38      82.92
Apr 1       250.91    244.28     121.34
Apr 15      301.53    188.07      68.86
May 1       308.61     73.06     318.10
May 15        0.35     17.97     266.74
Jun 1       166.60     54.50     307.80
Jun 15      219.12      0.19     257.21
Jul 1       227.76     246.76    148.03
Jul 15      280.10     192.27     97.27
Aug 1        85.86     228.33    137.84
Aug 15      136.93     172.58     85.82
Sep 1       300.78     206.73    124.49
Sep 15      350.18     149.32     70.81
Oct 1       354.38      31.46    317.20
Oct 15       42.46     332.73    262.19
Nov 1       203.13       3.69    297.68
Nov 15      250.41     304.16    241.87
Dec 1       252.80     184.48    126.44
Dec 15      229.81     124.68     70.37
Jan 1        99.80     154.96    105.17 

These values are based on The IAU Report on Cartographic Coordinates, M.E. 
Davies et al., Celest. Mech., 29, 309-321, 1983.