396 
ficial axial symmetry and the oblateness are related to 
this fact. The periods can be measured by two inde- 
pendent methods: 
1. Extremely accurate average periods have been 
obtained from definite markings, these averages ex- 
tending over weeks or months. The probable error of 
some such determinations is only a fraction of a second. 
This corresponds to errors of speed of rotation of a few 
tenths of a meter per second. Related to this method is 
the use of the periodic light variation of the more dis- 
tant planets. 
2. When the spectroscope slit is set parallel to a 
latitude circle of a planet, the planet’s rotation causes 
the spectral lines to be inclined to the vertical. The 
tangent of the angle of inclination is proportional to 
the speed of rotation. This method gives the instan- 
taneous speed of the layers of the atmosphere which re- 
flect the light. The probable error of this type of meas- 
urement is of the order of 100 m sec, a quantity which 
is small from an astronomical point of view, but large 
in comparison with normal meteorological experience. 
For Jupiter, the spectral speed of rotation agrees 
with the speeds of the spots. In view of the large error 
of spectral velocities, this does not exclude the pos- 
sibility that the spots may move with speeds differing 
from those of the general current by 50 m sec™ or more. 
On Saturn, the period of rotation determined by spots 
near the equator is about 10 hr 14 min. A very careful 
spectral investigation resulted in a period of 10 hr 2 
min [5]. These values differ from each other by three 
times more than the probable error of the spectroscopic 
value. If real, the discrepancy probably means that the 
general currents on Saturn move faster than the mark- 
ings. This interpretation is tenable if, for example, the 
markings were of the nature of storms (which usually 
do not move with the wind speeds). 
Even though the nature of the “spots” is not known— 
they appear to be large cloud systems—the spot periods 
are usually interpreted to approximate the period of 
the “air” in the neighborhood of the spots. The heavy 
crosses in Fig. 3 show the normal distribution of the 
periods of spots and markings on Jupiter as function of 
Jovicentric latitude. The same figure, with the sign 
reversed, would also show the relative velocity dis- 
tribution. In the bright equatorial zone the periods are 
short, corresponding to fast westerly winds. A transi- 
tion from short to long periods takes place in the dark 
equatorial belts. Spots of nearly the same relatively 
long period oceur in the tropical zones, temperate belts, 
temperate zones, and even the so-called polar regions 
near latitude 45°. 
The short periods in the equatorial zone might lead 
to the interpretation that the equatorial spots are situ- 
ated at a higher level than the markings in the other 
zones and belts. However, spots on Jupiter occasionally 
move around each other but never cover each other up 
as might be expected if they were at different levels. 
1. Jupiter data are collected in various Memoirs of the 
British Astronomical Association, Jupiter Section. For Saturn, 
see [10] for example. 
COSMICAL METEOROLOGY 
It is interesting to make an attempt to estimate the 
period of the core of Jupiter from the given distribution 
of the spot periods with latitude. One might assume, for 
example, that the net torque of the currents about the 
planet should vanish. In that case, the period of the 
planet would be near 9 hr 52 min, with westerlies near 
the equator and easterlies between latitudes 20° and 
60° in both hemispheres. Such speculation, however, is 
misleading. After all, we are looking at a high level in 
Jupiter’s atmosphere. If we observed only high levels 
of Earth’s atmosphere, we would find westerlies at 
almost all latitudes. If, as is likely, the meridional tem- 
perature gradient on Jupiter is in the same direction as 
that on Earth, the high-level winds on Jupiter should 
also nearly all be westerlies. In that case the period of 
the core may be longer than that of any of the spots, 
that is, about 10 hours. 
A wealth of information is available concerning de- 
tails in the spot periods of Jupiter. Occasionally, spots 
near the boundaries of the currents are found, with 
periods differing from those of the main currents by 
as much as 5 min (100 m sec). These spots are indi- 
cated by light circles in Fig. 3. Again, one might expect 
them to be located at levels different from those used 
in the determination of the periods of the principal 
currents, yet these unusual spots often move around 
the normal spots. An analysis of these unusual spots 
was made under the assumption that they were situated 
at approximately the same level as the normal markings. 
This analysis indicated systematically that the bright 
zones are regions of anticyclonic shear, and the dark 
belts are regions of cyclonic shear (see the line in Fig. 
3). This idea is substantiated by the behavior of the 
SSSSRE TTY 
WAN 
DASAMSA AAAS 
SHR. 55 MIN. 
‘D 
WARABREBBBAREERE 
SHR.SOMIN. 
: 
Bi 
d 
g 
g 
g 
g 
g 
g 
a 
KYU SSS SSE EEDA 
IX YAAALA LALLY LIRA 
-50 -—40 -30 —20 —-l0 oO 10 20 30 
JOVIGENTRIG LATITUDE 
nS 
fo) 
50 
Fig. 3.—Distribution of spot periods on Jupiter with lati- 
tude. Heavy crosses indicate average periods for the latitude 
band; light circles indicate occasional periods. The hypothet- 
ical current distribution is represented by a line. Hatched areas 
are dark belts; clear areas are light zones. 
“Teversing spots.”’ These spots are some of the few mark- 
ings on Jupiter which show meridional movement in 
addition to zonal motion; they actually describe an 
anticyclonic orbit relative to the bright tropical zone 
in the Southern Hemisphere. First they move more 
slowly than the main current, at its northern edge, then 
they traverse the main current, and finally they move 
faster than the main current at its southern edge. 
