Prior to the visitations of spacecraft to Saturn, there had been many observational problems with ascertaining the true rotational period. This is certainly due to the lack of any long-term distinguished atmospheric features and the little colour contrast available to visual observers.
It was in 1789 that William Herschel saw some faint features moving across the rings, and so he figured out for the first time that the ring rotation was 10 hours and 32 minutes. The estimation appears fairly good, as it also corresponds roughly to the rotation of the atmosphere on the equator adopted today. Similarly in 1793, Herschel used another two features and found the little smaller period of 10h 16.0m. This latter result is also interesting, as the agrees with the 1876 value found by Ashley Hall (1829-1907) of the average equatorial planetary rotation of 10h 14m 23.8±2.30s. Hall used an active outburst of several white spots. Again in 1894, Arthur Stanley Williams of the British Astronomical Association determined 10h 12.6m for the equator and at 20° latitude as 10h 14.3m. Yet another spot was seen at +36° latitude in 1903 by William Frederick Denning and Edward Emerson Barnard (1887-1923), and both declared they found the mean period of 10h 38.4m than and clearly different than the 1876 spot observed by Hall. Another in 1910 was seen in the southern latitudes at -36° by Philips, Hough and Denning, giving their periods from about 10h 38.0m to 10h 38.5m.
The next major determination was with the appearance of the Saturnian “Great White Spot” in 1930 that was discovered by Will Hay. In longitude this spot covered about a third to quarter of the Saturnian disk being about 5″ to 7″ (seconds of arc) across. Aligned at latitude +15°, both Wright and Rowland found the period as 10h 14m 07s, by timing the edges of the white area.
One of the greatest advances in the Saturnian rotation was achieved by Moore in 1938. (See Ref.10) This was done with spectroscopic analysis during various ring crossings and measured the Doppler velocities of the rotation at various latitudes. It was here that the acceleration of the atmosphere towards the equator was ascertained, and this agreed well with the rates observed with the earlier spot values.
During 1950 to 1960, observations of various features were found to change velocities significantly over three months or so. Modern values for the Saturnian rotation quote System III as 10.656 hours or 10h 39m 22s which surprisingly agrees well with Denning and Barnard!
Note: In 1990, the Hubble Space
Telescope produced an animation showing that the general
Saturnian atmospheric motion and cloud patterns.
See
NASA Astronomy Picture of the Day.
The International Astronomical Union (IAU) originally decided to agreed on the rotation rate of Saturn in the mid-1960’s. To save having the very complicated differential rotation rate, Saturn like Jupiter was simply divided into the visual phenomena of System I and System II. These values were based on visual observations of Saturn whose sole purpose was to identify and measure any new atmospheric phenomena for proper study. This was mainly applied to observed active features often seen within the NEB(S), EZ and SEB(N) zones.
In reality, as long as the standard rotational period is applied, then some surface feature relative to other surrounding belt makes little difference — even if the rotation period were slightly inaccurate. This also procedure was again applied directly to the features known as the Great Red Spot (GRS) on Jupiter. For example, the GRS drifts through its assigned latitude at different rates than the surrounds, and is predictable via an ephemerides.
When radiometric measures were made for Saturn, the resultant rotation period was found to be different than the earlier System I and II. Radio waves from Saturn are produced by the magnetic field produced by a metallic hydrogen region surrounding the small solid core. Named System III, it is presently preferred to describe Saturnian atmospheric features. Astronomers also now favour System III because it is likely reflecting the true rotation of the inner core and is not subject to atmospheric changes based on the latitude of the observed phenomena in question.
System III is used in all other regions and has a drift rate standardised as 10h 39m 22s. This produces 810.80236° per day. System III is generally rounded as a drift rate of 810° per day. [Based on eight times at 810° per day or four times 10h 39m 22s]
System I is used for the equatorial regions for the zones to about ±30°. Using the rotational ‘day’ of 10h 14m produces 844.29967° per day. System I is generally rounded to have a drift rate of 844° each day. [Based on three rotations times at 844° per day.]
Since late 1990’s the IAU no longer supports nor gives information on any of the parameters for System I, and System III replaces the old visual System II. System III parameters are now exclusively supported by the IAU. (See: Celestial Mechanics, 63, p.127-148 (1996)).
01. Barnard, E.E., AJ., 23, 180 (1903)
02. Campbell, W.W., AJ., 2, 416 (1895)
03. Cragg, T.A.; “Rotation of Saturn”,
PASP, 73, 314. (19xx)
04. Denning, W.F., J.BAA., 14, 176 (1904)
05. Gurnette, R.L., Woolley, R.v.d.R, “Explanatory
Supplement to the Ephemeris”, Pub. USNO (1961)
06. Hall, A., A.N., 90, 45 (1877)
07. Hellad, R., Schubert, G., Anderson, J.D.,
“Jupiter
and Saturn Rotation Period”, arvix:0907.3418v1,
[astro-ph.EP](20 Jul 2009)
08. Herschel, W., Phil. Trans. Roy. Soc., 84,
48 (1794)
09. Hough, G.W., J.BAA., 14, 176 (1904)
10. Keeler, J.E., A.J., 1, 127 (1895)
11. Read, P.L., P.L, Dowling, T.E., Schubert,
G.,“Saturn’s rotation period from its
atmospheric planetary-wave configuration”;
Nature, 460, pg.608-610 (Letter)(30 July
2009)
12. Moore, J.H., PASP, 51, 274 (1939)
13. Pannekoek, A., “A History of Astronomy”,
Dover Pub. (1961)
14. Rowland, J.P., MNRAS, 94, 86 (1933)
15. Sheenhan, William.,“Planets and Perception :
Telescopic Views and Interpretation.” Arizona Press
(1988)
16. Williams, A.S., MNRAS, 54, 297 (1894)
NOTE: A sample of the information on the Central Meridian Ephemeris 2005 and corrections for Saturn can be found at; or in the B.A.A. Handbook.
The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for regarding either legal or medical purposes. Although the data is accurate as possible some errors might be present. The onus of its use is place solely with the user.
