S A T U R N : Part 3



Prior to the vistation of spacecraft to Saturn, there have been many observational problems with ascertaining the rotational period. This is certainly due to the lack of any long-term distinguished atmospheric features and little colour contrast.

In 1789, based on some features seen across the rings, William Herschel figured that the ring rotation was 10 hours and 32 minutes - which also corresponds roughly to today’s rotation of the atmosphere on the equator. Similarly in 1793, he used two features and found the period of 10h 16.0m. This latter value is interesting as the average equatorial rotation of the planet was determined in 1876 as 10h 14m 23.8±2.30s by Ashley Hall (1829-1907) He used an active outburst of several white spots in 1894 Arthur Stanley Williams of the British Astronomical Association determined 10h 12.6m for the equator and at 20o latitude as 10h 14.3m. In 1903 another spot was seen in 1903 at +36o latitude by William Frederick Denning and Edward Emerson Barnard (1887-1923), both declared they had found a different mean period of 10h 38.4m than with Hall’s spot. Another was seen in the southern latitudes of -36o in 1910 by Philips, Hough and Denning, giving periods from about 10h 38.0m to 10h 38.5m. The next major determination was 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 +15o, both Wright and Rowland found a period to be 10h 14m 07s, by timing the edges of the white area.

One of the greatest advances in Saturn’s rotation was achieved by Moore in 1938 (Ref. 10). This was done with a spectroscopic analysis during a ring crossing 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 that this agreed well with the rates observed with the earlier spot values.

During the 1950’s and 1960’s observations of various features were found to change velocities significantly over three or so months. 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. Saturn has three systems of rotation are recognised that are named System I, II and III. (Jupiter has only System I and System II.)

Note: An Hubble Space Telescope animation taken in 1990 of the atmospheric motion and general cloud pattern of Saturn can be found at the NASA Astronomy Picture of the Day.

Central Meridian Systems for Saturn

The IAU originally agreed on the rotation rate of Saturn in the mid-1960’s. To save a very complicated differential rotation rate, Saturn like Jupiter was divided into the visual phenomena of System I and System II. These were based on visual observations of the planet whose purpose was to identify and measure atmospheric phenomena for proper study and was applied to features within the NEB(S), EZ and SEB(N) zones. In reality, as long as a standard rotational period is applied, then a surface feature relative to other surrounding belt makes little difference - even if the rotation period is slightly inaccurate. This applies to the Great Red Spot on Jupiter for example, that drifts through its assigned latitude at a different rate than the surrounds.

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 in rotation based on the latitude of the phenomena in question.

System III is used in all other regions and has a drift rate standardised as 10h 39m 22s. This produces 810.80236o per day. System III is generally rounded as a drift rate of 810o per day. [Based on eight times at 810o per day or four times 10h 39m 22s]

System I is used for the equatorial regions for the zones to about ±30o. Using the rotational ‘day’ of 10h 14m produces 844.29967 per day. System I is generally rounded to have a drift rate of 844o each day. [Based on three rotations times at 844o 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)).

Rotation References

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. Herschel, W., Phil. Trans. Roy. Soc., 84, 48 (1794)
08. Hough, G.W., J.BAA., 14, 176 (1904)
09. Keeler, J.E., A.J., 1, 127 (1895)
10. Moore, J.H., PASP, 51, 274 (1939)
11. Pannekoek, A., “A History of Astronomy”, Dover Pub. (1961)
12. Rowland, J.P., MNRAS, 94, 86 (1933)
13. Sheenhan, William.,“Planets and Perception : Telescopic Views and Interpretation.” Arizona Press (1988)
14. Williams, A.S., MNRAS, 54, 297 (1894)
NOTE: 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.


Last Update : 29th October 2005

Southern Astronomical Delights © (2005)


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