SPA Planetary Section

Saturn is a 'Gas Giant Planet' like Jupiter its visible surface being only clouds appearing as brightish zones and darkish belts, but Saturn is somewhat smaller and at a much greater distance from earth so its surface features which are also generally more delicate than Jupiter's are less conspicuous. However, Saturn is particularly interesting to observe because of its fine Ring System and interesting satellites which can be seen with moderate sized telescopes. In 1997 Saturn will be best placed for observation about Opposition on October 10 then appearing at apparent magnitude +0.2 with a telescopic disc of 18 arc seconds polar diameter, the ring system being 44.7 arc seconds across the Major Axis. It will then be 8.4 Astronomical Units from Earth. As seen from earth Saturn's axial tilt varies significantly from one apparition to another . At intervals of 13.7 and 15.2 years the rings are seen edge on as occurred in 1995 and again in 2008 between times the rings are seen at varying degrees of openess the maximum being 25 degrees. Whilst even throughout any given apparition Saturn's axial tilt is continually slowly changing in 1997 variations are as follows...

    Mid Month    May   June   July  August  September October November December 
    Degrees      -9.3  -11.0  -11.6 -11.6    -10.7     -9.8    -9.0     -8.7 

Saturn not being truly spherical but significantly oblate it is desirable to make drawings of the planet's globe the appropriate correct shape. Standard observation report forms for Saturn are available to SPA members on which an outline of the Saturn's globe is represented by an oval 40mm across the polar axis, the outline of the rings also being shown for a 10 degree tilt which is about average for 1997, though Saturn's axial tilt varies between 8.4 degrees and 11.6 degrees during the apparition. Strictly speaking the standard form only provides an accurate representation of the rings in mid May and mid October however for most general observation purposes the report form outlines are acceptable. Saturn's north is towards the bottom on drawing blanks as seen in most astronomical telescopes. Anyone wishing to make particularly accurate observation drawings (such as needed to assess the latitudes of Saturn's features) should make their own more accurate representation of the ring system as appropriate on separate drawings. Observers should draw Saturn's features preferably using ordinary 'lead' pencils of suitable grades ( e.g. H, B, 2B ) to represent the shading of intensity actually observed irrespective of whether or not the surrounding area is blacked out to represent the night sky adjacent the planet. Please do not render Saturn's features in a negative form (i.e. do not show bright areas shaded dark, dark features light) as is frequently done by observers making drawings of Deep Sky Objects.

Observers may also make intensity estimates of Saturn's features. To do so, allocate to each feature the appropriate number between 0 and 10 that represents the features relative brightness (where 0 is extremely bright and 10 is the blackness of the adjacent night sky). In the event of any specific planetary colours being seen these may also be noted but preferably by making written descriptions with other details in the observation notes section of the form. It is generally preferable to make written descriptions of colours rather than attempting to make coloured drawings whilst at the telescope because it is difficult to select colouring impliments and render colours very satisfactorily when drawing at the telesubdued light. Any of Saturn's satellites observed should be recorded on the appropriate section of the report form. Saturn has more known satellites than any other planet and several of them can be seen with good moderate sized telescopes. The brightest satellites attain the following apparent magnitudes at average Oppositions :-

Titan +8.4,   Rhea +9,          lapetus +10 to +12, Tethys +10.3, 
Dione +10.4,  Enceladus +11.8,  Hyperion+14.2.
 

Maintained by Michael Oates Email: mike@ph.u-net.com Last modified 3rd July 1997