Saturn Observing Programme
With its spectacular rings, few would doubt the unique fascination of the planet Saturn to the astronomer and layman alike. Most of what has been learned in recent decades concerning the Saturn system has come from spacecraft encounters but the dynamic nature of the planet and satellites dictates that Earth-based studies though limited in resolution, are still useful. More so, the dedicated amateur astronomer familiar with the general appearance of the Saturn still has a role to play and their contribution assumes a greater significance when communicated to a central body such as the Saturn Section of the British Astronomical Association. Founded in 1891, the Section maintains a programme intended to provide interesting and useful work for those with access to astronomical telescopes.
Opinion will always differ with regard to choice of telescope. Traditional reflectors and refractors work well but in recent years, compound designs have provided an excellent compromise between aperture and physical size. Good quality optics are an essential prerequisite and it has to be said that the greater resolving power and enhanced contrast offered by a large instrument will often be needed to confirm the vague spots which occasionally appear on Saturn. It must be stressed that changes in the appearance of Saturn’s belts and zones are usually slight or gradual and considerable experience on the part of the observer is needed to recognise them. The degree to which each observer can follow the programme will vary but it is hoped that he or she will try to carry out a general inspection of the globe and rings as often as possible.
Besides visual observing there is considerable scope for high-resolution photography, CCD imaging and photometry. Indeed, given the rapid development of affordable CCD equipment, the modern amateur can today conduct studies which only a few years ago would have been considered the sole province of the professional.
General Inspection of the Globe and Rings
A general inspection of the globe and rings of Saturn should be undertaken on each occasion the planet is observed. Points to be noted could include the following;
a) Position and shape of shadows of globe on rings (ShGR) and rings on globe (ShRG).
b) Any change in the position or form of the belts on the globe.
c) Any star that appears likely to be occulted by the globe or rings. These events are very rare and may or may not be predicted far in advance.
d) Position of any bright or dark spots within the belts and zones which appear to rotate with the planet.
e) Any change in the visibility or any irregularity of the outline of Ring C.
f) Any indication of a faint ring inside or outside of the principle rings.
g) Position of any dusky patches or subdivisions on the ansae of the rings.
Observers with small telescopes should carry out items ‘a’ to ‘c’, with ‘d’ to ‘g’ being in the remit of larger apertures. Items ‘c’ and ‘d’ should be accorded immediate attention and should be dealt with as suggested in sections 6 and 7 to follow.
A set of blank outlines covering the complete presentation of Saturn as seen from Earth in degree increments from 0 to 28° is available either from the Director or by clicking here. Individual drawing styles will differ but the golden rules are to sketch only detail which can be seen with certainty and never sacrifice accuracy for artistic license.
Visual estimates of relative intensity from a long-term part of the Section programme. Intensity estimates of rings, belts, zones and the various parts of these are made on a scale from ‘1’ for the bright, outer part of Ring B to ’10’ which would equate to the black of a very dark sky or shadow. Caution should be exercised at an apparition where the rings are presented at a shallow angle where the outer part of Ring B is apt to appear darker than the ‘assigned’ value. Exceptionally, a value of ‘0’ may be assigned to a very bright feature such as a rare, prominent white spot. Novice observers are advised to mark intensity estimates on their drawings, as precise identification of belts and zones is not always straightforward. Given steady seeing, intensity estimates could be made at every observation but care should be taken to avoid bias by previous estimates. When collated with the work of other observers, significant variation over one or more apparitions may be evident.
Though to an extent subjective, estimates of the colour of the various belts, zones and rings are useful, especially by those equipped with large reflectors or suitably corrected refractors. Colour on the disk of Saturn is usually less obvious than on Jupiter but careful use of colour filters can identify tints in the belts and zones of Saturn and the Wratten range of filters are suitable for use at the telescope. Intensity estimates are made with filters in the same way as in integrated light (see section 3). Filters may also be used to investigate the bicolour aspect of the rings, where one ansa may appear brighter than the other when examined with say, a red or blue filter.
Photometry and Astrometry of the Satellites
Visual magnitude estimates of Saturn’s satellites have been largely superseded by CCD photometry. In this respect, there is an opportunity to do work of lasting value. To observers proficient in photometric and astrometric techniques there is the opportunity to work directly with the professional community.
These events are very rare and are not always predicted beforehand. If possible, every fluctuation in brightness, with the time and the star’s exact position should be recorded during its passage behind the rings, or as it disappears behind or emerges from the limb of the globe.
Unlike Jupiter, definite bright or dark spots are seldom seen in the belts and zones of Saturn, and on the rare occasions when they do, every effort should be made to establish their rotation period by means of visual transit estimate, or by measurement from high resolution photographs or CCD images. On discovery of a spot, observers are requested to contact the Director without delay, to allow other observers to be alerted. The method used to make visual estimates is straightforward and consists of estimating to the nearest minute, the time at which the preceding end, centre and following end of the spot appears to cross the central meridian. Naturally for a small spot it may not always prove possible to transit the preceding and following ends, and an estimate for the centre will suffice.
Two rotation systems are accorded to Saturn and are in general use by the Section. System I is assigned to the equatorial regions and assumes a drift rate of 844°×3 per day (period 10h 14m) while System III is assigned to the remainder of the planet and assumes a drift rate of 810°×8 per day (period 10h 39m 22s×4). System I is based on the observed rotation of visual phenomena in the equatorial zone of Saturn while System III is based on radio emissions from the planet and has now replaced an earlier visual reference frame, System II, as the periods for the two rotation periods are similar. Tables providing central meridian values and increments in Systems III and I are provided in the BAA Handbook.
Physiography of the Rings
Observers equipped with large telescopes may care to inspect the classical rings for delicate albedo variation and subdivisions. These may be recorded by drawing, high-resolution photography or CCD imaging. Visual observers can make intensity estimates or carefully estimate the position of subdivision while photographs and CCD images may be directly measured. The principle division is Cassini’s division which may be seen with relatively small apertures while Encke’s division seems to represent a variation in albedo roughly central in Ring A. The far more elusive Keeler gap lies close to the outer edge of Ring A and is a true division, but is unlikely to be glimpsed other than with large apertures under the very best conditions. Observers should be aware that the visual impression of subdivision could be created by bordering areas of different albedo within a ring component. Likewise they can also result from the over processing of CCD images. The possibility of recording material outside the bounds of the classical ring system should not be discounted, especially at an edgewise apparition.
Methods exist for measuring or estimating the position of belt edges on the globe of Saturn. Visual observers skilled in the use of the micrometer can obtain reliable results but measurements from accurate drawings are also useful, the measures being reduced to Saturnicentric latitude by means of the Crommelin formula. Measurements required are the polar radius ‘r’ and the distance ‘y’ of each belt edge (or in some cases, belt centre) from the centre of the globe taken along the meridian. Increasingly, qualitative results can be derived by measurement of correctly exposed photographs or CCD images and computer software is available to assist in this. In the long term, latitude estimates can reveal change in latitude, while in the short term they can aid the identification of belts.
Prior to opposition, the shadow of the globe on the rings is found on the preceding ansa. It will gradually diminish in the period leading to opposition to reappear on the following ansa after opposition. An interesting exercise is to determine the times for the disappearance and reappearance of the ShGR. Depending on the respective Saturnicentric latitudes of the Earth and Sun, the shadow of the rings on the globe can be to the north or south of the rings, and when on the inner side of the rings it can be difficult to distinguish the ShRG from Ring C superimposed against the globe.
Edgewise Presentation of the Rings
About every 15 years, the Earth will make either one or three passages through the ring-plane of Saturn and the Sun will make one. Edgewise apparitions provide a unique opportunity to explore the nature of the rings and to observe faint satellites of Saturn. Near the time of passage of the Earth through the ring-plane, the rings are extremely difficult if not impossible to see visually. There may also be a period during which the unilluminated face of the rings is presented towards the Earth, this being the case when the Sun and Earth are on opposite sides of the ring-plane. With the glare of the rings absent, it is no coincidence that satellite discoveries have been made during edgewise apparitions.
In addition to the aesthetic appeal of following the transit and shadow-transit of the larger moons across the globe of Saturn, our knowledge of their orbits may be refined from accurate observations of their eclipses and occultations by Saturn. Eclipses, occultations and mutual phenomena of the satellites are most effectively followed with large apertures and CCD’s present an especially effective means to record them.
Use of the Section proforma is encouraged as it promotes a consistent observing method, maximises the value of observations and speeds their reduction. Available from the Director on request is a standard report form for the making and reporting of individual observations. Spaces are provided for name, observing station, date, UT start and finish, instrument and seeing (the latter on Antoniadi’s five-point scale, ranging from ‘I’ [excellent] to ‘V’ [very poor]). A blank outline of Saturn correct for the date of observation can be traced from the range of outlines available through the Section, on to the space provided on the form. Latitudes can later be measured from the drawing and entered in one of two columns, the other being for the listing of intensity estimates.
A second form may be used to summarise estimates of intensity, colour and latitude at the end of the apparition. Features are arranged on the summary sheet in vertical order, starting at the south of the globe and working north. Similarly, ring details are listed starting with Ring A and working inwards. Once listed, intensity and latitude estimates are averaged.
The Director would appreciate receiving copies of observations at regular intervals throughout an apparition but please do not send the originals as the Section cannot be responsible for their safety while in transit.