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Summary

The major event of this apparition was the appearance of two lightspots in the southern hemisphere. Both lay on the northern com-ponent of the South Temperate Belt at an approximate latitude of41°S (planetographic) and extended into the South Tropical Zone.The derived longitudinal drifts over 30 days with respect to Sys-tem 3 are 11.3° and 10.4°. The equivalent rotation periods are 10h39m 40.2s and 10h 39m 38.8s respectively.

A light spot was also detected in the zone between the compo-nents of the South Equatorial Belt (SEB). The derived drift over 30days with respect to System 3 is −212.3° (equivalent to 10h 33m50.4s). Although it is not completely certain, this spot could havebeen identical to one observed at a similar latitude during the 2006/2007 apparition.

The North Equatorial Belt (NEB) had a bluish colour whereasthe SEB had a warmer colour.

Very high resolution observations made by Damian Peach dur-ing 2007 December from Barbados revealed a very narrow lightstrip which extended longitudinally across the planet’s disk withinthe shadow of the rings on the globe (SH R on G). This light stripwas due to sunlight passing through Cassini’s Division and illumi-nating the planet. The detection of this feature may be a first for anamateur observer.

Two transits of Enceladus were recorded by Chris Go.Introduction

Shortly after solar conjunction on 2007 August 21,1 Saturn’s east-erly motion carried it north of Leo’s brightest star, Regulus. Theplanet then lay to the east of Regulus for the remainder of the appa-rition. Following quadrature on 2007 December 20,1 Saturn’s appar-ent retrograde motion carried it back towards Regulus. The twoobjects were at their closest by the time Saturn reached its secondstationary point on 2008 May 32 and the pair made a fine sight withthe naked eye or binoculars. At this time, Saturn’s apparent magni-tude was +0.6 compared to the fainter mag +1.35 of Regulus.

Saturn was at opposition on 2008 February 24 at 10h UT2,3 withan apparent magnitude of +0.2. The major axis of the rings was45.3" across and the planet’s equatorial diameter was 20.0".

The variation of the apparent inclination of its pole and ringswith respect to both the Earth and the Sun during this apparition isshown in Figure 1. The inclination with respect to Earth was ap-proximately −6.6° in mid-December but increased to −9.9° at thebeginning of May.

At opposition, the rings were inclined to the Earth at approxi-mately −8.4° and to the Sun at −8.2°. A couple of days beforeopposition, the inclinations with respect to the Sun and Earthwere identical. Solar conjunction occurred on 2008 Sept 4 at 02h.2,3

Saturn during the 2007/2008Saturn during the 2007/2008Saturn during the 2007/2008Saturn during the 2007/2008Saturn during the 2007/2008apparitionapparitionapparitionapparitionapparitionMike Foulkes

A report of the Saturn Section. Director: Mike Foulkes

Figure 2. 2007 Dec 10d, 07h 38m UT. CM1= 289.3, CM2= 104.5, CM3= 141.2.(Peach, Barbados). This is one of the highest resolution observations made duringthis apparition. Tethys and its shadow were in transit at the time with the satelliteitself appearing as a light spot. Tethys was approaching the CM and projectedagainst the NEBn. Its shadow lay further north and was approaching the Np. limb. Inparticular, this image shows a narrow bright strip within the SH R on G which is theregion of the planet illuminated by sunlight passing through the Cassini Division.Other features are described in the text. This and the SH R on G are shown at a largerimage scale in the inset.

Figure 1. The variations in Saturn’s polar and ring inclination as seen fromEarth (parameter B) and from the Sun (parameter B') during the 2007/2008apparition. Negative values indicate that the south pole and rings were inclinedtowards the Earth and Sun.

This report provides an analysis of the Saturn observations made bymembers of the BAA Saturn Section during the 2007/2008 apparition.

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Observations

The observers who contributed visual observations during thisapparition are shown in Table 1 and those who contributed digitaland photographic observations are shown in Table 2.

The first observation of the apparition was made by Gray on2007 October 20 and the final observation was made on 2008 July 5by Giuntoli.

Gray and Abel each provided a large number of visual observa-tions. The majority of drawings received were monochrome al-though Abel and Frassati also made several in colour.

Although a large number of visual observations was submit-ted, the majority of observations for this apparition were made bydigital imaging techniques. All figures used in this report are im-ages except where otherwise noted.

Peach, Sharp and Tyler spent the period from 2007 Dec 1 untilDecember 15 on Barbados, in an attempt to obtain better seeingthan from their respective home locations. Their primary goalwas to image Mars when it came to opposition on 2007 Dec 24.However images were also taken of Saturn and some of thesewere of very high resolution such as are shown in Figures 2, 3(a)and 5(b). Details of a previous imaging expedition to Barbadosby these observers are given in Ref. 4.

When seeing permitted, many other observers routinely producedimages with good resolution. Some generated colour images withcolour cameras but others used monochrome cameras and producedcolour composites from images taken with red, green and blue filtersor in combination with a luminance channel. The typical appearanceof the planet in red, green and blue filters during this apparition isshown in Figure 3. The appearance of individual features in differentwavelengths is discussed further below.

Several observers combined images taken on a given night intoshort animations. These showed the movements due to the planet’srotation of any spots present and so helped confirm the reality ofsuch spots. They also showed the movement of any satellites andtheir shadows when in transit across the planet’s disk.

Although the vast majority of images were taken with medium

Figure 3. The appearance of Saturn during this apparition with red (R), green (G)and blue (B) filters. The colour composites (RGB) made from these images are alsoshown for comparison.(a, top). 2007 Dec 06d, 08h 40m UT. CM1= 188.2, CM2= 131.2, CM3= 172.7.(Sharp, in Barbados).(b, below). 2008 Feb 12d, 00h 19m UT. CM1= 72.7, CM2= 350.3, CM3=310.2.(Arditti). This shows STB/STropZ spot no. 2 near the CM.

Figure 4. Belt and zone nomenclature used in this report. The figure isbased upon the image in Figure 2.

Figure 5. A comparison of the belt and zone structure during the 2006/2007 and 2007/2008 apparitions based on images taken by Peach.(a). 2007 Apr 18d, 19h 57m UT. CM1= 207.6, CM2= 69.0, CM3= 29.7.(b). 2008 Dec 06d, 09h 50m UT. CM1= 229.3, CM2= 69.0, CM3= 212.1.(taken in Barbados). Also shows the narrow strip of illuminated planetwithin the SH R on G due to sunlight passing through the Cassini Division.

Table 1. Visual observers, 2007/2008

Observer Location Telescope*

Abel, Paul G. Leicester & Selsey, UK 203mm Newt., 312mmNewt., 375mm Newt. &400mm Cass.

Adamoli, Gianluigi Verona, Italy 125mm Mak.Bradley, Herbie Great Malvern, UK 114mm Newt.Colombo, Emilio Cambio, Italy. 150mm Newt.Foulkes, Mike Henlow, Beds., UK 203mm SCTFrassati, Mario Crescentino, Italy 203mm SCTGraham, David Richmond, N. Yorks, UK 150mm OG & 230mm

Mak.Gray, David Kirk Merrington, 415mm DK

Co. Durham, UKGiuntoli, Massimo Monte Catini Terme, Italy 203mm SCTHeath, Alan Long Eaton, Notts., UK 203mm SCT & 250mm

Newt.Leatherbarrow, W. J. Sheffield, UK 127mm OG & 235mm

SCTLine, Ray Wellingborough, UK 212mm Newt.Lyon, Peter Quinton, Birmingham, UK 203mm SCTMcKim, Richard Upper Benefield, 410mm DK

Northants., UKPhelps, Ian Warrington, Cheshire, UK 215mm Newt.

*Cass.= Cassegrain; DK= Dall−Kirkham Cass.; Mak.= Maksutov;Newt.= Newtonian; OG= Refractor; SCT= Schmidt−Cass.

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to large aperture amateur instruments, on April 3Tyler showed that the major features (SEB, EZ,NEB and Cassini’s division) could be imaged withonly an 80mm refractor.

The WinJUPOS software5 (version 7.5.14) wasagain used to derive spot longitudes, average spotdrifts and belt latitudes. These positions plus visualcentral meridian transits are stored in a WinJUPOSdatabase which has been created for Saturn Sec-tion observations.

Nomenclature andterminology

The belt/zone nomenclature used in this report isshown in Figure 4. This is similar to that used in theprevious apparition report6 but has been extendedfor the northern hemisphere features that becamevisible during this apparition.

This report also uses the abbreviations and ter-minology used in the previous apparition report.6

All images shown in this report are orientedwith south upwards and with the preceding (p.)edge to the left. This is the view seen in an invert-ing telescope from the northern hemisphere.

Latitudes

2007/2008 latitudes

The WinJUPOS software5 was used to measurethe latitudes of the belt edges using some of thebest colour images. This software only generatesplanetographic latitudes. However some publica-tions quote planetocentric latitudes.

Table 3 shows the average latitudes (bothplanetographic and planetocentric) derived fromthese measurements. The planetocentric valueswere computed from the WinJUPOS derived val-ues using the formula given in Table 3.

Planetographic latitudes are used within thisreport unless noted.

In some cases, the measurement of some of thebelt edges proved to be difficult to achieve as thesewere not sharply defined, even in some of the bestimages. This in turn added to the potential uncer-tainty in the derived latitude measurements. Therelated standard deviations of the measurementsare also shown in Table 3. Generally, the standarddeviations are less than one degree, although it ispossible that the inaccuracy of the measurementsmay be higher than this value.

The higher resolution observations revealed anumber of belts in both hemispheres. As noted inthe previous apparition report,6 the nomenclaturefor some of these belts is not fully certain and lati-tude measurements made over a number of appari-tions may help to clarify the nomenclature further.

Table 2. Photographic and digital imaging observers, 2007/2008

Observer Location Telescope* Camera Filters** (IRblocking plus:)

Arditti, David Edgeware, Middx., UK 356mm SCT Lumenera Skynyx 2.0 R, G, B, BaaderUV (320−390nm)

Barry, Trevor Broken Hill, Australia 406mm Newt. ToucamBoots, Graham & Selsey, UK 380mm Newt. Toucam IR/UV Peters, KeithBuda, Stefan Melbourne, Australia 405mm DK Toucam 740 UV blockingCasquinha, Paolo Palmela, Portugal 356mm SCT Lumenera Skynyx 2.0M R, G, BFoulkes, Mike Henlow, Beds., UK 203mm SCT DMK 21 IRTGo, Chris Cebu City, Philippines 280mm SCT DMK21 BF04Hill, Richard Tucson, AZ, USA 356mm SCT SPC900NC UV blockingLawrence, Peter Selsey, UK 356mm SCT Lumenera Skynyx 2.0M R, G, BLeatherbarrow, W. J. Sheffield, UK 127mm OG DMK21

& 235mm SCTLewis, Martin St Albans, Herts., UK 222mm Newt. DMK 21Little,Trevor Southwick, UK 280mm SCT DMK 21 FirewireLyon, Peter Birmingham, UK 203mm SCT Fuji S09600 dig. camera UV blockingMeredith, Cliff Prestwich, Manchester 214mm Newt. ToucamPeach, Damian Barbados 356mm SCT Lumenera Skynyx 2.0M R, G, BPhillips, Jim Charleston, S.Carolina, 203mm OG Lumenera Skynyx 2.0M

USASampson, Ed Goring on Sea, UK 203mm SCT DMK21Sharp, Ian Barbados & Ham, 280mm SCT Atik 1HS R, G, B

W. Sussex, UKTatum, Randy Richmond, VA, USA 254mm Newt. Toucam ProTyler, David Barbados 280mm SCT, Lumenera Skynyx 2.0M R, G, B

& High Wycombe, UK 356mm SCT & 80mm OG

Wesley, Anthony Murrumbateman, 333mm Newt. PGR Dragonfly 2 Australia

*Cass.= Cassegrain; DK= Dall−Kirkham Cass.; Mak.= Maksutov; Newt.= Newtonian; OG= Refractor;SCT= Schmidt−Cass. **Filters: R= Red; Y= Yellow; G= Green; B= Blue; IRT= Infrared transmission

Table 3. Latitudes of the belts, 2007/2008

Measured planetographic latitudes

Belt Planeto- No of Standard Derived planeto-graphic measure deviation centric latitudeslatitude (°) ments (°)

SPR Band s −66.9 6 0.6 −61.8SPR Band n −60.2 15 1.1 −54.4

SSTBs −55.7 14 0.8 −49.5SSTBn −52.8 14 0.5 −46.4

STB(S)s −48.1 6 0.4 −41.7STB(S)n −45.0 11 0.7 −38.5STB(N)s −42.3 8 0.5 −36.0STB(N)n −40.5 15 0.6 −34.2

SEB(S) southern component s. edge −35.5 18 1.1 −29.6SEB(S) southern component n. edge −34.3 4 0.6 −28.5SEB(S) northern component s. edge −32.1 5 0.3 −26.6SEB(S) northern component n. edge −30.0 18 0.6 −24.7SEB(N) southern component s. edge −25.5 17 0.9 −20.8SEB(N) southern component n. edge −24.7 3 0.2 −20.2SEB(N) northern component s. edge −21.6 5 0.7 −17.5SEB(N) northern component n. edge −19.3 18 1.3 −15.6

C of narrow belt in the southern EZ −15.3 10 0.5 −12.3

EBs −10.4 17 0.6 −8.3EB northern component s. edge −4.0 7 0.8 −3.2EB northern component n. edge −1.1 13 0.6 −0.9

NEB(S) s. component s. edge 19.4 15 0.9 15.7NEB(S) s. component n. edge 21.3 10 0.9 17.3NEB(S) n. component s. edge 22.1 2 0.2 17.9NEB(S) n. component n. edge 24.3 2 0.3 19.8NEB(N) southern component s.edge 26.9 16 0.8 22.0NEB(N) southern component n. edge 29.3 3 0.6 24.1NEB(N) northern component s. edge 32.4 5 0.6 26.8NEB(N) northern component n. edge 36.0 18 0.4 30.1

NTBs 43.6 15 1.1 37.2NTBn 51.8 15 1.3 45.4

S edge of dark region near the planet’s 58.6 12 1.3 52.5 northern limb

Latitude measurements were made from images taken by Arditti, Buda, Casquinha, Go, Hill,Lawrence, Lewis, Sharp, Tyler & Wesley over the period from 2007 Dec 6 until 2008 May 5.Positive values indicate a northern latitude, negative values indicate a southern latitude.The WinJUPOS software provides planetographic latitudes.Planetocentric latitudes were derived from the planetographic latitudes by the followingformula: tan(Latc) = tan(Latg)/(1.12)2

where Latc is the planetocentric latitude and Latg is the planetographic latitude.

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A comparison of latitudes with those derivedduring the previous apparition

It is interesting to compare the appearance of the beltsand zones observed during this apparition with thoseobserved during the previous (2006/2007) apparitionas shown in Figure 5. A comparison of the belt lati-tudes during these two apparitions can be made bycomparing Table 3 of this report with Table 5 of Ref. 6.

Figure 5 shows that the structure of the belts andzones in the southern hemisphere was very similarduring both apparitions, although there were somechanges in intensity and colour which are discussedfurther below.

In addition, the derived latitudes of the southernhemisphere belts were very similar over both appari-tions allowing for the scatter in the measurements.For example, the derived latitude for the SEB(N)n wasat 19.0°S (±0.8° 1σ) during the 2006/2007 apparitioncompared to 19.3°S (±1.3° 1σ) during this apparition.

The South Polar Region Band showed a greaterdifference in average values and measurement scatterover the two apparitions. As this belt lay close to thesouthern limb during both apparitions, foreshorteningeffects were expected to impact on the measurement

Figure 6. A comparison of the derived northern hemisphere belt latitudes made during the 2006/2007and 2007/2008 apparitions. The belt and zone nomenclature used respectively during each apparitionis shown. The latitude measurements are taken from Table 3 of this report and Table 3 of Ref. 6. Theerror bars show the derived standard deviations of each of the latitude measurements.

Table 4. Average visual intensity estimates, 2007/2008

Notes:Intensities are made on the scale: 0= bright white, 10= black. The number of observations made by each observer is shown in ().Gray and McKim consistently showed the Cassini Divison to be much lighter than black.Gray often recorded Ring A in the f. ansa to be brighter than in the p. ansa. He also recorded Ring C to be brighter in the p. ansa than in the f.Where an observer only provided an integrated intensity for two or more featues, this is shown in }. These values are included in the apparitionaverage as indicated below:1. These intensities have been allocated to the average for the SPR Zone; 2. Allocated to the average of the STB(N); 3. Allocated to the average ofthe SEB(N); 4. Allocated to the average of Ring A1; 5. Allocated to the average of Ring B1.

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uncertainty. The latitude of northern edge of the EB also showed agreater difference between the two apparitions but it is not certain ifthis difference was due to a real change or to measurement error.

Latitude measurements showed changes in the structure of thenorthern hemisphere belts and zones between the two apparitionsas shown in diagrammatic form in Figure 6.

One major belt (designated as the NTB) was observed in thenorthern hemisphere during the 2006/2007 apparition (Figure 5). Thissometimes showed a double structure and a bright turquoise col-oured zone, designated as the NTropZ, lay immediately to the south.Latitude measurements showed that this spanned the latitudes ofthe NTB, NTropZ and NEB(N) of the 2007/2008 apparition.

Further, the NTropZ and NTropZ Band of the 2006/2007 appa-rition spanned the latitudes derived for the NEB(N) and NEB(S)of the 2007/2008 apparition. These changes may have been aprecursor to the further changes observed in the NEB duringlater apparitions, which will be discussed in the appropriate ap-parition report.

The consistency of the southern hemisphere latitude measure-ments over both apparitions indicates that the northern hemispherelatitude changes were not due to measurement errors. Howeveradditional checks were made to confirm this including:

– Re-measurement of a number of images from both apparitionsboth using WinJUPOS and by hand measurement plus handcalculation. These gave results consistent with those given inthis and the previous apparition report.6

– The introduction of small offsets and image scale variations ofeach image relative to the WinJUPOS measurement frame. Thisresulted in small differences relative to the reported results.However the differences were much less than the observedlatitude variations between the two apparitions.

Visual intensity and colourobservations

Table 4 shows the average visual intensity estimates for the beltsand zones made during the apparition. No colour filters were usedfor these observations.

For each belt or zone, this table gives the average value derivedby each observer plus the overall average based on the observa-tions of all observers.

The table shows some differences between the average values

Table 5. Colour estimates, 2007/2008

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Figure 7. The appearance of the STB(N)/STropZspots during 2007/2008.(a). 2007 Dec 3d, 19h 42m UT. CM1= 203.3,CM2= 228.4, CM3= 272.9. (Go). STB(N)/STropZ spot no. 2 on the CM.(b). 2008 Jan 18d, 02h 28m UT. CM1= 114.9,CM2= 79.8, CM3= 81.8. (Casquinha). STB(N)/STropZ spot no. 2 approaching the CM.(c). 2008 Jan 26d, 02h 11m UT. CM1= 183.7,CM2= 287.9, CM3= 268.2. (Lawrence). STB(N)/STropZ spot no. 2 approaching the CM.(d). 2008 Feb 7d, 23h 50m UT. CM1= 278.1,CM2= 325.6, CM3= 290.3. (Casquinha).STB(N)/STropZ spot no. 2 approaching the CMbut this appeared very faint. The SEBZ spot is p.the CM, and also appeared faint.(e). 2008 Feb 28d, 20h 15m UT. CM1= 244.0,CM2= 338.0, CM3= 277.6. (Drawing by Gray,×365). STB(N)/STropZ spot no. 2 approachingthe CM.(f). 2008 Mar 03d, 00h 03m UT. CM1= 30.8,CM2= 22.8, CM3= 318.6. (Casquinha). STB(N)/STropZ spot no. 2 p. the CM plus a small spot(no. 3) in the STB(N) on the CM.(g). 2008 Mar 18, 12h 52m UT. CM1= 186.9,CM2= 37.1, CM3= 314.1. (Go). Shows the 3light spots that were observed in the STBn/STropZ and the single light spot that was ob-served in the SEBZ. STB(N)/STropZ spot no. 1is approaching the p. limb, spot no. 2 on the CMand spot no. 3 is f. the CM. The SEB(Z) spot isnear the f. limb.(h). 2008 Apr 19, 10h 50m UT. CM1= 132.6,CM2= 32.1, CM3= 270.6. (Barry). STB(N)/STropZ spot no. 1 approaching the CM.(i). 2008 Apr 28, 20h 33m UT. CM1= 152.6,CM2= 108.3, CM3= 335.5. (Sharp). STB(N)/STropZ spots no. 1 and no. 2.(j). 2008 May 01, 01h 23m UT. CM1= 211.1,CM2= 95.7, CM3= 320.3. (Phillips). STB(N)/STropZ spots no. 1 and no. 2.(k). 2008 May 01, 11h 15m UT. CM1= 198.2,CM2= 69.5, CM3= 293.6. (Go). Compare thisimage with Figure 7(j) which was taken almostone rotation earlier. This image shows STB(N)/STropZ spot no. 1 on the CM followed by spotno. 2. Note the structure of each spot with twobrighter cores within spot no. 1.(l). 2008 May 5, 02h 30m UT. CM1= 27.3,CM2= 141.2, CM3= 0.9. (Hill). STB(N)/STropZspot no. 2 approaching the p. limb, and SEBZspot approaching the CM.(m). 2008 May 06, 20h 50m UT. CM1= 76.4,CM2= 133.3, CM3= 350.9. (Drawing by Gray,×365). STB(N)/STropZ spots nos. 1 and 2 ap-proaching the p. limb.(n). 2008 June 10, 21h 48m UT. CM1= 136.9,CM2= 142.9, CM3= 317.5 (Drawing by Abel,×222). STB(N)/STropZ spot no. 1 on the CM.

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derived by each observer. However the results show that visually,the SEB(N) and SPB were the darkest belts and that the EZ(S) wasthe brightest zone.

Table 5 shows the visual colour estimates for the belts, zonesand rings. This table also shows a visual interpretation by theDirector of the colours of the major belts and zones shown in someof the best colour images.

Both visual and digital observations showed a noticeable col-our difference between the two hemispheres.

Visually, the northern hemisphere was noted to be a green tinge(Abel), blue grey (Graham), bluish to green (Gray), a cold bluecolour (Heath), and slate blue (McKim). This contrasted with thewarm tone of the southern hemisphere

These colour contrasts are also shown in the colour images ofthis report.

The planet

South Polar Region (SPR)

Saturn’s high southern latitudes and its South Pole were presentedat a more oblique angle during this apparition compared to theprevious apparition. During 2007 December, the polar inclinationto the Earth was only −5.6° (Figure 1). This increased to −9.9° atthe beginning of May, therefore allowing a slightly improved viewof this region during the spring.

The most distinct feature of the high southern latitudes was theSouth Polar Region Band which formed a dark northern border tothe SPR.

Visual intensity estimates (Table 4) and many digital imagesshowed this belt to be dark and of a similar intensity to the SEB(N).The Band generally appeared dark grey. It also appeared very darkin red light images but less obvious in green and blue light images(Figure 3). Peach’s high resolution images taken in December (Fig-ures 2 and 5(b)) showed the Band to be double with a faint narrownorthern component and a darker broader southern component.

At lower resolution, the SPR generally appeared uniform andgrey. In higher resolution observations, a lighter zone (the SPRzone) was sometimes detected immediately to the south of theBand with a darker region further south which was designatedas the South Polar Cap (SPC) by a number of observers. Thesefeatures were generally detected in the spring when the polarinclination to the Earth was larger, although Gray recorded theSPC throughout the apparition; sometimes with a darker rim.The SPC sometimes showed a very slight ochre colour in someimages.

During the previous apparition,6 the SPR zone had a reddishcolouration as shown in Figure 5(a). During this apparition, it ap-peared grey in most images (e.g. Figure 5(b)) although Gray gener-ally assigned a brown/ochre colour to this region.

South South and South Temperate Belts and Zones

A narrow grey SSTB was sometimes detected. This belt was vis-ible in red light images and sometimes in green light images but notin blue (Figure 3).

The SSTZ appeared slightly more shaded than the STZ andboth appeared light grey/dull white.

South Temperate Belt (STB)

This was a distinct belt which appeared double, separated by alighter pale yellowish zone in higher resolution observations. Thesouthern component appeared darker. Both components were vis-ible in many red and green light images but only a single belt wasgenerally visible in blue light images (Figure 3).

On 2007 Oct 18, Gray observed the f. end of a dark section ofSTB at L3= 331. He further observed a f. end of a dark section ofSTB(N) at L3= 105.8 on Dec 11.

STB(N)/STropZ light spots

ObservationsThe major event of this apparition was the appearance of two lightspots centred within the STB(N). These often extended furthersouth into the STB(S) and further north into the South TropicalZone (STropZ).

These are designated as STB(N)/STropZ spots no. 1 and 2.Both spots were well observed with the majority of the obser-

vations made by digital imaging techniques with telescopes of200mm or larger. However both were detected visually by Gray andspot no. 1 was once detected visually by Abel.

Figure 7 shows the appearance of each of these spots duringthis apparition. Spot no.2 is also shown in Figures 3(b) and 15(d).

Spot no. 1 was first recorded by Go on 2008 March 18 when itwas only faintly visible (Figure 7(g)). Casquinha imaged a similarSystem 3 longitude on March 14 but this observation only showedspot no. 2 but not no.1. Consequently spot no. 1 seems to haveappeared between these dates.

Spot no. 2 was longer lived and the first observation recordingthis spot in the Section archive was also made by Go on 2007December 3 (Figure 7(a)).

Both appeared as light circular or oval spots. However a highresolution observation by Go on May 1 showed that each had amore complex internal structure (Figure 7(k)). Both spots some-times appeared brighter in green light images.

The longitudinal extension of spot no. 1 was 7.5° (±2.1° 1σ)from 14 observations, whereas the longitudinal extension of spotno. 2 was 7.5° (±3.2° 1σ) from 13 observations. The measure-ments suggest that both spots showed some changes in sizeduring the apparition. Changes in intensity were also observedas shown in Figure 7.

Figure 8 shows the positions of the centre of each spot vs. timerelative to longitude System 3.

Table 6 shows the latitudes, longitudinal drifts and rotationperiods derived for both of these spots. This table shows that theaverage drift derived for spot no. 1 with respect to System 3 wasslightly greater than the derived average drift for spot no. 2 but thedrifts were very similar allowing for measurement error.

During February and March, there were four additional ob-servations of light spots or breaks in the STB(N) (as shown inFigures 7(f) and 7(g)). The derived positions for these spots arealso shown in Figure 8 and they lie on a linear track. If theseobservations were indeed of the same object (designated spotno.3), then it had a greater retrograde drift with respect to Sys-tem 3 (23.3° (± 1.9 1σ) over 30 days) than derived for spots1 and 2 even though it lay at a similar latitude (40.9°S ± 1.5 (1σ)from 4 observations).

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Interestingly, the potential track of spot 3 intercepted that ofspot no 2 in early February, when spot 2 was only faintly visible(Figure 7d). There were no observations of any spots along theprojected track before February.

Correlation with atmospheric electricity observationsThe Cassini spacecraft includes a Radio and Plasma Wave Sci-ence (RPWS) instrument. It is used to receive and measure radiosignals coming from Saturn and its environment and continuessimilar work made by the Voyager spacecraft.

Observations by this instrument have measured the so-calledSaturn Electrostatic Discharges (SED).7 These are the radio signa-tures of lightning flashes. Observations by the Cassini ImagingScience Subsystem7 have shown that during 2008, the SED epi-sodes were strongly correlated with the two major spots describedabove and so indicated these to be convective thunderstorms.

South Tropical Zone (STropZ)

This was one of the brightest zones and generally appeared yel-low or cream in colour.

South Equatorial Belt (SEB)

The South Equatorial Belt was one of the most prominent beltsand this is shown in the various figures of this report and the

average visual intensity estimates (Table 4). It was easily visible insmall telescopes. Bradley was able to see this and the NEB visu-ally in a 114mm telescope and Tyler was able to image both beltsusing an 80mm refractor.

As during the previous apparition, this belt was noticeablydouble and both components were visible in relatively small aper-ture telescopes. The SEB(N) was darker than the SEB(S) and theintermediate zone was similar in appearance to the STropZ. Thecolour contrast between the SEB and NEB has already been dis-cussed earlier in this report.

Higher resolution observations (Figure 2) showed the SEB(S)had two distinct components of similar intensity. The SEB(N) wasalso closely double with a darker northern component.

On October 18, Gray observed an indefinite p. end of a darksection of SEB(S) at L3= 318. On February 9, McKim observedsome condensations on the SEB(N) p and f the CM with CM3= 16.On Oct 20, Gray recorded a small projection on the SEBn at L3=122. Abel also sometimes recorded some projections on the SEBn.

However the most interesting feature of this region was a lightspot in the SEB(Z). This is shown in Figures 7(d), 7(g), 7(l) and 9.Thisspot never appeared prominent and was often difficult to detect

Figure 8. Drift chart of the centres of the two light spots (labelled 1 and 2)observed in the STB(N)/STropZ during 2007/2008. The positions are given relativeto longitude System 3 and were derived from image measurements and visual centralmeridian transits by Abel, Arditti, Buda, Casquinha, Foulkes, Go, Hill, Lawrence,Phillips, Sampson, Sharp, Tatum and Tyler. Also shown are the four observations ofwhat may have been another single light spot at a similar latitude (labelled no. 3).

Figure 9. The SEBZ spot.(a). 2008 Jan 12, 02h 55m UT. CM1= 267.9, CM2= 103.4, CM3= 100.6.(Casquinha). The SEBZ spot is just p. the CM.(b). 2008 Feb 03, 01h 20m UT. CM1= 344.1, CM2= 292.6, CM3= 353.7.(Arditti). The SEBZ spot is just p. the CM.

Notes:L3(O) is the System 3 longitude at opposition on 2008 Feb 24. () indicates the estimated opposition longitude for any spot not observed atopposition. DL3 is the drift relative to System 3 in degrees of longitude per 30 days.

Table 6. Longitudes and drifts, 2007/2008

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even on the best images. It was not detected visually. This faint-ness sometimes made it difficult to derive its extent in longitudebut an approximate value of 4° to 6° is indicated.

Table 6 gives the derived drift and latitude for this spot. It had arapidly prograding drift with respect to System 3 and this drift wassimilar to that of a spot observed at this latitude during the previ-ous apparition.6

Figure 10 shows the longitude of this spot vs time in the samelongitude system used for the SEBZ spot during 2006/2007 appa-rition6 (i.e. −7.1° per day with respect to System 3). The positionsof the 2006/2007 spot are also shown.

The first observation of this spot on 2008 January 12 lay within9.6° of the extrapolated track of the SEBZ spot observed in the2006/2007 apparition. Further, both of these spots showed similarrapidly prograding longitudinal drifts with respect to System 3.Although it is not completely certain, these results indicate thatthe two spots could have been identical.

The Equatorial Zone (EZ)

The EZ(S) was the brightest zone on the planet and generallyappeared yellow or cream in colour.

A faint grey belt was visible in the southern part of this zone ata latitude of approximately 15.3°S. This belt was also observedduring the previous apparition.

A broad grey EZ Band (EZ B) was visible and sometimes thishad a darker northern edge.

The southern section of the EZ(N) was visible between the EZB and either the southern edge of Ring C or when seen throughring C, the southern edge of the SH R on G.

A narrow strip of the northern EZ(N) was visible to the north ofthe northern edge of the SH R on G when visible, or the northernedge of Ring A where projected onto the planet. This too appearedcream or yellowish.

Figure 10. Drift chart of the SEB and SEBZ spots during 2006 to 2008.This chart has been generated in a longitude system that moves at 7.1° perday faster than System 3, as in Ref. 6. During 2007/2008 the SEBZ spotpositions were derived from observations made by Arditti, Go, Hill, Law-rence and Tyler.

North Equatorial Belt (NEB)

When Saturn reappeared in the morning sky in late 2007, observ-ers noted that a very broad NEB was visible. Subsequent observa-tions made at higher resolution showed the NEB had a narrowfainter southern component and a much broader darker northerncomponent. This is shown in many of the figures of this report.High resolution observations such as Figure 5(b) showed thatboth the NEB(S) and NEB(N) had a double structure.

The colour contrast between the NEB and the SEB has beennoted earlier in this report.

Table 4 shows that the latitude range of the NEB was similar tothat of the SEB.

North Tropical Zone (NTropZ)

This zone showed a bluish colouration.

North Temperate Belt (NTB)

This belt had a similar colouration to the NEB. Some high resolu-tion observations showed it to be closely double with a darkersouthern component.

On October 24, Gray recorded an indefinite f. end to a darkersection of this belt at L3= 144.

North Temperate Zone (NTZ)

This zone was similar in colour to the NTropZ.A darker grey region was recorded between the north edge of

the NTZ and the planet’s northern limb. This may have been partof the North Polar Region (NPR). Gray reported a darker southernedge to this region.

The rings

Introduction

The rings were at a lower inclination to the Earth compared to theprevious apparition so any ring features were generally only vis-ible in the ansae. The appearance of the rings during this appari-tion is shown in many of the figures of this report. Figure 11 showsa mixture of both visual and imaging observations illustrating theappearance of the rings with different apertures.

Ring A

Ring A was divided into a darker outer section (A1) and an innerlighter section (A2). The width of A1 was greater than that of A2 asshown in Figures 11(a) and 11(c).

Higher resolution observations showed that A1 was dividedinto a brighter but narrow outer section and a broader darker innersection (Figures 11(a) and 11(c)). Such observations also revealedthe Encke gap at the outer edge of this broader darker section (SeeFigures 2, 7(k) and 11(a)).

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Ring B

Ring B is divided into a brighter outersegment (B1) and a darker inner segment(B2). McKim sometimes recorded agradual brightening from B2 to B1 ratherthan a distinct boundary between thesefeatures. Gray often recorded a darker in-ner segment to ring B2 (designated B3).Gray also sometimes suspected some‘spoke’ activity on Ring B.

During April, some visual observersrecorded Ring B as ‘dull’.

Ring C

Before opposition, Ring C was readily vis-ible when projected over the planet(C(M)). After opposition, the ring shadowon the globe (SH R on G) was visiblethrough C(M).

Many images showed ring C in each ansa. Visually McKimnoted it to be rather light. A number of visual observers were ableto detect Ring C in each ansa with apertures of approximately200mm (Figures 7(n), 11(b) and 11(d)). Adamoli reported a vagueindication of this with an aperture of only 125mm.

Figure 12. Opposition effect. (Tyler).(a). 2008 Feb 18d, 23h 22m UT. CM1= 189.9, CM2= 242.8, CM3= 194.2.(b). 2008 Feb 23d, 00h 08m UT. CM1= 354.4, CM2= 277.0, CM3= 223.6.(c). 2008 Feb 24d, 23h 30m UT. CM1= 220.9, CM2= 79.7, CM3= 24.0.

Figure 11. Ring detail.(a). 2008 Feb 12d, 14h 10m UT. CM1= 200.0, CM2= 99.0, CM3= 58.1. (Image by Wesley). This shows theEncke gap and Tethys Nf.(b). 2008 April 16d, 00h 05m UT. CM1= 101.7, CM2= 112.5, CM3= 355.2 (Drawing by Phelps, ×290).(c). 2008 Apr 17d, 19h 45m UT. CM1= 197.8, CM2= 149.9, CM3= 30.4. (Drawing by McKim, ×256 & ×410).(d). 2008 May 1, 19h 45m UT. CM1= 146.0, CM2= 5.5, CM3= 229.1 (Drawing by Line, ×300).

At lower resolution some observers recorded a ‘division’ at theboundary of A1/A2 (Figure 11(b)). This may have been due todetection of the darker segment of A1 described above, as no truedivision exists here.

Cassini’s Division

Cassini’s Division was seen in each ansa. Only the highest resolu-tion observations showed the division where projected againstthe planet (such as Figures 2, 7(e), 7(g), 9(a) and 11(a)).

Most visual observers and all images showed the division ineach ansa to be black. However both Gray and McKim recorded itas dark but not black. Each only assigned it an intensity of lessthan 8 (Table 4). Both of these observers are very experiencedvisual observers and used large instruments (415mm and 410mmDall−Kirkhams respectively). It is possible that this effect mayhave been due to irradiation from the bright rings becoming moreobtrusive with a narrower ring inclination.

Sunlight passing through the Cassini Division illuminates a rela-tively narrow strip of the planet within the shadow cast by therings onto the globe (SH R on G). Although this illuminated strip isoften hidden by the rings from the perspective of an observer onthe Earth, under certain circumstances it is theoretically visible.During the 2006/2007 apparition,6 this illuminated strip was ob-served through the Cassini Division itself.

During the early part of this apparition, the ring shadow wasprojected onto the planet’s northern hemisphere, north of Ring A.This narrow illuminated strip was theoretically visible to observ-ers on the Earth, lying within the shadow to the north of the pro-jection of Ring A against the planet.

The width of this strip is very small in angular terms and sowould be difficult to detect even with larger telescopes.

However Peach managed to record this strip in his imagestaken from Barbados on 2007 Dec 6 (Figure 5(c)) and Dec 10(Figure 2). This detection of this feature may be a first for anamateur observer.

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The opposition effect

For a few days around the time of opposition when the phaseangle is small, the rings showed a noticeable brightening (Figure12). This is the so-called ‘opposition effect’.

A report of Arditti’s observations of the ring brightening forthis opposition has already been given in Reference 8. Ardittimade observations in the infrared, red, green, blue and ultravio-let wavebands and noted that the brightening of the rings rela-tive to the planet’s globe was at its greatest in the infrared andultraviolet wavebands.

The shadows

The Globe Shadow on the Rings (SH G on R)

Before opposition, the SH G on R was projected onto the southernarm of the p. ansa. After opposition, it was projected onto thesouthern arm of the f. ansa.

The Ring Shadow on the Globe (SH R on G)

During the early part of the apparition, the Earth lay further northwith respect to the ring plane than the Sun (Figure 1) and so the SHR on G was seen projected onto the northern hemisphere as shownin Figure 2.

A few days before opposition the inclination of the rings to theEarth and Sun was identical. The SH R on G was not visible in Tyler’simage taken shortly after opposition but this was taken in less thanfavourable seeing (Figure 12(c)). After this time, when the Earth layfurther south with respectto the ring plane than theSun, the SH R on G becamevisible through Ring C(M).On February 28, it was nar-rowly visible throughC(M) (Figure 7(e)) andbecame wider within C(M)as the inclination of therings with respect to theEarth increased, as shownin Figure 7(f).

The satellites

Many observers were able to de-tect the satellites Titan, Rhea,Iapetus, Dione and Tethys, eithervisually or by digital imaging.Heath recorded Titan with a yel-lowish colour during April. A fewobservers were also able to detectthe fainter inner satellites Mimasand Enceladus, as shown in Fig-ure 13.

The inclination of the orbits ofseveral inner satellites was suchthat transits, occultations andeclipses occurred during this ap-parition.

Enceladus is a small body witha diameter of approximately500km. This is only a tenth the angular size of Titan, making it avery challenging object to observe in transit.

However on two separate nights, Go was able to image a lightspot exactly in the position predicted for Enceladus by theWinJUPOS software. In both cases, the shadow of Enceladus wasalso in transit but not detected. Details of these observations aregiven in Table 7. Figure 14 shows the image taken on April 4, withthe WinJUPOS predicted position for comparison.

A number of transits of Tethys and its shadow was imaged byseveral observers and details are given in Table 7. The highestresolution observation of such an event was made by Peach on2007 Dec 10 (as shown in Figure 1) where the disk and shadow ofthe satellite were clearly resolved.

Other typical images of these transits are shown in Figure 15.These events were only recorded by digital means and not de-tected visually.

Transits of Dione occurred from early 2007 October until theend of 2008 February (based on WinJUPOS predictions). Only oneobservation of a Dione transit was made, by Gray, as shown inTable 7 and Figure 16.

Figure 13. Saturn’s satellites.(a). 2008 Mar 30d, 23h 30m UT (Lyon). Negative image showing three ofSaturn’s satellites including Enceladus.(b). 2008 Apr 04d, 03h 48m UT (Hill). The satellites from Mimas to Titan.

Figure 15. Tethys and its shadow in transit.(a). 2007 Dec 12d, 04h 50m UT. CM1= 79.5, CM2= 193.8, CM3= 228.3. (Arditti).Green light image. Tethys and its shadow in transit (highlighted).(b). 2008 Jan 17d, 02h 38m UT. CM1= 160.0, CM2= 194.3, CM3= 185.4. (Cas-quinha). Green light image. Tethys in transit (highlighted).(c). 20087 Apr 12d, 22h 47m UT. CM1= 43.2, CM2= 152.6, CM3= 39.0. (Law-rence). The shadow of Tethys in transit with Tethys itself p. the planet’s disk.(d). 20087 Apr 20d, 10h 15m UT. CM1= 236.4, CM2= 104.3, CM3= 341.7.(Buda). Tethys and its shadow in transit. STB(N)/STropZ spot no. 2 is approachingthe p. limb.

Figure 14. Enceladus in transit.(a). 2008 Apr 4d, 11h 38m UT. CM1=97.6, CM2= 119.6, CM3= 16.2. (Go). Theposition of Enceladus is indicated.(b). The position of Enceladus in transitat the time of the above observation aspredicted by the WinJUPOS software.

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Acknowledgment

The author would like to thank Dr Georg Fischer for providing acopy of the paper ‘Atmospheric Electricity at Saturn’.7

Address: 2 The Hawthorns, Henlow, Beds. SG16 6BW. [mike.foulkes@btinternet.com]

References

1 Astronomical Almanac, 20072 Astronomical Almanac, 2008

Figure 16. 2007 Dec 09d, 04h 00m UT. CM1= 37.1, CM2=249.5, CM3= 287.5. (Drawing by Gray, ×365). Dione enteringtransit (highlighted).

3 Handbook of the BAA, 20084 Peach D. A., ‘Planetary observing missions to Barbados in 2005 and

2006’, J. Brit. Astron. Assoc., 117(6), 301−308 (2007)5 WinJUPOS free software available for download from: http://

jupos.privat.t-online.de/index.htm6 Foulkes M., ‘Saturn during the 2006/2007 apparition’, J. Brit. Astron.

Assoc., 120(4), 206−218 (2010)7 Georg Fischer et al., ‘Atmospheric Electricity at Saturn’, Space Science

Review, 137, 271−285 (2008)8 Arditti D., ‘Saturn and the ‘opposition effect’’, J. Brit. Astron. Assoc.,

118(2), 106 (2008)

Received 2012 January 30; accepted 2012 May 30

Table 7. Satellite and shadow transit observations during 2007/2008

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