Saturn at its magnificent best.
observers can expect to see as the ringed Planet approaches its two closest
oppositions in over 15 years.
Damian A. Peach
Saturn Section, Assistant Director.
Above is an image of Saturn obtained by the Author in October of 2000 using his
30cm reflector and CCD camera. The Planet was 1.25 billion Km’s distant at the
time. The magnificent ring system dominates the view, bisected by the dark
many times has been said, that the rings of Saturn are one of the most memorable
and magnificent telescopic sights in the night sky?. Well, many would agree this
is certainly true, and the good news is for observers both old and new, now has
never been a better time to study Saturn and its rings as it now approaches its
two best oppositions for northern hemisphere observers since 1974. The last time
Saturn was tilted at its maximum toward us was back in 1988, though the
Planet’s declination during this time was unfavourable for Northern hemisphere
observers, as the Planet passed through Scorpius into Sagittarius. Now, once
again after a very long wait, the “ringed Planet” is again fast approaching
its best for observers across the northern hemisphere.
is the Solar System’s second largest Planet with an Equatorial diameter of
120, 536 Km’s, making it some 9.5 x the diameter of the Earth. Its polar
diameter is almost 10% smaller at the poles making it the most flatted Planet in
the Solar system. It is composed of various gases, primarily hyrdrogen, helium,
methane and ammonia. It has no solid surface beneath its thick clouds, which at
its equator are swept around by winds of well over 1000kph – far stronger than
the winds on Jupiter. However, despite this it is a much less active world than
its giant neighbour, often appearing devoid of large storms and spots, unlike
Jupiter – though large bright storms do occasionally occur, such as back in
1990, which I will cover in more detail later.
like Jupiter is divided into rotation periods, due to the varying motions of the
observed atmosphere at different latitudes. These periods are called System 1
(which is assigned to the equatorial regions) and assumes a drift rate of
844.3 degrees per day (period 10h 14m) and System III (which is assigned to the
remainder of the planet) and assumes a drift rate of 810.8 degrees per day
(period 10h 39m 22s.4). Most of the transient Saturnian features the amateur
will observe occur in the Equatorial Zone of Saturn (System I.) Only very
occasionally are small spots followed in areas outside the Equatorial Zone (EZ.)
It takes Saturn over 29 years to orbit the Sun once, meaning its position
in the sky changes quite slowly from year to year, meaning it can remain well
placed for many years, but for northerly observers, it can of course also remain
poorly placed, as was the case during the mid-late 1980s into the early 1990s.
The rings are without doubt Saturn’s most well-known, and well-observed feature, ever since Galileo first turned his small telescopes on the Planet back in 1610. The rings are divided into 5 main rings. There are two main rings, called rings A and B, of which B is the brightest. Then ring C or the Crepe ring, which is inside the B ring and appears very dusky. Rings D and F are faint, and not observable to amateurs. Two main divisions dark division are observable to amateurs. The Cassini Division (a 4,700 km gap) which separates the bright A and B rings is easily observed in Telescopes as small as 2.4 inches (60mm) when the rings are well presented, as is currently the case. The other division (which is located near the outer edge of Ring A) is know as the Encke Division, a small 325km gap actually discovered by Astronomer, James Keeler in 1888. The division is much smaller, and will require a telescope of at least 9 inches (235mm) aperture, and very steady atmospheric seeing conditions to resolve. Though the rings our very wide (almost 300,000km) they are extremely thin (just 1 km) making them almost invisible when they are presented edge on to us, as was last the case back in 1995.
Fig 01a: A simulated view of Saturn with the major belts, zones, rings and divisions indicated.
Another well known feature of the Saturnian system is its many moons (28 in total.) Several of these small worlds are observable to amateur observers. Equipped with a typical 200mm telescope, the observer will be able to follow Titan, Dione, Enceladus, Iapetus, Mimas, Rhea, and Tethys. A 300mm telescopes will reveal the faint Hyperion. There movements can be easily followed from night to night. Titan is the largest, brightest and most famous moon. It has a thick atmosphere of Nitrogen, giving the moon an orange cast. Telescopes of 200mm or greater aperture under very stable seeing will reveal Titan as a small disk, spanning just 0.9” arc seconds in angular diameter.
coming 2002 and 2003 apparitions.
coming two apparitions of Saturn, as previously mentioned are the most
favourable to occur in many years for those observers located in the Northern
hemisphere. The Planet remains in Gemini throughout this period at over +20
declination. The Earth will is now passing over 26 degrees south of the ring
plane, presenting us with a fine open view of the Southern hemisphere of the
Planet, peaking over the next 18 months. The Earth will be at its greatest
distance South of the Ring plane in March of 2003, when it will be just over 27
degrees South. Also, the Planet is passing closest to Earth during the next two
apparitions, which will mean the Planet will appear larger in the telescope and
brighter than at any other time in the last 25 years. This peaks on Dec 31st,
2003 when Saturn will appear 20.55” arc seconds in diameter and shining at
distance between Earth and Saturn 1988-2005.
1988 Jun 20
1989 Jul 02
2005 Jan 13 8.0756 +21° 20'
table listing all Saturn-Earth minimum distances during its last 16 apparitions.
Note, not all actually occurred on opposition night during that apparition. Note
that the coming 2002 and 2003 oppositions are the closest in many years.
views of Saturn on the next two opposition nights in 2002 and 2003. These years
will present us all with an excellent opportunity to study the ring system in
worthwhile visual observations.
what can the observer expect to see during the next two apparitions. Well, or
course this will depend upon the telescope he/she is using, and the experience
of the observer. In my opinion a 100mm (4 inch) refractor or 150mm (6 inch)
reflector is the smallest telescope with which detailed observations can be
made. 250mm aperture (10 inch) and larger telescopes will provide glorious views
under stable seeing conditions, and very useful, and valuable observations can
be made using such instruments.
the start of making an observation the observer should ensure that there
telescope is properly collimated, and they have a pencil and pad at hand to
sketch and note the details of there observation. Its best to start with a blank
outline of the correct aspect (available from the BAA or ALPO Saturn sections)
and slowly fill in the details that can be seen. Points that should be noted by
the observer are in general:
Position and shape of shadows of globe on rings (ShGR)and rings on globe (ShRG).
Any change in the position or form of the belts on the globe.
Any star that appears likely to be occulted by the globe or rings. These events
are very rare and may not be predicted far in advance.
Position of any bright or dark spots within the belts and zones which appear to
rotate with the planet.
Any change in the visibility or any irregularity of the outline of Ring C.
Any indications of a faint ring inside or outside the principle rings.
Position of any dusky patches or subdivisions on the ansae of the rings.
brightness or colour asymmetry visible across the ring system.
with smaller telescopes should carry out items '1' to '3' with items '4' to '8'
being in the remit of larger apertures.
also worthwhile to make intensity estimates of Belts, Zones and Ring system to
help monitor any changes that may occur. Colour estimates are also worthwhile by
those using telescopes of 200mm (8”) and larger. Transit estimates of any
bright or dark spots seen on the Planet is also very important, but again this
is usually only within reach of larger telescopes. Colour filters, as with the
other Planets, are of great use on Saturn. I would recommend Wratten #21
(orange), #58 (green) and #38A (blue) filters for use on 200mm – 400mm
aperture telescopes. Unfiltered views in larger telescopes are also very
worthwhile, as stubble shades of colour can often be noted, both on the Planet
itself, and the ring system.
the observer should note with their observation the date and time (UT),
Telescope, magnification, filters used, and the quality of the seeing conditions
using either the Pickering or Antonaidi scales.
would very much encourage observers to submit their work to the Saturn sections
of the British Astronomical Association (BAA) and Association of Lunar and
Planetary Observers (ALPO.) These organisations specialise in producing detailed
reports each Saturn apparition based on the work of amateur observers worldwide.
drawing based on an actual visual observation made in 1997 by experienced
planetary observer Carlos Hernandez of FL, USA. The drawing accurately
represents the visual appearance of Saturn in a typical amateur telescope under
good seeing conditions.
imaging and photography.
well as being a rewarding object for the visual observer to study, it is perhaps
equally (or perhaps more so) rewarding an object for today’s telescopes
equipped with CCD cameras. Though a bright object visually, for CCD cameras,
Saturn is perhaps the most challenging Planet to capture high quality images of.
Its low surface brightness means that fairly long exposures will be required. In
general a telescope of at least 200m (8”) is required to produce high quality
results. Those equipped with larger apertures will find it easier as exposure
times will be reduced.
a night of CCD imaging I would recommend that the observer use a set of colour
filters to image the Planet to produce a set of Red, Green and Blue filtered
images – which must be used in conjunction with a filter to block IR light. If
these images are obtained correctly within a ~5min window the may be composited
to create a true colour image of the Planet. The CCD can record allot more
detail on Saturn’s low contrast disk than is visible to the eye, as we can
sharpen images to give us a clearer picture of the Planet. Red filtered images
are especially useful for viewing the belts and zones of Saturn, and high
quality images often reveal many belts and zones are present. High quality true
colour images often reveal stubble colour shades of the belts and zones,
occasionally ranging from red to green!. CCD images also give us a very clear
picture of the ring system. Images of the highest resolution obtained by
amateurs reveal the tiny 325km Encke Division in Ring A – which subtends a
width of less than 0.1” arc seconds. As many amateurs know, CCD imaging is a
very extensive subject, and I would advise the reader to contact the affore
mentioned organisations for further advice on this subject.
of Saturn through the telescope is a very challenging operation, and is only
recommended using telescopes of at least 300mm (12”) aperture. Photograpy has
now been almost completely superseded by CCD imaging. During the late 1980’s
observers such as Don Parker and Isao Miyazaki produced high quality photographs
of Saturn using large telescopes under excellent seeing conditions.
with visual observations, I greatly encourage observers to submit their CCD
images of Saturn to the BAA and ALPO Saturn sections for analysis. The great
majority of data obtained by amateurs today is by the use of CCD cameras, and
they have greatly aided in our study of the ringed Planet.
set of 3 images obtained by the author, showing the different appearance of the
Planet in Red (left), Green (centre) and Blue (right) filters.
activity on Saturn – what can we expect in 2002-03?
recent years, the activity on Saturn has been quite low, but there have been
some very interesting periods. During 1999 the band surrounding the South Polar
Cap exhibited a strong red colouration, discovered by Toshihiko Ikemura and the
author. This is the first time such a strong colouration has been noted, and it
was followed by amateurs both visually and with CCD cameras during the autumn of
1999. Also, distinct green coloured bands were also observed in high-resolution
images. These colours have been present on Saturn every apparition since then,
though now not as distinctly. Some transient spots have been followed in recent
years, especially during 1998 and 2000.
interesting phenomenon noted last year in high quality images, was a distinct
colour asymmetry observed in Ring A. This asymmetry is still under investigation
by observers, and more observations are needed to fully understand its nature.
coming apparitions I would expect to see a continuation of the present level of
activity. Occasionally huge outbursts of activity can occur, known as great
white spots (GWS.) Theses spots erupt in Saturn’s equatorial zone (EZ),
usually once every thirty years. This last occurred in September of 1990, when
observers were treated to such an outburst, lasting several weeks, and
encircling the entire EZ. Notable activity also occurred in 1994 and 1996,
though not to the degree of 1990. Observers should always be on the lookout for
periods of increased activity across the Equatorial Zone of the Planet.
Images of Saturn obtained in 1990 by the Pic du Midi observatory 1 metre
cassegrain, showing the Great White Spot outbreak that occurred that year. This
was the biggest such eruption to occur on the Planet in 30 years.
Mission to Saturn.
over the next few years that interest, especially from the professional
community, will greatly increase as NASA’s Cassini spacecraft finally reaches
Saturn in July 2004. It was launched back on October 15th, 1997, and
has been en-route to Saturn ever since. It passed Jupiter in late December of
2000 and sent back some of the best images of the Planet to date. We can only
hope, (and expect it) to the do the same when it reaches Saturn.
Fig 06: NASA’s
Cassini spacecraft was launched back in 1997, and is due to spend 4 years
studying Saturn from 2004 to 2008. This artists view depicts the release of the
Huygens probe toward Titan.
craft itself consists of two main sections. The Cassini orbiter, which will stay
in orbit around Saturn to study its atmosphere, rings, and general environment.
The second main section (and mission objective) is to deploy the Huygens probe
down through the Atmosphere of Saturn’s largest moon, Titan, to the surface.
This is perhaps the most fascinating and ambitious part of the whole mission, as
astronomers currently do not know what the surface of Titan is like, as it is
permanently hidden from view by the thick nitrogen atmosphere that covers Titan.
By the time the spacecraft reaches Saturn it will have travelled some 3.2
billion Km’s to reach it. The mission is planned to last until 2008, but this
will probably be extended, as was the case with Galileo at Jupiter.
Now has never been a better time for the amateur observer to make a real contribution to the study of Saturn. The novice observer equipped with a typical 150mm telescope can make detailed observations with practice, and today’s advanced amateur, equipped with powerful modern telescopes and CCD cameras, can produce data that is useful not only to amateur organisations, but professionals as well. As Cassini draws ever nearer to its destination in 2004, we come into a very interesting and exciting time as everyone turns there attention to the Solar System’s most beautiful Planet.
Peach. July, 2002.