.
SATURN RING CROSSINGS
 -------------------
 John Pazmino
 NYSkies Astronomy Inc 
 nyskies@nyskies.org
 www.nyskies.org 
 2006 January 26 initial
 2021 September 7 current 

Introduction
 ----------
    In summer 2005 Saturn's rings closed up enough to reveal the 
planet's north pole, covered by the rings during the fully open phase. 
The rings are gently closing toward their edgeon aspect in 2009. This 
revelation of the north pole sparked some NYSkies banter about the 
changing tilt of the rings. The dropdead views of the rings presented 
by the Cassini probe as it orbits Saturn also fueled this discussion. 
The general chat drifted to the situation where the rings are turned 
edgeon to Earth. Many NYSkiers recall the edgeon phase of the rings in 
1995-1996;.sOme remember those of 1979-1980 and 1965-1966. 
    Since then we enhoyed the edgen phase of 2009 and look forward to 
that of 2025. 

Brief history
 -----------
    When Galileo inspected Saturn with his simple telescope, he 
thought the planet was made of three globes. One was a large central 
globe, presumably the very orb of Saturn himself. The others were 
lateral orbs, smaller and dimmer, one to the east and one to the west. 
Anthropomorphicly they assisted Saturn in his slow gait thru the 
zodiac. 
    Other astronomers examined Saturn with scopes similar to Galileo's 
and came up with alternative interpretations. To home astronomers 
today these notions seem hilarious. What blew the astronomers's minds 
was the slow atrophy of these appendages after a couple years! Were 
they deceived somehow for their earlier existence? Are these ears, 
wings, handles, flaps really so mutable? Did the main body of Saturn 
somehow eat them? 
    Then just as mysteriously they came back! First as dim smudges, 
then as small round blobs, finally as the fullsize globes or wedges or 
triangles. And so for three rounds of coming and going, the side lobes 
of Saturn puzzled astronomers to no end. 
    this bizarre behavior of Saturn occurred several times.Yhe 
appendages disappaeared in 1612, 1627, 1642, 1656. Halfway between 
thee years they bloomed to full prominence. 
    Huygens in the 1650s studied Saturn with improved telescopes and 
finally sussed out that Saturn is surrounded by a thin flat ring 
concentric wiyh yhe planet's globe.. He then correctly explained why 
the lateral whatevers vanished and returned. The ring is seen at 
various tilts as Saturn went around the Sun.  When it turned edgeon. 
to Earth's sightline, It was too thin to see! 
    Since then the edgeon phase of the rings was eagerly awaited, 
according as the general interest in planets waxed and waned in the 
late 17th and whole 18th, 19th, and 20th  centuries. 

Ring system
 ---------
    In small scopes Saturn has two rings. A broad inner band, the B 
ring, and a narrower, slightly darker, outer band, the A ring. The two 
bands are separated by a thin gap, Cassini division. The rings are 
accurately circular, centered on the planet. Larger scopes may 
discern a gauze or tissue band against the inner edge of the B ring. 
This is the C ring. None of the other rings, D to whatever, found by 
spaceprobes. are within reach of small instruments. 
    The rings are made of distinct separate particles of boulder to 
house size. Each is in its own orbit around Saturn, with some 
resonance effects. The orbital speed of the ring particles decreases 
in a Kepler function from the inner edge to the outer. 
    Here are the sizes of the rings:

  Ring    Ratio  Km rad   Discoverer 
  ------  -----  -------  ----------
  Saturn   1.00   60,330  prehistoric 
  C ring   1.23   74,206  W Bond, G Bond, W Dawes, 1850 
  B ring   1.52   91,702  C Huygens, 1655 (A & B as one ring) 
  Cassini  1.95  115,230  G Cassini, 1675 (gap in ring) 
  A ring   2.02  121,867  G Cassini, 1675 (A & B as two rings) 
  outer    2.26  136,346  several rings found by spacecraft visits 

    Over the decades, slightly different ratios were cited; the ones 
here are from the Observer's Handbook of 1996. The discrepancies seem 
too large for measurement error, yet I'm not aware of any serious idea 
that the rings over time change size or proportion. 

Ring crossing 
 -----------
    The alternate fluffing and folding of the rings is purely an 
effect of perspective. We are seeing the stable rings from different 
angles as Saturn orbits the Sun. For home astronomy needs, the rings 
are fixed in space, altho they with the whole planet have a very slow 
precession. 
    During an orbit of Saturn, Earth sees the rings on the north side, 
along their edge, on the south side, edge, and back to north. The 
complete cycle takes a Saturn year, 29-1/2 Earth years. It's the 
combination of Saturn's motion in his orbit and Earth in hers that 
bring the Earth from one side to the other, to make the 'crossing', as 
the event is usually called. The interacting Earth-Saturn motions 
cause the interval between edgeon phases to wander a bit. It averages 
out to half the Saturn year, about 14-2/3 Earth years. 

A few details
 -----------
    A proper treatment of the ring crossings requires fancy work with 
vectors and geometry.  Some solar system softwares can do the maths 
and graphics to accuately depit the ring crossings. 
    The bottom result is that on a rings's edgeon aspect, Earth can 
enjoy either one or three crossings, but not two. There can not be two 
because Earth starts on one side of the rings and ends on the other. A 
doublle crossing would put Earth back on the initial side. 
    It so happened that the last three edgeon phases were triple 
crossings! these were in 1966, 1980, 1995. Thisset of triples  may 
have jaded some of us about these events! In fact, te first two in the 
21st century are singlec rossings, 2009 and 2025. 
    When the edgeon phase has only one crossing, it occurs near 
Saturn's superior conjunction. Single crossings are difficult or 
impossible to observe with typical home optical methods. Saturn is 
then in twilight or d aylight. 
    Triple crossings straddle Saturn's opposition.  The first one 
occurs when Saturn is in the morning sky; middle one, all night sky; 
last one, evening sky. Generally all three are observable in nightt, 
away from twilight. Once in a while either the first or third is 
hidden in twilight.  
    The crossings occur when Saturn isin Aquarius-Pisces or Leo-Virgo 
(constellations, not signs). It is just where Saturn's ring plane 
lines up in space to cut across Earth's orit..Since crossings take 
place near Saturn's opposition and superior conjunction, they occur in 
or near March and september. That thee months are near Earth's vernal 
and autumnal equinox is s plsin coincidencee. 
    The fully fluffed ring phase occurs near the winter and summer 
solstices. Saturn is then  in Scorpius-Sagittarius and Taurus-Gemini. 
from this mechanics of ring rossings it is easy to look for them. Note 
where in the zodiac is Saturn and figure ow long is he from the 
crossing 'seasonal points'. The chart here lays out the process. 

 -----------------------------------------
  Saturn in        Opposition  Tilt     Crossings 
  ---------------  ----------  -------  ---------------------------
  Leo-Virgo        March       small    triple; try prev Dec & next 
                                        Jun for 1st & 3rd events 
                               ---------------------------------
                               large    single; try prev and next Sep 
                                        (super conjtn) for event 
 ----------------------------------------------------------------
  Aquarius-Pisces  September   small    triple; try prev Jun & next                  

                                       Dec for 1st & 3rd events 
                              ---------------------------------
                               large   single; try prev and next Mar 
                                        (sup  conjtn) for event event 
 ------------------------------------------------------------

    The actual hunt is made with a solar system software that 
accuratelymodels  the Saturn ting system. 

Missed triple crossings 
 ---------------------
    The ring crossing of 1936 was a single one, but was almost a 
triple. Or it was barely a triple. On 1937 Feb 21 Earth crossed from 
north to south for a clean event. It was hard to observe due to 
Saturn's small elongation from the Sun in evening twilight. 
    The tricky calculation is for the crossings in June 1936. By some 
calcs, there was none. On 1936 June 28 Earth approached the rings from 
the north, hovered a split-degree away, then backed away. 
    Other calcs show there was barely a crossing on 1937 June 26, 
north to south, and an other on June 30, south to north. Earth between 
the two events stayed within a millidegree of the rings. Greatest tilt 
was on June 28.
    A similar case is the crossing in 1685. In December 1684 Warth 
approached a fraction-degree from the ring plane, heading north to 
south.  Observations thru the crude scopes of the era describe the 
event as a crossing. Earth nevr reached the rings, with proximity on 
22 December 1984.  The single crossing occurred on 20 August 1685.    

Exposed ring face 
 ---------------
    The side of the rings exposed to view alternate north, south, 
north, south, and so on from the one crossing to the next. A south 
face covers part of the north half of the planet's globe.  The rings 
with north face exposed covers part of the planrt's south half. 

    That's why in 2005, with the south face in view, the north part of 
Saturn's sphere is hidden. The rings were then slowly collapsing so 
that by summer 2005 they slideh off the of the planet's north polar 
region. Catching sight of the pole after bing hidden for several years 
can be a thrill for some observers. 
    The rings are aligned with Saturn's equator. Their latitude on 
Saturn is zero degrees. On Saturn Earth has a latitude, according as 
where she is in the planet's oen sky. When this latitude is zero, 
Earth is crossing the ring plane. The technical measure of a crossing 
event is the moment when Earth's saturnographic latitude is zero. 

Sun's ring crossing
 -----------------
    The Sun also croses the ring plane  and has his oen saturnographic 
latitude. This is zero at the ring crossing. The or other side of the 
rings, away from the corssing, is seen from the Sun and is lighted by 
him.
    During a ring crossing Earth and Sun may be on the same or 
opposite sides of the rings. If on the same side we see the that side 
ighted by the Sun. If on opposite sides we see the unlighted side. 
This is not fully dark because sunlight filters thru the thin ring 
material. The face in view is merely darker than if it was lighted. 

Observing edgeon rings 
 --------------------
    In small telescopes the rings completely disappear about a week 
before the crossing and reappear about a week afterwards. During the 
crossing, the planet is naked of rings! The e aspect of the ringless 
period depends on the aperture and quality of telescope, acuity of the 
the observer's eyesight, clarity and stability of the local air. An 
edgeon event can sustain higher magnification than the casual view of 
the planet. Use the highest power the scope and air can handle, while 
keeping the image sharp and steady. It may be tough to pick out any 
Saturn moons along the edge on the rings, they being mere points in 
small scopes. 
    The rings are thin! From the visits by spaceprobes they are now 
known to be only a kilometer or so thick! If the entire breadth of the 
rings is scaled to a US letter paper, the paper would have to be about 
four nanometer thick to maintain the same proportions for the rings. 
    The rings dwindle to a narrow needle, then a hairline filament 
during the crossing season. Home astronomers in the last three 
crossings  of the 20th century eere filled with delight to watch the 
rings collapse, barely open, collapse again, before finally opening up 
for real. 

Recent and future crossings 
 -------------------------
    I give here the crossings of 1966 thru 2039.  They span the 
careers of veteran and newcomer astronomers. I include for context the 
superior conjunctions and oppositions associated with the crossings. 

  date         event         elong  comments 
  -----------  ------------  -----  ------------------
  1966 Mar 10  super conjtn    0    north face exposed 
  1966 Apr  2  1st crossing   19 W  north to south, morning 
  1966 Jun 16  Sun crossing   85 W  north face lighted 
  1966 Sep 16  Opposition    180    Saturn in Aquarius-Pisces 
  1966 Oct 28  2nd crossing  139 E  south to north, all night 
  1966 Dec 18  3rd crossing   88 E  north to south, evening 
  1967 Mar 22  super conjtn    0    south face exposed 
 -----------------------------------------------------
  1979 Sep 11  super conjtn    0    south face exposed 
  1979 Oct 27  1st crossing   29 W  south to north, morning 
 1 980 Mar  3  Sun crossing  168 W  south face lighted 
  1980 Mar 12  2nd crossing  178 W  north to south; all night z/  1980 
Mar 15  opposition    180    Saturn in Leo-Virgo 
  1980 Jul 23  3rd crossing   52 E  south to north, evenin g 
  1980 Sep 24  super conjtn    0    north face exposed 
 ---------------------------------------------------------
  1995 Mar  5  super conjtn    0    north face exposed 
  1995 May 22  1st crossing   67 W  north to south, morning 
  1995 Aug 10  2nd crossing  144 W  south to north, all night  
  1995 Sep 13  opposition    180    Saturn in Aquarius-Pisces 
  1995 Nov 19  Sun crossing   12 E  north face lighted 
  1996 Feb 12  3rd crossing   31 E  north to south, evening 
  1996 Mar 16  super conjtn    0    south face exposed 
 -----------------------------------------------------
  2008 Sep  4  super conjtn    0    south face exposed 
  2009 Mar  9  opposition    180    Saturn in Leo-Virgo 
  2009 Aug 11  Sun crossing   32 E  south face lighted 
  2009 Sep  4  one crossing   12 E  south to north, evening 
  2009 Sep 18  super conjtn    0    north face exposed 
 -----------------------------------------------------
  2025 Mar 11  super conjtn     0   north face exposed 
  2025 Mar 23  one crossing     9 W north to south, morning 
  2025 Sep 25  Opposition    180    Saturn in aquarius-Pisces 
  2025 Nov  6  Sun crossing  132 E  south face lighted 
  2026 Mar 24  Supeer conjtn   0    south face exposed 
  ----------------------------------------------------
  2038 Sep 12   super conjtn    0   south face exposed 
  2038 Oct 15  1st crossing   28 W  south to north, morning 
  2039 Jan 22  Sun crossing  123 W  south face lighted 
  2039 Mar 17  Opposition    180    Saturn in Leo-virgo 
  2039 Apr  1  2nd crossing   75 E  north to south, evening 
  2039 Jul  9  3rd crossing   44 E  south to north, evening 
  2039 Sep 26  super conjtn    0    north face exposed 
 -------------------------------------------------

Moons and crossings 
 -----------------
    Saturn's rings present severe impediments against examination of 
the sky around the planet. When they at or near edgeon, the planet 
enjoys dark sky around him. Hence, a traditional exercise for ring 
crossings was the search for new satellites. The table here gives 
those found from Earth during various crossings. All but one of the 
classical nine Saturn moons were found around an edgeon phase. With 
the visits of spaceprobes to Saturn, the discovery of moons is now 
independent of the ring tilt as seen from Earth. 
    Only one moon was ever found from ground observation well away 
from an edgeon phase. This is Phoebe, found by W Pickering in 1898. 
The rings were then almost fully open at 26 degree tilt! The discovery 
photograph is on display at the Harvard plate collection in Phillips 
Auditorium, Harvard University, Cambridge MA. The atom-sized dot of 
the moon, inside an inked circle on the glass, is not at all easy to 
spot. 

  crossing   moon        discoverer 
  ---------  ---------   ----------
  1655       Titan       C Huygens 

  1671-1672  Iapetus     G Cassini 
             Rhea        G Cassini 

  1685       Dione       G Cassini 
             Tethys      G Cassini 

  1789-1790  Enceladus   W Herschel 
             Mimas       W Herschel 

  1848-1849  Hyperion    G Bond, W Bond, W Lassel 

  1898       Phoebe      W Pickering, between crossings 

The abovee moons are the traditional nine satellites of Saturn thru 
the mid 20th century. 

  1966-1967  Epimetheus  S Larsen, j Fountain, R Walker 
             Janus       A Dollfus 

  1979-1980  Calypso     W Baum, D Currie, D Pasen, P Seidelmann 
             Helene      P Laques, J Lecacheux 
             Telesto     S Larsen, B Smith, R Walker 

    After 1980 donzens more moons were discoverd by spaceprobe visits 
at Saturn.  An edgeon phase of the rings was no longer requied.. 
    Some crossings coughed up no new moons, like 1995-1996. Others 
were unobservable for occurring too close to the Sun, like 1950. 
    I haven't found any other category of observation requiring an 
edgeon rings. Whole-globe studies are best done then. On the other 
hand, studies of the poles of Saturn require wide open rings. 

Conclusion 
 --------
    Saturn is commonly called a static, tho immensely beautiful, 
planet. It is hardly dynamic like mars or Jupiter and wanting of 
sugnificant phases like Venus and Mercury. Casually for a given 
apparition saturn looks the same for every viewing.
    Over the years, the planet, by its axis tilt.t prsents the rings 
to ys at various angles. The fully open rings are some 27 degrees 
inclined to our sightline, folding up to nothingness, then opemnng 
again to show the opposite face. the cycle takes about 29-182 years 
per cycle. when the rings are edgeon, they become more dynamic, 
altering aspect in timescale of weeks.