SATURN RING CROSSINGS
 -------------------
 John Pazmino
 NYSkies
 nyskiesastronomy@earthlink.net
 2006 January 26
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. 
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. 
    Huygens in the 1650s studied Saturn with improved telescopes and 
finally sussed out that the planet is surrounded by a thin flat ring. 
He then correctly explained why the lateral whatevers vanished and 
returned. The ring simply tilted edgeon so 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
    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. This I leave out here. The bottom result is that 
on each instance of the rings's edgeon aspect, Earth can suffer one or 
three crossings. The number must be odd because the Earth starts on 
one side of the rings and ends on the other. An even number of 
crossings would put Earth back on the initial side. This is 
impossible. 
    It so happens that the last three edgeon phases were triple 
crossings! This could make some of us jaded about the events! The next 
one is only a single event. The longterm trend is about half triples 
and half singles. We were just awfully lucky in the late 20th century! 
    When the edgeon phase has only one crossing, it occurs near the 
superior conjunction, with Saturn on the far side of the Sun. Single 
crossings are difficult or impossible to observe with classical 
optical methods from the ground because Saturn is then in twilight or 
daylight. Triple crossings have the first one when Saturn is in the 
morning sky; middle one, all night sky; last one, evening sky. 
Generally all three are observable in nighttime. Once in a while 
either the first or third is in twilight.  
    The crossings occur when Saturn is near the vernal or autumnal 
equinox in the ecliptic. There is nothing at all connected to Earth 
seasons; it just works out that way. So we must have Saturn in 
Aquarius-Pisces or Leo-Virgo for the crossings. The fully fluffed ring 
phase occurs near the winter and summer solstices in Scorpius-
Sagittarius and Taurus-Gemini. 
Hunting the crossings 
 ------------------- 
    With Saturn in Leo-Virgo, opposition occurs in March. Saturn in 
Aquarius-Pisces puts opposition in September. I then calculated the 
ring tilt for the opposition, using March 15 or September 15 as a 
starting date. If the rings were closed up, only a couple degrees 
tilt, I knew that year experienced a triple event. A binary search 
around opposition turned up the central of the three crossings. 
    Finding the other two was slippery due to their irregular 
spacings. I started with three months before and after opposition. A 
binary search around these dates rustled up the 1st and 3rd crossings. 
    If the rings were quite open, there was only a single crossing. I 
then explored around the superior conjunction. This is, for a starting 
point, six months before or after opposition. (Saturn moves slowly 
thru the zodiac.) One of them is quickly seen to be way off; the rings 
keep opening up. The other closes them. 
    A home computer and astronomy software accomplished in one weekend 
what used to take weeks of time and teams of mathematicians. The 
scenarios are easiest seen in table form. 
  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 
                                        (super conjtn) for event 
1936-1937 crossing 
 ----------------
    The ring crossing of 1936 was a single one, but almost a triple. 
Or it was barely a triple. On 1937 Feb 20 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 millidegree or so north of them, then backed 
away. 
    Other calcs show there was barely a crossing on 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, with greatest tilt on 
June 28. 
    I give my own figures based on modern parameters for Saturn and 
    Earth. Accounts from the 1936-1937 event generally state there was 
only the one crossing on 20 February 1937 and a kiss, as it were, on 
28 June 1936. 
    A similar situation occurred in 1684-1685. With the crude scopes 
of 
that era, reports show a crossing in December 1684. There was none. 
Earth came within two-tenth degree of the rings, then receded. A 
modern scope would have easily caught the missed crossing. The real 
edgeon event came on 29 August 1685. 
Table of crossings 
 ----------------
    I give here the last few crossings and the upcoming one. The 
dates are in New York time. I include for context the superior 
conjunctions and oppositions associated with the crossings. 
  date         event         elong  comments 
  -----------  ------------  -----  ------------------
  1936 Mar  3  super conjtn    0    north face exposed 
  1936 Jun 26  1st crossing  102 W  north to south, morning
  1936 Jun 30  2nd crossing  106 W  south to north, morning
  1936 Sep 10  opposition    180    Saturn in Aquarius-Pisces 
  1937 Feb 20  3rd crossing   21 E  north to south, evening
  1937 Mat 15  super conjtn    0    south face exposed 
  (some calcs show no actual crossing in June 1936)
  1949 Sep  4  super conjtn    0    south face exposed 
  1950 Mar  8  opposition    180    Saturn in Leo-Virgo  
  1950 Sep 14  one crossing    1 E  south to north, evening
  1950 Sep 17  super conjtn    0    north face exposed 
  1966 Mar 10  super conjtn    0    north face exposed 
  1966 Apr  5  1st crossing   19 W  north to south, morning 
  1966 Sep 18  opposition    180    Saturn in Aquarius-Pisces 
  1966 Oct 26  2nd crossing  139 E  south to north, all night 
  1966 Dec 17  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 26  1st crossing   29 W  south to north, morning 
  1980 Mar 12  2nd crossing  178 W  north to south, all night 
  1980 Mar 15  opposition    180    Saturn in Leo-Virgo 
  1980 Jul 22  3rd crossing   52 E  south to north, evening 
  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 11  2nd crossing  144 W  south to north, all night  
  1995 Sep 13  opposition    180    Saturn in Aquarius-Pisces 
  1996 Feb 11  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 Sep  4  one crossing   12 E  south to north, evening 
  2009 Sep 18  super conjtn    0    north face exposed 
Elongation 
 --------
    Elongation is the angular distance along the ecliptic of Saturn 
from the Sun. Within 120 degrees east of the Sun I note that Saturn is 
in the evening sky. Between 120 E and 120 W, all night sky; within 120 
W, morning sky. Note that the direction refers to the displacement 
from the Sun, NOT the location in the sky. This can be confusing at 
first, being that Saturn in small eastern elongation is in the western 
sky in evening; western elongation, eastern sky in morning. 
    The nominal elongation for superior conjunction is zero degrees; 
Saturn is more or less in line with the Sun and is quite invisible by 
ordinary optical means from the ground. We can 'see' Saturn near 
conjunction via the SOHO satellite, which photographs the Sun 
continuously in a 14 degree field from far outside the atmosphere. The 
field shows the background star, otherwise totally swamped by 
terrestrial daylight, and any planets near their superior or inferior 
conjunctions. 
    The nominal elongation for opposition is 180 degrees. Saturn rises 
near sunset and sets near sunrise; it's in the sky all night long. 
    The edgeon event of 1950 was totally invisible for occurring 
within a couple days of superior conjunction. At that time the only 
observation possible was from the ground and there was no hope of 
seeing the planet in good detail so close to the Sun. 
    The crossing of 2009 will be pretty tough to observe by ordinary 
earthly methods. Assuming SOHO is still running by then, its image is 
like a binocular view, with too low magnification to show the rings. 
By 2009 there may be electronic gadgets for home astronomers to remove 
the bright twilight and reveal Saturn in an artificially darkened sky. 
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. That's why 
in 2005 with the south face in view, the north part of Saturn's sphere 
is hidden. The rings ar slowly collapsing so that by summer 2005 they 
will slide off the of the planet's north polar region/ There is 
nothing special about this event, but the joy of seeing this part of 
Saturn after bing hidden for several years. 
    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 local sky. When this latitude is zero, Earth is in 
line with the rings. The technical measure of a crossing event is the 
moment when Earth's saturnographic latitude is zero. That's how I 
verified the accounts of the crossings from past decades thru 
historical almanacs. 
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 above 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 
 
    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, altho plausibly whole globe studies are best done then. On the 
other hand, studies of the poles of Saturn require wide open rings. 
Sun crossings
 -----------
    I'm leaving out details of the Sun's ring crossings, but here are 
a couple notes. The rings are visible by reflected sunlight. If the 
Sun and Earth are on opposite sides of the rings, the rings are far 
darker than if the Sun and Earth were on the same side. The rings are 
not totally opaque. They are more like translucent or diffusing 
sheets, with the A ring transpiring a bit more sunlight than the B 
ring. 
    There is only one Sun crossing for each round of Earth crossings, 
so there is the chance for opposed latitudes for the two bodies for 
some time within the edgeon episode. During the era of small imperfect 
telescopes, the reports could have mistaken such a dark ring for an 
edgeon ring. 
Edgeon view
 ---------
    In small telescopes the rings quite disappear completely about a 
week before the crossing date and reappear about a week afterwards. 
During the crossing, the planet is naked of rings! The ringless period 
depends on the aperture and quality of the telescope, acuity of the 
the observer's eyesight, and clarity and stability of the local air. 
    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 size 60-gram bond paper, the paper is 
way too thick to scale the thickness. 
    The rings dwindle to a narrow needle, then a hair-thin filament 
during the crossing season. Home astronomers in the last three 
crossings had delight to watch the rings collapse, barely open, 
collapse again, before finally opening up for real.