SATURN RING CROSSINGS ------------------- John Pazmino NYSkies firstname.lastname@example.org 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.