SATURN EDGE-ON PHASE
-------------------
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
1980 Mar 15 opposition 180 Saturn in Leo-Virgo
1980 Jul 23 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 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.