SkyEye

Last night I saw the rings of Saturn
for the first time, that brilliant band
of icy crystals and dust. Mirrors
shepherding the light, collecting it
like pollen or manna
or pails of sweet clear water drawn
from the depths of an ancient well.
— Ellen Bass, "Saturn's Rings", 2014

Planetary Rings

A ring system is simply a disc of material, usually ice or dust particles, orbiting an astronomical object such as a planet. Saturn's ring system is certainly the most eye-catching but the other three gas giants in our solar system are also encircled by rings. There is also evidence that a number of minor planets may have small rings and it is probable that many large gaseous exoplanets have planetary ring systems.

Although the planet Saturn has been known since ancient times, its rings were not seen until 1610 when Galileo Galilei turned his primitive telescope to the skies and noted that Saturn had two large companions either side of the disk. After Galileo's groundbreaking discoveries, improved telescope optics led Dutch astronomer Christiaan Huygens to conclude in 1655 that the 'arms' or 'moons' extending from Saturn were actually a ring system. Twenty years later, Giovanni Cassini proposed that the ring system was comprised of multiple smaller rings with gaps in between them. The largest of these gaps, between the A and B rings, is now called the Cassini Division in his honour. The nature of the rings remained unclear until the nineteenth century when Scottish scientist James Clerk Maxwell demonstrated that both a solid ring and a continuous fluid ring would be unstable and thus, the rings must be composed of many small particles or moonlets.

Ever improving ground-based imaging techniques and the advent of the space age have spurred on discoveries of planetary rings:

• Astronomers James Elliot, Edward Dunham and Jessica Mink discovered dark rings around Uranus in 1977. More rings have since been discovered by Voyager 2 and the Hubble Space Telescope.
• In 1979, Pioneer 11 discovered the F ring of Saturn and Voyager 1 observed the rings of Jupiter. The later Galileo mission to Jupiter revealed further details of the Jovian ring system.
• During its 1980 fly-by of Saturn, Voyager 1 discovered the G ring.
• Neptune finally joined the ringed crowd in 1984 when ring 'arcs' were detected by Patrice Bouchet, Reinhold Häfner and Jean Manfroid at the La Silla Observatory and by Faith Vilas and L.-R. Elicer at the Cerro Tololo Inter-American Observatory. The rings were eventually photographed by Voyager 2 five years later.
• The discoveries from the highly successful Cassini mission are too numerous to mention but include many revelations relating to the dynamics, structure and composition of the rings.

Ring Plane Crossings

The following diagram shows how the appearance of Saturn changes over the course of 15 years, beginning in 2017 when the north pole of Saturn was most inclined toward Earth and ending in 2032 when the south pole of Saturn will be most inclined toward Earth. During this interval, the Earth will pass through the plane of the rings. This event will occur in March 2025.

The obliquity of Saturn is 26.7°. Because the rings are in the equatorial plane of Saturn, the angle of the rings relative to the Sun also varies between 0° and 26.7° as the planet completes its orbit around our star. Twice during the Saturnian year of 29.4 Earth years, the ringed planet reaches an equinox when the Sun is directly over the equator and thus, directly over the rings. This is when the Sun sees a ring plane crossing.

Earth also observes ring plane crossings twice a Saturnian year but because the orbit of Saturn is inclined slightly to the ecliptic, these crossings are not on exactly the same dates as the Sun-based ring plane crossings. Also, the Earth either experiences a single crossing event or a triple crossing event. If Earth is on the opposite side of the Sun from Saturn, then there is a single ring plane crossing near the time Saturn is at conjunction, rendering the event largely unobservable. However, if Earth and Saturn are on the same side of the solar system, then a triple ring plane crossing takes place, with the middle one occurring near opposition.

The periodic disappearance of Saturn's rings was noticed soon after the invention of the telescope. Galileo was flabbergasted to find that Saturn's companions, which he had first observed in 1610, were completely absent in 1612. Even more confusing was the reappearance of the 'moons' later on. What Galileo had observed, of course, was the period close to the 1612/3 ring plane crossing when the rings were nearly edge-on.

The rings of Saturn can add considerably to the overall apparent brightness of the planet. The wider open the rings, the brighter the overall effect. However, near ring plane crossing, the rings can suddenly go quite dark. It depends whether we are observing the lit or unlit side of the rings. Consider the diagram below which shows the 2068 ring plane crossing. This crossing proceeds from south to north so before the crossing, we are observing the south or underside of the rings (first figure). The solar ring plane crossing occurs first, in June. During this time, the Sun goes from shining on the south side of the rings to shining on the north or upperside of the rings. This is a slow process and because the Sun is not a point source of light as seen from Saturn, both sides of the rings are briefly illuminated either side of the moment the Sun is directly over the edge of the rings. However, the view from Earth is still from the south so we end up seeing the 'unlit' rings (second figure). The rings appear much darker from the unlit side as little of the sunlight filters through. The B ring, normally the brightest, is virtually opaque to sunlight and appears almost black from the unlit side. This situation continues as the ring tilt approaches zero and then in August, at the terrestrial ring plane crossing, the rings vanish as they appear edge on (third figure). From this point onwards, the view from Earth is on the north side of the rings which is the sunlit side (fourth figure).

Calculating Ring Plane Crossing Dates

In order to calculate the date and time when the Earth or Sun crosses the ring plane of Saturn, we first need to know the orientation of the ring plane in space. The first accurate determination was made by the Russian-German astronomer Georg Struve as part of his work on the orbits of the major satellite of Saturn, published in 1930. More recently, photometric measurements of the brightness of the rings during the 1966 and 1980 ring plane crossings and observations of occultations of stars by the rings by Earth-based telescopes and the Voyager and Cassini spacecraft have further improved the accuracy with which the orientation of the ring plane is known.

Knowing the orientation of the ring plane in space, we can then calculate the angle between the ring plane and the Saturn-Earth or Saturn-Sun direction at intervals using high-precision planetary ephemerides such as the JPL Development Ephemeris (DE) data. A ring plane crossing event occurs when the angle passes through zero.

The following tables list ring plane crossings since the first observations by Galileo. The first column gives the date of the crossing in the Gregorian calendar. The second column is the elongation of Saturn from the Sun as seen from the Earth. Following conjunction, Saturn is a morning object and is west of the Sun. At opposition, the elongation changes from west to east, and Saturn becomes an evening object until the next conjunction. The third and fourth columns indicate whether it is the Earth or the Sun which is crossing the ring plane, and they also give the direction of the crossing. The last two columns show the length of the interval between consecutive ring plane crossing events and whether the lit or the unlit side of the rings is visible from the Earth after the listed event. The shorter intervals during a ring plane crossing season are listed on the left, whilst the longer intervals are on the right.

Ring Plane Crossings in the Seventeenth Century

Ring plane crossings are an excellent time to look for new satellites since the blinding brightness of the ring is much diminished or absent. Cassini took advantage of the 1671/2 ring plane crossing to discover two new moons, Iapetus and Rhea. The single ring plane crossing of 1685 was nearly a triple one starting at the end of the previous year. Earth was less than a degree south of the ring plane on 22 December 1684 but then widened the gap afterwards. Just before the ring plane crossing which finally took place eight months later, Cassini discovered another two satellites, Dione and Tethys.

Ring Plane Crossings in the Eighteenth Century

William Herschel discovered the moon Mimas during the ring plane crossing of 1789/90 and confirmed his earlier discovery of Enceladus.

Ring Plane Crossings in the Nineteenth Century

Hyperion was the next moon discovered during a ring plane crossing with William and George Bond, and William Lassell finding the satellite during the 1848/9 event. Astronomers James Carpenter, Otto Struve and William Wray made close observations of the unlit side of Saturn's rings during the 1861/2 ring plane crossing.

Ring Plane Crossings in the Twentieth Century

The years 1936/7 nearly saw a triple ring plane crossing when Earth came to within 0.01° of the ring plane on 28 June 1936 before retreating northward. The actual Earth-based ring plane crossing occurred the following February. Two new moons were discovered during the 1966 ring plane crossing: Epimetheus (Richard Walker) and Janus (Audouin Dollfus). Walker believed he had photographed Janus, but John Fountain and Stephen Larson identified it in 1978 as a separate satellite on an almost identical orbit. Three more moons joined the family during the 1979/80 ring plane crossing: Calypso (William Baum, Douglas Currie, Dan Pascu, Kenneth Seidelmann), Helene (Pierre Laques, Jean Lecacheux) and Telesto (Stephen Larsen, Harold Reitsema, Brad Smith, Richard Walker). The 1995/6 crossing was significant as it was observed from space by the Hubble Space Telescope.

Ring Plane Crossings in the Twenty-First Century

The first two ring plane crossings of the twenty-first century are singles, with Saturn located near the Sun in both cases. The first event that will be observable is the 2038/9 triple ring plane crossing, with Saturn at opposition in mid-March 2039.

Ring Plane Crossings in the Twenty-Second Century

This century begins and ends with two triple ring plane crossings.