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Ring Plane Crossings of Saturn

Planetary Rings

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

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:

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. Diagram showing the variation of tilt of Saturn's rings

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).

Diagram showing the difference between the lit and unlit sides of the rings

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 fourth and fifth 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.

Date (UT)ElongationEarthSunRing Visibility
30 December 1612 63° E North → South Unlit for 49 days
17 February 1613 19° E North → South Lit for 13.5 years
26 August 1626 14° E South → North Unlit for 9 days
4 September 1626 7° E South → North Lit for 15.6 years
29 March 1642 21° W North → South Unlit for 78 days
15 June 1642 90° W North → South Lit for 126 days
20 October 1642 142° E South → North Unlit for 58 days
16 December 1642 84° E North → South Lit for 12.8 years
20 October 1655 38° W South → North Unlit for 124 days
20 February 1656 161° W South → North Lit for 19 days
10 March 1656 177° E North → South Unlit for 128 days
16 July 1656 55° E South → North Lit for 14.9 years
28 May 1671 79° W North → South Unlit for 56 days
23 July 1671 131° W South → North Lit for 135 days
5 December 1671 89° E North → South Unlit for 69 days
12 February 1672 25° E North → South Lit for 13.5 years
5 August 1685 32° E South → North Unlit for 25 days
30 August 1685 11° E South → North Lit for 15.6 years

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.

Date (UT)ElongationEarthSunRing Visibility
23 March 1701 14° W North → South Unlit for 57 days
19 May 1701 63° W North → South Lit for 13.4 years
14 October 1714 32° W South → North Unlit for 107 days
29 January 1715 136° W South → North Lit for 52 days
23 March 1715 168° E North → South Unlit for 109 days
10 July 1715 62° E South → North Lit for 14.9 years
14 May 1730 64° W North → South Unlit for 89 days
11 August 1730 148° W South → North Lit for 89 days
8 November 1730 119° E North → South Unlit for 90 days
6 February 1731 33° E North → South Lit for 13.4 years
14 July 1744 53° E South → North Unlit for 43 days
25 August 1744 17° E South → North Lit for 15.6 years
16 March 1760 7° W North → South Unlit for 33 days
17 April 1760 35° W North → South Lit for 13.5 years
7 October 1773 25° W South → North Unlit for 88 days
3 January 1774 107° W South → North Lit for 93 days
6 April 1774 155° E North → South Unlit for 84 days
29 June 1774 73° E South → North Lit for 14.8 years
1 May 1789 52° W North → South Unlit for 118 days
27 August 1789 164° W South → North Lit for 40 days
6 October 1789 154° E North → South Unlit for 114 days
28 January 1790 42° E North → South Lit for 12.9 years

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.

Date (UT)ElongationEarthSunRing Visibility
21 December 1802 98° W South → North Unlit for 11 days
1 January 1803 109° W North → South Lit for 168 days
18 June 1803 79° E South → North Unlit for 64 days
21 August 1803 24° E South → North Lit for 15.6 years
10 March 1819 3° E North → South Unlit for 6 days
16 March 1819 4° W North → South Lit for 13.5 years
30 September 1832 17° W South → North Unlit for 65 days
3 December 1832 75° W South → North Lit for 146 days
28 April 1833 133° E North → South Unlit for 43 days
11 June 1833 92° E South → North Lit for 14.9 years
21 April 1848 41° W North → South Unlit for 113 days
1 September 1848 166° W North → South Lit for 12 days
13 September 1848 177° W South → North Unlit for 128 days
19 January 1849 52° E North → South Lit for 12.8 years
22 November 1861 69° W South → North Unlit for 71 days
2 February 1862 141° W North → South Lit for 104 days
17 May 1862 110° E South → North Unlit for 88 days
12 August 1862 32° E South → North Lit for 15.5 years
6 February 1878 31° E North → South Unlit for 23 days
1 March 1878 11° E North → South Lit for 13.6 years
22 September 1891 8° W South → North Unlit for 38 days
30 October 1891 41° W South → North Lit for 15.5 years

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.

Date (UT)ElongationEarthSunRing Visibility
13 April 1907 30° W North → South Unlit for 105 days
27 July 1907 126° W North → South Lit for 69 days
3 October 1907 163° E South → North Unlit for 96 days
7 January 1908 66° E North → South Lit for 12.8 years
8 November 1920 54° W South → North Unlit for 106 days
22 February 1921 160° W North → South Lit for 49 days
12 April 1921 147° E South → North Unlit for 114 days
4 August 1921 41° E South → North Lit for 15.4 years
29 December 1936 69° E North → South Unlit for 54 days
21 February 1937 20° E North → South Lit for 13.6 years
14 September 1950 2° E South → North Unlit for 8 days
21 September 1950 5° W South → North Lit for 15.5 years
2 April 1966 20° W North → South Unlit for 75 days
16 June 1966 86° W North → South Lit for 134 days
28 October 1966 139° E South → North Unlit for 51 days
18 December 1966 87° E North → South Lit for 12.9 years
27 October 1979 41° W South → North Unlit for 128 days
3 March 1980 169° W South → North Lit for 9 days
12 March 1980 177° W North → South Unlit for 133 days
23 July 1980 53° E South → North Lit for 14.8 years
22 May 1995 68° W North → South Unlit for 81 days
10 August 1995 144° W South → North Lit for 101 days
19 November 1995 111° E North → South Unlit for 84 days
12 February 1996 31° E North → South Lit for 13.5 years

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.

Date (UT)ElongationEarthSunRing Visibility
11 August 2009 32° E South → North Unlit for 25 days
4 September 2009 11° E South → North Lit for 15.6 years
23 March 2025 10° W North → South Unlit for 44 days
6 May 2025 48° W North → South Lit for 13.4 years
15 October 2038 29° W South → North Unlit for 99 days
22 January 2039 124° W South → North Lit for 69 days
1 April 2039 163° E North → South Unlit for 99 days
9 July 2039 67° E South → North Lit for 14.8 years
6 May 2054 52° W North → South Unlit for 117 days
31 August 2054 163° W South → North Lit for 40 days
10 October 2054 154° E North → South Unlit for 114 days
1 February 2055 42° E North → South Lit for 13.4 years
29 June 2068 70° E South → North Unlit for 57 days
25 August 2068 21° E South → North Lit for 15.6 years
14 March 2084 2° W North → South Unlit for 13 days
27 March 2084 12° W North → South Lit for 13.5 years
5 October 2097 18° W South → North Unlit for 69 days
13 December 2097 81° W South → North Lit for 134 days
26 April 2098 139° E North → South Unlit for 53 days
18 June 2098 87° E South → North Lit for 15.4 years

Ring Plane Crossings in the Twenty-Second Century

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

Date (UT)ElongationEarthSunRing Visibility
25 April 2113 40° W North → South Unlit for 131 days
3 September 2113 163° W North → South Lit for 16 days
18 September 2113 178° E South → North Unlit for 125 days
22 January 2114 53° E North → South Lit for 12.8 years
26 November 2126 69° W South → North Unlit for 72 days
6 February 2127 141° W North → South Lit for 104 days
21 May 2127 110° E South → North Unlit for 88 days
16 August 2127 32° E South → North Lit for 15.5 years
19 February 2143 23° E North → South Unlit for 16 days
7 March 2143 9° E North → South Lit for 13.6 years
26 September 2156 9° W South → North Unlit for 40 days
5 November 2156 43° W South → North Lit for 15.4 years
15 April 2172 30° W North → South Unlit for 105 days
29 July 2172 126° W North → South Lit for 69 days
5 October 2172 163° E South → North Unlit for 96 days
9 January 2173 66° E North → South Lit for 12.8 years
10 November 2185 53° W South → North Unlit for 108 days
26 February 2186 161° W North → South Lit for 46 days
12 April 2186 150° E South → North Unlit for 116 days
6 August 2186 42° E South → North Lit for 15.4 years

Sources

The IAU 1989 model for Saturn was taken from Report of the IAU/IAG/COSPAR working group on cartographic coordinates and rotational elements of the planets and satellites — 1988, M.E. Davies, et al, Celestial Mechanics and Dynamical Astronomy, 46, 187. Planetary positions were calculated using the JPL DE406 ephemeris.