by Peter Anderson

Approximately every 15 years, Saturn’s rings are presented edgewise as the Earth and the Sun pass through the plane of the rings. In the planet’s 29.4 year orbit, we first see one face of the rings and then 15 years later the other, after passing through the plane of the rings. As Saturn has an elliptical orbit, these edgewise presentations alternate at intervals of approximately 13.75 and 15.75 years.

Sometimes there are two extra passages through the ring plane by the Earth as it whizzes from one side of the Sun to the other in six months first above and then below the Sun-Saturn plane and the angles change, and this is the reason for the three passages. For example, when the edge on phase occurs close to Saturn’s solar conjunction as in 2025, the rings are edge on to the earth only once. When the edge on phase occurs near opposition, the rings are edge on to the Earth three times. One ring plane crossing occurs close to opposition, the other two occur close to quadrature, but the Sun passes through the ring plane only once.

Now is the time to consider how you would like to observe these coming events in 2024 and 2025 as the rings narrow. I have used data from Jan Meeus (Astronomical Tables of the Sun, Moon, and Planets). On this next occasion the Earth crosses the ring plane on 23rd March 2025 when Saturn will be only 9.5 degrees from the Sun. The Sun then crosses the ring plane from North to South on May 6th 2025 and Saturn is then 47.5 degrees distant in the morning sky.

Saturn will be unobservable around the time of ring passage that occurs as it has just passed conjunction with the Sun on 12th March 2025. So some very interesting phenomena will be missed when the rings disappear for at least a day or so. Either side of this event the rings are seen incredibly foreshortened and appear as spikes either side of the planet. Over this period with very low and then backlit illumination until 6th May, the rings may well appear very patchy with brighter and darker areas. Their shadow may even be all that is visible as a dark line across the planet.

Though observations will not be possible at the time of the Earth’s ring plane crossing, many of the unusual related effects would be expected to continue until after the Sun has also crossed the ring plane in early May. Saturn is then in the pre-dawn sky but less than 50 degrees from the Sun so the window for good observations at such low altitudes in the morning sky is probably just an hour or so. Nevertheless we are fortunate that the relative positions are favourable and in early May at our latitude Saturn rises to be nearly directly over the sunrise position. This ensures an acceptable altitude for early morning observations. Without the obvious rings as a distraction it also presents an opportunity to clearly see that the disc of Saturn is much more oblate than that of the other giant planet, Jupiter.

But in 2024 Saturn’s rings also provide interesting opportunities. Due to the dynamics of the relative movement of the Earth and Saturn during 2024, the rings will at best come within 1.9 degrees of edgewise around 25th June. One phenomenon reported at the low angle of presentation of the rings (as will occur in mid 2024) is the possible occurrence of transient radial spokes in the rings. Of course many of the phenomena being described require good conditions and moderately high magnifications to be fully appreciated. Saturn then comes to opposition with the Sun on 8th September 2024.

Then in 2025 Saturn is in opposition on 21st September, and aside from the ring plane passages earlier in the year as previously described, Saturn’s rings close to less than 0.4 of a degree of edge on between 17th November and 1st December 2025 . You can easily use the Stellarium Computer Programme to check the appearance. Also refer to the attached plot. Because, in this case the planet was in opposition only two months earlier, this period in late 2025 will be very suitable for convenient evening observations of the nearly edge on rings. Though the angle of the rings will be very low, they will be bright because the angle of the Sun is higher.

Saturn ringless, or with very thin rings presents great satellite observing opportunities with the moons strung out in a line each side of the planet and passing close to and at times even occulting/eclipsing each other. The four major Jovian Moons have apparent diameters between 1.0 and 1.7 arc seconds (Europa 1.0”, Io 1.2”, Ganymede 1.7” and Callisto 1.5”) that can be resolved as discs. However of Saturn’s moons, only VI, Titan, second only to Ganymede in size, but at a much greater distance, displays an observable disc. This is around 0.8 arc seconds in diameter and therefore requires a 6” (15cm) aperture telescope or greater to comfortably resolve as a disc.

The next two largest moons of Saturn only subtend a quarter of an arc second so any hope of resolving any further discs can be dismissed. However transits and video imaging of four of these – Enceladus, Tethys, Dione, and Rhea – have been observed and conducted with very large amateur instruments but the observations have generally been described as ‘very very difficult’.

In my view one of the most satisfying observations has been that of a shadow transit of Titan when the moon and its shadow (of the same approximate size) pass together over the surface of Saturn taking several hours to do so. My predictions using the Occult computer program list eleven of these shadow events potentially partially visible from Eastern Australia. These commence on 28th March 2025 and end on 4th September 2025, but each is concluded before the rising Saturn has reached more than a few degrees in altitude as seen from the east coast. (The dates are (UT) 28th March, 13th April, 29th April, 15th May, 31st May, 16th June, 2nd July, 18th July, 3rd August, 19th August, and 4th September.) They are effectively unobservable from Brisbane at 153 degrees east, and further west in Australia nothing will be seen at all.

I wondered why these events occurred under similar annoying circumstances and the answer is simple. Titan has an orbit of 15.945 days or 15 days 22.68 hours. So for the next orbit of Titan some 16 days later the phenomena would occur just over and hour earlier. The orbit of the Earth around the Sun results in the stars etc. rising nearly four minutes earlier every night. Saturn’s slow orbital motion also tends to work against you and you are effectively back where you started. So the successive shadow events essentially stay put over the six months, ending around the time Saturn rises. However there is a very slow drift and the ending of these shadow transits slowly rises in altitude (as seen from Brisbane), occurring at 1, 2, 3.5, 4.5 and 5 degrees altitude for the last on September 4th 2025.

Of the Titan occultation and transit events involving the disc of Saturn, the situation is more promising than for the shadow events. Again we are constrained by the parameters already described so the events are the latter parts and conclusions of the transits and emergences from occultation. The following events may be observable.
Timings were derived via the Stellarium program that from my experience is quite accurate. No account is taken for limb darkening or any phase effect. (NOTE: Any daylight saving adjustment – if applicable- is not included in EAST times quoted.) I would also suggest commencing a little early in case there is some ‘drift’.

• 28th March 2025 (UT) 29th March EAST. Transit emergence. Titan’s emerging disc is bisected by the planet’s limb at 5.33am at 8 degrees altitude, and is clear at 5.41am at 10 degrees altitude.
• 13th April 2025 (UT) 14th April EAST. Transit emergence. Titan’s emerging disc is bisected by the planet’s limb at 5.42am at 22 degrees altitude, and is clear at 5.53am at 25 degrees altitude. *Readily observable
• 21st April 2025 (UT) 22nd April EAST. Occultation re-appearance. Titan’s emerging disc is bisected by the planet’s limb at 4.20am at 11 degrees altitude and is clear at 4.31am at 13 degrees.
• 29th April 2025 (UT) 30th April EAST. Transit emergence. Titan’s emerging disc is bisected by Saturn’s limb at 5.08am at 27 degrees altitude and is clear at 5.27am at 31 degrees altitude. *Readily Observable
• 7th May 2025 (UT) 8th May EAST. Occultation reappearance. Titan’s emerging disc is bisected by Saturn’s limb at 3.25 am at 11 degrees altitude and is clear at 3.43am at 15 degrees altitude.
• 4th September 2025 Around 7.12pm. Titan grazes the northern limb of Saturn and at this same time its shadow leaves the disc. At only 5 degrees altitude start a few minutes earlier if you have a good horizon but this observation, whilst interesting on the Stellarium program is very marginal at best at such low altitude.
• 20th September 2025 Transit emergence. Titan’s emerging disc is bisected by Saturn’s limb at 6.22pm at 9 degrees altitude and is clear at 6.37pm at 12 degrees altitude.

Of course the ‘Occult’ program lists many events by the inner moons numbers III, then IV, then V, as the angle of the plane of the rings decreases and then the reverse as they increase again. This occurs between the wider timeframe 20th February 2023 and 2nd April 2027. Centrally to these are the mutual occultations and eclipses between moons ranging from I to V and two unobservable events with satellite VII, and cover the periods between 29th September 2024 and 3rd December 2024, and then between 1st April 2025 and 20th January 2026. However these bodies are quite faint and neither they nor their shadows could be seen in amateur telescopes when projected against the disc of Saturn, nor likely seen when very close to its bright limb. They would simply be faint starlike points passing each other, or in the case of mutual events, merging and separating.

In any case only the brighter ones would be reasonably visible. Their visual magnitudes: I Mimas 12.9, II Encelades 11.7, III Tethys 10.2, IV Dione 10.4, V Rhea 9.7 VII Hyperion 14.2. (VI Titan is 8.3.)

During 2024/2025 several occultations of Saturn by the Moon are predicted to occur for the Brisbane area:

At 9.27am (closest) on May 4th 2024. This is predicted as an occultation by some sources but the edge of the occultation track passes nearly 300km away to the south-east. The 23% illuminated waning crescent Moon will be 53 degrees distant from the mid morning Sun at the time. Because of its low surface brightness and the bright sky, Saturn would be a very difficult object to locate and observe as the northern lunar limb passes close by.

The next is between 10.50pm and 11.47pm on June 27th 2024. The occultation is unfavourable because Saturn is rising in the east and is only 3 degrees altitude when it is occulted. But it is at 16 degrees altitude at re-emergence from behind the dark limb of the 64% waning Moon and this would be readily observable and worthwhile to attempt.

The final occultation occurs between 6.26pm and 6.53pm on September 17th 2024. At first it appears more favourable with an altitude of 20 degrees at commencement at 6.26pm and 26 degrees altitude at re-emergence at 6.53pm. (From my site 10km due west of central Brisbane.) However the Moon is 99% illuminated and the following day is full Moon. Predictions show that both events occur behind the fully illuminated southern polar regions of the Moon. The extremely bright lunar limb relative to Saturn would make meaningful observations/timings very difficult. Further north the event lasts longer and circumstances are different.

There are no lunar occultation events for Saturn visible from Brisbane in 2025.
(Sources are ‘Occult’, Stellarium, and Astronomy 2024.)

The assistance of Dominic Ford (data tables) and Michael Foulkes, Director Saturn, Uranus and Neptune Section, British Astronomical Association, is much appreciated.