by Peter Anderson
In its 29.4 year orbit, the edge on presentation of Saturn’s rings occur at intervals of 13.75 and 15.75 years with the next occasion being in early 2025. For this article I researched historical works including ‘The Planet Saturn by A F O’D Alexander’ (1962) and a paper with Patrick Moore listed as the first author, describing the 1966 occasion. In the latter case the telescopes used were of the size employed by amateur astronomers and so quite relevant to this article. It then occurred to me that I had observed and documented this same 1966 presentation. Comparing my observations I found that they were entirely consistent and thus may serve as a reasonable guide for 2025.
In my previous article on Saturn from late 2023 (and published in the AAQ newsletter in April 2024), I provided an overview of Saturn during 2024 and 2025 for observers in south-east Queensland.
On this occasion I will deal specifically with phenomena associated with the passage of the Earth and Sun though the ring plane. This passage by the Earth occurs on 23rd March 2025 is only eleven days after solar conjunction. With Saturn being 9.5 degrees away from the Sun on this date, it is regrettably impossible to observe Saturn around the time of this passage.
However the Sun then later follows the Earth in crossing the ring plane from North to South on May 6th 2025. Between 23rd March and 6th May 2025, the Earth and the Sun will therefore be on opposite sides of the ring plane and so the rings will appear very faint, only visible by the light filtering through from the sunlit side (which is often very patchy and can give rise to very uneven illumination) and by the light reflected from the body of the planet. In smaller instruments the rings may even disappear from view for a few days around the time of solar crossing on 6th May 2025 though there would still be the light reflected by the body of the planet. An interesting exercise would be to follow the visibility of the thin line of the ring shadow across the planet throughout the period. This shadow would be exceptionally thin because of the extremely low angle of the Sun to the rings.
Though Saturn is in the general direction of the Sun in the sky at this time, it is 10.5 times further away, so unlike comets, the phenomena being observed from our vantage point is not driven by solar influence, but simply by the interplay of the very low angles of illumination as the Earth and Saturn progress in their orbits. The events being described occur in our early morning sky and we are very fortunate that from our latitude and its position Saturn rises quite steeply from the horizon after conjunction. However it is not until around 22nd April 2025 that it will attain an altitude of 30 degrees by the time of commencement of civil twilight (Sun -6 degrees). For high resolution observing, the window for reasonable observing therefore commences in mid April and most of the highly interesting phenomena would then occur over the following month.
Some figures: At the time that the Earth passes through the ring plane on 23rd March 2025, the Sun is at an angle of 0.65 degrees above the rings on the opposite side. This angle steadily decreases until the Sun crosses the ring plane on 6th May 2025 by which time the Earth is then 2.2 degrees above it and is now on the same side as the Sun. By 9th June 2025 the Earth is 3.3 degrees and the Sun over half a degree above the ring plane. Later in the year between 17th November 2025 and 1st December 2025 the movement of the Earth brings it to within 0.4 degrees of the ring plane but as the Sun is then some 3 degrees above the ring system it will be brightly illuminated, even though the appearance is very nearly edgewise.
The following 1967 article and supporting drawings (as originally simplified for publication), may be helpful for intending observers of Saturn in April and May 2025 to appreciate what to expect. (Extracts from pages 12 and 13, and complete page 14, Astronomical Society of Queensland ‘Astroquest’ Vol 2 No 2. April-May 1967).
