Has Saturn devoured his own sons?

Galileo Galilei, Third Letter On Sunspots (1 December 1612)

As this post goes live, the seasons have just turned on Saturn. The autumnal equinox for its northern hemisphere came on 6 May 2025, and its north pole is now moving into a period of darkness that will last for 13½ years. After that, it will tilt towards the sun again, and enjoy 15½ years of uninterrupted daylight.*

I can plot the Saturnian latitude at which the sun is overhead on any given date, like this:

As on Earth, the sun roams back and forth across the equator during the course of a Saturnian year. Because the axial tilt of Saturn is 26.7°, that’s how far the sun strays north and south.^† It crossed into the northern hemisphere on 10 August 2009; it will return from its excursion south of the equator on 22 January 2039. The asymmetrical 15½:13½ seasonal split between north and south is easily discernable on the chart.

On 6 May 2025, as on the other equinox dates listed above, the sun shone down precisely above Saturn’s equator—which meant that it also illuminated Saturn’s rings edge-on, giving them minimal illumination.

And, because the Earth always stays close to the sun in Saturn’s sky, we are treated to an edge-on view of Saturn’s rings during the Saturnian equinoxes.

But, because the Earth can stray as much as six degrees either side of the sun, as seen from Saturn, the dates of our edge-on views don’t correspond precisely to the Saturnian equinoxes. I can make this clearer with another chart, adding the latitude at which the Earth is overhead to my plot of solar latitudes:

That sinusoidal green line, marking the Earth’s latitude, cycles around the solar latitude with a period of 378 days—the extra 13 days beyond the length of a calendar year are the time it takes the Earth to catch up with Saturn’s own, slow movement around the sun.

It’s perhaps a little puzzling why the amplitude of the oscillation in Earth latitude is maximal during the equinoxes, as sun and Earth cross the Saturnian equator, and damps down to almost zero at the solstices, when the sun and Earth reach their maximum Saturnian latitude. But a couple of views of Saturn at different seasons will give the explanation.

Here’s a view from the dark, far side of Saturn during its northern summer solstice, looking towards the sun, with Earth’s orbit marked in red.

With Saturn’s rotation axis tilted towards the sun, the Earth’s orbit runs more or less parallel to Saturn’s lines of latitude, and the Earth’s latitude always closely matches that of the sun.

Contrast that with the situation during the northern autumnal equinox:

With Saturn’s axis tilted sideways relative to the direction to the sun, Earth moves back and forth at an angle to Saturn’s lines of latitude, alternately increasing and decreasing its latitude by several degrees.

Now let’s zoom in on the chart for the current equinox.

The Earth’s oscillation in latitude actually carried it across the Saturnian equator some time before the equinox on 6 May 2025. This event is called a ring plane crossing, and it happened on 23 March 2025. That’s the time at which we, here on Earth, had an edge-on view of Saturn’s rings—at which point, being less than a kilometre thick, they became invisible even to powerful telescopes.

Between 23 March and 6 May, there was another complication. During that period the Earth had a sliver of a view of the south side of the rings; but the sun was still illuminating the north side. Our usual view of the rings is in bright reflected light from their illuminated surface—but they’re much dimmer in transmitted light, something we only experience around the time of ring plane crossings.

For several weeks either side of that period of abnormal illumination, the rings are still hard to see because of the narrow viewing angle, particularly for small telescopes. And for this particular ring plane crossing, there was another complication—Saturn was very close to the sun, as seen from Earth, and so the event was difficult to observe.

But notice what happens towards the end of 2025—Earth oscillates back towards the Saturnian equator again, reaching its closest on 23 November. It’s not quite a ring plane crossing, but it’s close. And Saturn will be in a much more favourable position in the night sky, far from the sun. It will appear ringless to anyone with a small telescope.

That excursion back to the equator raises an immediate question. Can the Earth ever actually recross the Saturnian equator? It sure can. Here’s the situation around the 2039 equinox, as the sun cross the equator from south to north:

The Earth makes its first ring plane crossing on 15 October 2038, well before the sun reaches the equator on 22 Jan 2039, by which time the Earth is starting to drop southwards again. It crosses the Saturnian equator from north to south on 2 April 2039 and then crosses back again on 9 July 2039. These triplets of ring plane crossings are actually relatively common, slightly outnumbering the single crossings. (Note that a doublet is impossible—the Earth always has to end up on the opposite side of the equator from where it started.)

We can get a better handle on how these triple crossings occur by looking down on the solar system from above:

In this diagram, Earth and Saturn are moving around the sun in an anticlockwise direction. The intersection between Saturn’s ring plane and the Earth’s orbital plane is a line, which sweeps upwards across the diagram as Saturn moves along its orbit. The Earth’s first crossing of the ring plane occurs when the Earth is moving in the opposite direction to the ring plane’s motion. But it then sweeps around the sun and overtakes the slowly moving ring plane for its second crossing. Finally, as the Earth begins to turn back around the sun to complete its orbit, the ring plane catches up with it, and a third crossing occurs.

The ring plane takes a year to cross the width of Earth’s orbit, and that’s why three ring plane crossings are the maximum possible during any given Saturnian equinox—there’s no time for the Earth to “go around again” and catch up with the ring plane for a second time.

So why is all this relevant to Galileo Galilei?

Galileo was the first person to see Saturn’s rings, in 1610. The combination of poor telescope optics and the completely unfamiliar idea of a planet with rings, led Galileo to interpret what he saw like this^‡:

Saturn is not single but a composite of three, which seem to touch each other and never change their relative position and never move among themselves or change

(The inset is Galileo’s drawing of what he had seen.)

But, having observed this constant triple appearance of Saturn for a couple of years, in December 1612 Galileo reported:

I found it solitary, without the presence of the accompanying stars and in the highest degree round and terminated like Jupiter, and thus it continues to remain.
Now what is to be said concerning such a strange metamorphosis? Have the two minor stars been consumed after the manner of solar spots? Have they disappeared and suddenly fled? Has Saturn devoured his own sons?

But now we know the answer to Galileo’s puzzle. He had begun observing Saturn during the closing years of its northern summer. The autumnal equinox came on 30 December 1612, and Earth made its single ring plane crossing on 17 February 1613. No wonder Galileo’s mysterious “accompanying stars” had disappeared!

Of course, after a few months, Galileo was able to report that the “two little lateral stars” had reappeared, and by 1616, with the ring plane more open, he was able to make another sketch, and write that he:

… saw it with two mitres in place of the round stars which reduced it to the figure of an olive. However the middle ball was quite easily seen distinct and particularly two obscure spots placed in the middle of the attachments of the mitres or if we choose to say of the ears.

I don’t know about you, but I want to shout with frustration at the fact that Galileo got so close, but never knew the truth. It wasn’t until 1655, thirteen years after Galileo’s death, that Christaan Huygens first hazarded the idea that Saturn might be surrounded by a ring.

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As a bonus, here’s a quick little (38 seconds) video of Saturn during the 2038-2039 triple ring crossing, produced using (as ever) Celestia. Events are speeded up by a factor of a million, so you’ll notice a constant flicker as some of Saturn’s satellites (and their shadows) come and go.

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* Like Earth, Saturn pursues a slightly elliptical orbit around the sun. It’s a little closer to the sun, and moving slightly faster, during its northern-hemisphere winter. Which means that summer lasts longer in the northern hemisphere than in the southern.
^† Latitudes plotted here are planetocentric—the angle between Saturn’s equatorial plane and a line connecting its centre to the sun (or Earth). Strictly speaking, the sun (or Earth) is not directly overhead at points on the planet’s surface corresponding to this latitude, because Saturn is significantly oblate, so that its surface isn’t orientated at right angles to lines radiating outwards from its centre, except at the poles and equator. Latitudes of Saturn’s surface features (cloud bands) are usually given in planetographic coordinates. The difference between planetocentric and planetographic is the same as that between geocentric and geodetic, which apply specifically to the Earth. For a detailed discussion of that topic, see my post “Finding Apollo Trajectory Data”.
^‡ This, and subsequent translated quotations, comes from Partridge & Whitaker, “Galileo’s Work on Saturn’s Rings” Popular Astronomy (1895) 3: 408-14