Happy Saturn equinox day!

Warm wishes to all denizens of Saturn's north pole (as well as those who live at the north poles of Saturn's main moons), who are seeing sunlight for the first time in almost 15 Earth years today. The Sun has finally crossed Saturn's equator; it's now the northern face of the rings that's sunlit, and the southern face that's in shadow.

I contributed a podcast to 365 Days of Astronomy on the topic of Saturn's equinox; there's a transcript below, but first, here's a roundup of links to past stories.

• First, you may want some background on the alphabet soup of Saturn's rings
• And it wouldn't hurt to review Saturn's moons
• Here are recent posts on equinox events: Daphnis' shadow and wake; Saturnshine and moon shadows; more on moon shadows; hints of equinox
• And here's some background on Cassini's extended mission, also known as the "Equinox Mission"

Hi, I'm Emily Lakdawalla, from The Planetary Society. I've been looking forward to today, August 11, 2009, for more than five years, because there's something thrilling going on at Saturn.

First, a little background. Saturn is the second largest planet in the solar system and a delightful target through even the smallest of telescopes. With just a little bit of magnification, its yellowish point of light resolves into a disk framed by a brilliant ring system, looking like a plate with two handles. Usually, that is. But anyone who points a small scope at Saturn in 2009 and expects to see its emblematic ringed shape, is in for a disappointment; the rings have vanished.

The same disappointment befell Galileo almost 400 years ago. In 1610 he pointed his telescope at Saturn, becoming the first to observe the rings. At the time, they looked to him like two enormous satellites nearly touching the main body. Two years later, the so-called satellites disappeared. He wrote, "I do not know what to say in a case so surprising, so unlooked for and so novel." A couple of years later, the rings appeared again. This event happened once every fifteen years, or twice a Saturn orbit.

It was nearly fifty years before Christiaan Huygens correctly explained this bizarre behavior of the sixth planet. He deduced that Saturn was surrounded by "a thin, flat ring, nowhere touching" the planet. That thin, flat ring, he said, was "inclined to the ecliptic," meaning that it tilted at an angle to the plane of Saturn's orbit. That tilt is what allows us to see the rings, as we look down on them from their north, or up from their south. But as Saturn progresses around the Sun in a stately orbit taking almost 30 Earth years, there are two special times per orbit when we gaze upon the rings edge-on. At those times, the rings virtually disappear from view. The next one of these ring plane crossing events happens in a little less than a month, on September 4; so if you were to point a telescope at Saturn today, you wouldn't be able to see the rings.

This year's ring plane crossing is a little hard to study from Earth, because Saturn is relatively close to the Sun in our sky. Fortunately, we do have an observer in a good position to study Saturn. The Cassini spacecraft has been there for five years, studying the planet, its rings, and its moons with twelve science instruments. Cassini's prime mission ended a year ago, but NASA gave it a two-year extension to let it watch the events of this summer unfold.

Now, Cassini's path around Saturn is presently quite tilted to the plane of the rings. That means that Cassini, unlike Earth-based observers, has an excellent vantage point high above and far below the rings, which are most certainly not disappearing from Cassini's view. Even so, over this year Cassini has seen the rings fade, becoming dimmer and dimmer. What's going on?

Here's the story: Earth passing through the ring plane isn't the only reason the rings are disappearing from view. Because Earth is always pretty close to the Sun, from Saturn's distant point of view anyway, the dates when Earth passes through Saturn's ring plane are almost identical to the dates when the Sun passes through Saturn's ring plane. Today, August 11, 2009, is the equinox, the day when the Sun crosses the plane of Saturn's equator, and therefore its rings. Throughout Cassini's five years at Saturn, the orbiter has seen the Sun shining up from the south, illuminating the southern hemisphere of the planet and the southern face of the rings. Today, that all changes. The Sun is setting on the southern face of the rings, and rising in the north. This special day marks a change in Saturn's seasons; southern summer is giving way to autumn. At the same time, the Sun is rising in Saturn's north, bringing the new light of spring.

For us on Earth, the date of the equinox isn't particularly notable unless you're standing close to the poles; on the day of the equinox, the Sun rises on one pole and sets on the other. But Earth has nothing at all like Saturn's ring plane, and for an observer standing on the rings, the months surrounding the equinox have been very strange.
magine yourself standing on the south side of the rings. For fifteen years, the Sun has been overhead, except when the bulk of Saturn blocks the Sun from your view. But as the equinox approached, the sun set, and its light came at you from a lower and lower angle. Just as happens during a sunset on Earth, the ground got darker, because it was being lit more obliquely.

Then, a few months ago, something special started to happen. The rings are very nearly but not quite perfectly flat. Here and there, gravitational forces from Saturn's moons and even from bunches of particles within the rings cause lumps and bumps where the rings deviate from perfect flatness. A couple of months ago, the Sun had gotten low enough that these structures started casting shadows on the rings. One particularly notable set of shadows formed on the edge of a narrow gap in the rings where a tiny moon named Daphnis clears out a space for itself. Daphnis' orbit of Saturn is tilted a very tiny bit from being coplanar with the rings, so on each orbit it bobs up and down. Its tiny gravity exerts up-and-down forces on the particles that line the edge of its gap, making a diminishing set of waves that travel around Saturn in front of and behind the moon. Those waves are now casting their own beautiful set of wavelike shadows. You can use trigonometry to calculate the height of Daphnis' waves; they stand roughly a thousand meters high.

Ring waves aren't the only things casting cool shadows in the Saturn system right now. Saturn has dozens of moons, all of which also cast shadows. Because the moons all orbit in the same plane as the rings, most of the time their shadows are cast off into space. But near the equinox, their shadows begin to collapse on each other. It starts with the moons casting shadows onto Saturn, making tiny eclipses that dart across the sunlit globe; on the opposite side of the planet, the little moons are in turn eclipsed by Saturn, dropping into Saturn's shadow once every orbit. As the Sun has set closer and closer to the ring plane, the moons actually started casting shadows on each other and on the rings.

You might think that the shadows only show up on the sunlit side of the rings. But, as is often the case in the Saturn system, reality is weirder, because parts of the rings are transparent. So moon shadows are visible on both faces of the rings.

All of this shadow play peaks today, with the Sun balanced between south and north on Saturn, with the rings lit almost directly from the side. On this day, almost no sunlight falls on the rings, but they're not totally dark, because they are lit with planetshine -- sunlight that falls on Saturn and bounces off to strike the rings.

From today onward, the Sun will climb northward, and shadows will shorten, in a series of events that more or less mirror those that heralded the arrival of the equinox. Cassini is there to watch all of it happen.

Over the next few years, as Saturn's seasons continue to shift, Cassini's attention will once again wander away from the rings. The Sun's northward climb allows its rays to fall on the north poles of Saturn and all of its icy moons for the first time since Cassini arrived. Cassini's cameras and spectrometers will have a lot of work to do to capture images that will map parts of the Saturn system not seen by human eyes since the Voyager spacecraft passed through, thirty Earth years, or one Saturn year ago. Of course, that means the cameras will no longer be able to see the south poles of Saturn or its moons. No matter how long the Cassini mission is extended, it's unlikely to last long enough -- another fifteen years -- to photograph the south poles again. Not in visible light anyway.

One of the most exciting discoveries made by Cassini at Saturn is the intense volcanic activity of the south pole of the tiny moon named Enceladus. You might think that with the arrival of winter to the south poles of Saturn and its moons, we'll no longer be able to study Enceladus' bizarre activity. But the removal of sunlight actually creates an opportunity. From now on Cassini will be able to gaze on Enceladus' south pole with infrared instruments that can see the heat radiating from inside the little Moon. The polar winter darkness allows study of this internal heat engine without complications caused by solar heating.

You can watch all of these exciting events unfold as they happen, because the Cassini mission publishes all of its camera's photos directly to the Internet. I check in on Cassini's website nearly every day, and regularly post some of the best images to my blog. Please visit me at planetary.org slash blog to see the latest bizarre views from the ringed planet. Thank you for listening!