SJAA Ephemeris January 2009 | SJAA Home | Contents | Previous | Next

The Shallow Sky

Seasons in the Sun?

Akkana Peck


January skies are surprisingly bare of planets.

Saturn rises in mid-evening and is up for the rest of the night, transiting a bit past 3 am at just under 60 degrees. The ring tilt is less than a degree – almost edge-on. How visible are they? The rings, composed of small rocky/icy particles ranging from dust sized to several meters in diameter, are less than a kilometer thick, so they disappear when edge-on. But at what angle do they disappear? Watch Saturn over the next month or two and find out.

The rings will widen from now until mid-summer, then they’ll rapidly close until they’re exactly edge-on during the first week of September. But here’s the bad news: Saturn is a daytime object in September. So January is actually your best chance to see Saturn with nearly edge-on rings.

In one sense they might be more interesting now than they would have been in September: the angle the rings present to us, on Earth, is different from the angle they present to the Sun. Why does that matter? Because as long as the rings aren’t edge-on to the sun, they’ll create a shadow on the planet. In September when we ourselves cross the ring plane, the rings will also be pretty much edge-on to the Sun (less than .4 degrees). But this month, the angle of the rings to the Sun is a bit over 3.3 degrees (compared to the roughly .8 degree tilt we’ll see from here). That means you might be able to see the shadow of the rings on the planet, perhaps without seeing the rings themselves. It’s not much of a shadow, but it might be enough to be visible, and we wouldn’t be able to see it in September even if the Sun didn’t get in the way.

Why does the ring tilt change with respect to the sun? Why are the rings’ shadows always changing like that? It’s Saturn’s “axial inclination” – the tilt of the planet’s axis with respect to the plane of its orbit. It’s the same effect that causes Earth’s seasons, but Earth’s axial tilt is only 23°, while Saturn’s is a healthy 26.7°. So when you watch changes in the shadow of Saturn’s rings on the planet, you’re watching Saturn’s slow progression through its seasons. And you thought that Mars was the only planet where we could see the seasons progress!

Meanwhile, Venus continues its excellent evening apparition, with dim Uranus also hovering in the evening twilight. Uranus is mostly too faint to see in twilight, but on the evenings of January 21st and 22nd, Venus passes about a degree and a half northwest of its dimmer neighbor. That might make it possible to find faint Uranus - it's worth a try anyway.


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