We are in a special time where Earth and Saturn’s tilts create two wonderful illusions for lovers of the night sky. Not only have Saturn’s rings disappeared from view, but its largest moon has cast its shadow upon the planet a handful of times this year, something you only have two more chances to see; the phenomenon only happens every 15 years.
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The dates for your diary are September 4 and September 20. In both cases, the moon and its shadow will appear across the surface of the planet in the Northern Hemisphere.
When to see Titan’s shadow transit Saturn
According to Sky & Telescope, the best date for viewing it is September 20. Saturn is going to be at opposition on September 21, so Earth and Saturn are at their closest. Saturn will then be at its brightest, allowing for the best chance to see the event.
On September 4, the transit will start at 5:25 am UTC (1:25 am ET) and last for 3 hours and 25 minutes. On September 20, it will start at 5:09 am UTC and end at 7:34 am, a much shorter transit time as the shadow will be cast at a much higher latitude, so it has less to cover. But Titan should also be visible near its shadow. Given the hours, people in America have the best chance to see it.
If those two dates are no good, you’ll have a very brief window on October 6, as the shadow will briefly appear on the Saturnian North Pole. If that is no good, then tough! You will have to wait until 2040 for the alignment to repeat itself.
The reason for the alignment is that Earth and Saturn have a similar tilt with respect to their orbital planes around the Sun. Ours is 23.5 degrees for our planet, and Saturn’s is 26.7 degrees. The inclinations of their planetary orbits are also close to each other, respectively 1.57 degrees and 0.93 degrees, with respect to the invariable plane of the Solar System.
Saturn’s rings are aligned with the planet’s equator and Titan’s orbit has just a 0.3 degree tilt. So, every 15 years or so, when Earth and Saturn’s respective tilts are canceled out by a favourable alignment, we can see Titan’s transit in front of the planet, we can see the moon eclipsing a bit of Saturn, and we can even see the rings disappearing.
If you can’t make the shadow transit, the next best thing will be trying to combine the disappearing rings with a Titan transit. Titan transits Saturn on November 22 (from 18:52 to 01:10 UT), and the rings will be at their narrowest on November 23, making that another great night for viewing.
It is recommended to use a 3-inch telescope with at least a 200x magnification to see the spectacle.