A rather odd event is transpiring in the Saturnian system, one that only happens a couple of times in our lifetime; its rings are vanishing. Not really, of course; we are merely passing through the super-fine ring plane as viewed from the Earth. The exact date of the “crossing” as viewed from Earth is Friday, September 4th, when the 20 meter thick rings will be exactly edge on and vanish from all but the largest telescopes. Just a few weeks ago, Saturn passed equinox, when the rings were edge on to the Sun and hence, not illuminated across their 100,000+ km expanse. This happens every 14 to 15 years during the planet’s 29.7 year orbit.
Now here’s the catch, and the challenge therein; Saturn is at magnitude +1.4 and almost on the opposite side of the Sun; spotting it in the dusk skies will be a tough, but not impossible challenge. Viewing a ring-less Saturn will mean sweeping the dusk horizon, directly after sunset. A +0.35 magnitude Mercury about 10 degrees to the left and south of the ringed planet will help with the challenge, as will absolutely clear skies; both planets will only be 6 degrees above the horizon at sunset on the 4th. Saturn’s rings have been nearly edge on for all of 2009 and will continue to be so when it emerges in the dawn skies late this fall for the remainder of the year… the actual superior conjunction of Saturn with the Sun occurs on September 17th.
The astro-term for the week is orbital resonance. Exert a steady gravitational force on an orbiting body long enough, and areas of stability will occur. These result in famous resonances such as the 2:3 ratio of the Pluto-Neptune systems and the Kirkwood Gaps in the asteroid belt. A 1:1 or 2:1 resonance however, is unstable, and these sort of orbital resonances are responsible for the gaps in Saturn’s rings. The most famous gap is the Cassini Division, and this empty void exists because of a resonance any object placed there would share with the moon Mimas, in a 2:1 ratio. Tinier “shepherd moons”, such as Pan and Atlas also give rise to the thousands of grooves we see in the Saturnian ring system spotted by the Voyager and Cassini spacecraft. These tiny effects do indeed add up!