Wednesday evening, September 13, 2017, at 9:59 p.m. CDT, Saturn reaches eastern quadrature as Saturn, Earth, and Sun form a right triangle. Eastern quadrature is so named as Saturn is 90° east of the Sun. This is the time when Saturn presents to us its most gibbous phase. Even so, Saturn will be 99.7% illuminated due to its great distance from us.
A more noticeable effect will be the shadow of Saturn on its rings, a phenomenon best seen at eastern or western quadrature.
Saturn will only be 12° above our horizon in SW Wisconsin at the exact moment of eastern quadrature Wednesday evening. Earlier that evening, Saturn crosses the celestial meridian at 6:51 p.m.—22 minutes before sunset. If it weren’t for daylight, that would be the best time to observe Saturn: when it is highest in the sky and we are seeing it through the least amount of atmosphere. If you have a telescope equipped with a polarizing filter, you can significantly darken the blue sky background around Saturn since the planet will be exactly 90° away from the Sun, where the scattered sunlight is most highly polarized. Rotate the polarizer until the sky is darkest around Saturn.
Speaking of Saturn, the Cassini mission will come to a bittersweet end on Friday, September 15 around 5:31 a.m. CDT when the storied spacecraft, which has been orbiting Saturn since June 30, 2004, will have plunged deep enough into Saturn’s atmosphere that it is no longer able to point its high gain antenna towards Earth. Soon after that, Cassini will burn up in Saturn’s massive atmosphere. We on Earth will not receive Cassini’s last radio transmission until 1h23m later—at around 6:54 a.m. CDT.
Emily Lakdawalla, who is arguably the best planetary science journalist in the world these days, includes the visual timeline of Cassini’s demise shown below and in her recent blog entry, “What to expect during Cassini’s final hours”.
Also, on Wednesday evening, don’t miss NOVA: Death Dive to Saturn, which will air on Wisconsin Public Television’s flagship channel at 8:00 p.m.
It may be a while before we visit ringed Saturn and its retinue of moons again. But further exploration of Titan and Enceladus is certain to feature prominently in humankind’s next mission to Saturn. Hopefully, that will be soon.
Ever wonder what is “quad” (four sides, rectangular) in “quadrature”? Perhaps this diagram will help…