As reported in previous Updates, the classical planet Saturn has been shining in the evening sky after sunset since early March. Since last year, Saturn has been passing slowly through the stars of the constellation Leo. Saturn is the outermost planet of the solar system visible to the unaided eye, and has been seen by skywatchers on every clear night since ancient times.
Since Saturn is so far away, it moves very slowly through the constellations, taking nearly 30 years for a complete circuit of the zodiac, corresponding to a single orbit around the Sun. Saturn takes over two years to pass through one of the 12 zodiac constellations, compared to Jupiter, which passes through one constellation a year, and completing its orbit around the Sun in 12 years.
In the current season, Leo is seen moving west in the early evenings, toward the sunset, following the Sun's apparent annual advance through the constellations. By August, Leo will vanish into the sunset, and Saturn will disappear with it. The current season is a good time to learn to find Saturn in the night sky. The waxing crescent Moon will be seen passing near Saturn on the evening of Saturday, June 27. This will be an excellent opportunity to learn to identify Saturn.
Ring Plane Crossing
2009 is an uncommon year in the cycle of Saturn. Every 15 years, or twice in each 30 year cycle of Saturn's orbit, the Earth crosses the plane of Saturn's rings. Like the Earth, the poles of Saturn are inclined to the plane of it's orbit. As Saturn move through its orbit, the tilt of Saturn remains constant. Saturn's north pole points very closely to the North Star, Polaris, so the orientation of the sky as seen from Saturn would be very similar to how we see the sky here on Earth.
Like the Earth, Saturn has solstices, corresponding to the times when the poles of the ringed planet are inclined at their maximums toward and away from the Sun. Similarly, Saturn also has equinoxes representing the "in between" times between solstices.
These events can be observed through a telescope. When Saturn is at its solstices, the rings are turned to their maximum angle to the plane of the solar system, and appear their largest and most open as seen from the Earth. Saturn last had a solstice in 2001, when the south pole was turned toward the Sun, and the rings were at their widest as seen through telescopes here at home. The next solstice of slow-moving Saturn will be in 2017, when its north pole will be pointed at the Sun.
In between the solstices, near the times of Saturn's equinoxes, the rings appear very thin, because the plane of Saturn's rings is seen from nearly edge-on. Saturn will reach its equinox on August 10, 2009, when the plane of Saturn's rings crosses the Sun. In the meantime, throughout the summer of 2009, Saturn's rings will appear very thin.
In most cycles of Saturn's orbit, it is possible to see Saturn's rings actually vanish! As the Earth crosses Saturn's ring plane, the rings are viewed perfectly edge-on, and since they are so thin, they become completely invisible! Unfortunately, this time around, the crossing of Saturn's ring plane falls on September 4. At this time, Saturn will be very close to the Sun, and only visible grazing the horizon in very strong twilight. It's not likely that many people will be able to see a ringless Saturn at this time.
The next ring crossing of Saturn will be in 2025, but unfortunately, Saturn will again be too close to the Sun. Saturn's rings disappeared in 1995, but if you didn't see it then, you missed your best chance for quite a while! Our next best chance to observe a ring crossing will be 2038! Maybe by that time, our homeschool kids can show it to their own kids!
more information about topics from Classical Astronomy,
please check out Signs & Seasons,
a homeschool astronomy curriculum!