Observing Projects 10E Saturns Inner Moons

Saturn has nine major moons that are brighter than magnitude +16, which is the photographic limit of an 8-inch telescope under a dark sky. Realistically under any level of light pollution a telescope in the 8-10 inch class will be able to pick out as many as five of Saturn's moons. One of these moons, Titan is within easy reach of any telescope. Three moons inside of Titan; Rhea, Dione and Tethys are about magnitude +10 and can be viewed with some effort in moderate light pollution with a medium aperture telescope. A fifth moon also can be magnitude +10 and orbits outside Titan, which we will deal with in another project.

Finding tenth magnitude specks of light circling Saturn sounds daunting if you have tried our observing project at Mars searching for Phobos. Finding Rhea, Dione and Tethys is much easier even though they are about the same brightness as Phobos. First, all four of these moons circle much farther from Saturn than Phobos does. Secondly, Saturn does not have nearly the same surface brightness as Mars does. These two facts make locating Saturn's three inner moons much easier than the more elusive moons of Mars. One thing you should avoid is going after these faint prizes when a bright moon is near by. As the moon makes its monthly passage of Saturn, its light can make viewing the moons very difficult. Also remember your eyes and take proper care to dark-adapt prior to taking on difficult targets.

When Saturn is favorably placed in the evening sky many astronomy publications will list out the times of greatest elongation for several of Saturn's moons. Sky & Telescope publishes a monthly chart of Saturn moon locations for Titan and the four brightest satellites that orbit inside Titan. This would also include Ence-ladus, which is marginally visible at magnitude +12 in an 8-inch telescope under the darkest of conditions. The best time to try for any of these moons is when they are either near their greatest elongation from the planet and thus easiest to see, or when they are near enough to Titan so the brighter moon can serve as a guidepost.

Saturn's gravity, like that of Jupiter, has a profound organizing effect on its flock of moons. Unfortunately for us, the resonances are not so easy to notice because they usually involve the much fainter moons. Tethys resonates on a 1:2 ratio with Mimas (magnitude +16) and Titan on a 3:4 ratio with Hyperion (also magnitude +16). The one pair of resonating moons that you might be able to track is Dione and Enceladus (magnitude +12). Enceladus will be well beyond the capabilities of an 8-inch telescope unless the skies are very dark.

As Saturn's moons circle the planet, you will notice one big difference from the moons of Jupiter. Instead of moving back and forth on a straight line, the satellites instead are tracing ovals around Saturn. This is because Saturn's axis is inclined 28 degrees to its orbit while the axis of Jupiter is inclined only three degrees. An important ramification of this is that the moons of Saturn do not engage in regular eclipses as those of Jupiter do. The shadows of the moons are usually aimed well off into space and only when Saturn's rings approach edge on to our line of sight do the moons begin to direct their shadows at the planet. Like the edge-on ring period, this only occurs about once each fifteen years.

The moons of Saturn present a moderately difficult challenge that will test your ability to plan, see and execute. The reward of seeing them and beginning to push your eyes and equipment to new levels is worth the work and will inspire you to push yourself further out into the universe.

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