4 Feb 2016

The Moons of Saturn during February


Click on the images to enlarge

Saturn is the showpiece of the sky. Apart from Jupiter, it is by far the largest member of the Sun's family; even though it is so far from the Sun (886,000,000 miles on average) it shines brightly, and even a modest telescope will show its superb system of rings. Also on view will be some of the remarkably interesting retinue of moons or satellites.
I have been asked many time what size telescope is needed to see the Moons now that Saturn is at a reasonable height in the dawn sky. Here are the average magnitudes of Saturn's principal satellites.
There are 62 satellites altogether. One of them, Titan Mag 9.7, is of planetary size, and is visible with a 2-inch telescope. Of the rest, Iapetus Mag 13, Rhea Mag 9.7, Dione Mag 10.6, and Tethys Mag 10.7 are moderately bright, but the others cannot be seen except with the help of powerful instruments.
Mimas Mag 12 and Enceladus Mag 11.6 are distinctly elusive (I find them difficult even with a 12-inch reflector), but are interesting because they seem to be only about as dense as water.
Next, reckoning outward from Saturn, we come to Dione Mag 10.6, which is much more massive than Tethys. My own observations indicate that it is a little brighter, but in any case a modest telescope will show both. Rhea Mag 9.7 is further out, and larger, so that it is quite easy to see in a small instrument.
Between the orbits of Titan and the next major satellite, Iapetus, there is a wide gap. In this region Bond, in 1848, found a small moon, Hyperion Mag 13, which is distinctly difficult to observe.
All the inner moons move practically in the plane of Saturn's equator, which is the plane of the ring-system. Iapetus, however, has an orbital tilt of more than 14 degrees. It is a long way from Saturn, and its movement’s are relatively slow. But its main point of interest lies in the fact that it is variable in brightness. When west of Saturn, it is conspicuous; my observations indicate that it can become brighter than the 9th magnitude.
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