Page 99 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO SEE

SATURN’S TOP 5 MOONS TO OBSERVE

TITAN RHEA IAPETUS DIONE TETHYS
The largest of Saturn’s The second largest moon This is the third largest This moon orbits up to This moon orbits about
moons has a 16-day of Saturn and currently and most distant of the 1.5 ring diameters from one ring diameter
orbit. At its farthest, the 20th catalogued in main moons of Saturn. Saturn, over 2.7 days. away from the planet
you’ll fi nd it about fi ve of distance out from the Its 79-day orbit takes it Its visual magnitude of and takes 1.9 days to
Saturn’s ring diameters planet. It makes an orbit out to 12 ring diameters +10.4 makes it visible on do so. It has a visual
from the planet, with in 4.5 days, reaching from the planet. The dark nights with a 3-inch magnitude of +10.3
a brightest visual just under two ring visual magnitude ranges refractor. This is the and so can be seen in a
magnitude of +8.4, diameters from Saturn. from +10.1 to +11.9, so densest of the moons, 3-inch refractor. Tethys
which makes it visible The visual magnitude is Iapetus needs about which means that it may has a great canyon that
in good binoculars. This +9.7, making Rhea an a 6-inch scope to see have a large rocky core. stretches three-quarters
large moon makes up easy target for a 3-inch it at its darkest. Unlike Two smaller moons, of the way round the
over 96 per cent of the refractor telescope. The the other, larger moons, Helene and Polydeuces, moon, and it is believed
mass of everything in moon is covered with Iapetus has quite an share the same orbit to be composed of
orbit around the planet. an icy surface. inclined orbit. as Dione. water-ice.

from your stars using planetarium software fainter. This is why it had been impossible
to check your observing fi eld before you to see it before. MOON DIAGRAMS
go out. The other alternative is a moon Cassini deduced, correctly, that this
One of the best ways of plotting the
locator diagram. For an explanation of what was because the moon has one very bright
positions of Saturn’s moons is with a
these are and how to use them see ‘Moon hemisphere and one very dark one, and is diagram that looks a bit like a double
diagrams’, right. also tidally locked to Saturn. This means, helix. The orbit of each of Saturn’s moons
like our Moon, it always shows the same is represented by a line curving either
MEET THE FAMILY face to its planet. It follows that we see a side of a central vertical strip, which
represents Saturn. An inner moon’s line
After Titan, the next brightest moon is different part of Iapetus from our Earthly
weaves back and forth close to the
Rhea, which shines at mag. +9.7. This is viewpoint when it is to the east or west of
central strip, while an outer moon’s line
quite a substantial drop in brightness, but Saturn. As a result, Iapetus varies between swings out wider from the strip depicting
Rhea will look its best through a 6-inch mag. +10.1 and mag +11.9. However, the Saturn’s movement. Dates are plotted
scope or larger, as will the rest of the faintness trophy goes to the moon Mimas, down the diagram and you simply draw
fainter, visible moons. which at mag. +12.9, needs perfect viewing a horizontal line across for the date you
want to observe. Each moon’s position
There are seven of Saturn’s moons within conditions without any light pollution to be
with respect to Saturn is indicated where
the grasp of a 6-inch scope. After Titan able to see it comfortably. Moving up to an
your line crosses a curve.
and Rhea, in decreasing brightness, come 8-inch aperture will improve your chances
Tethys at mag. +10.3, Dione at mag +10.4, of seeing this distant world.
Enceladus at mag +11.8 and Iapetus. To catch Saturn and its moons at their
The last moon mentioned is a very best, ideally you want to be out observing
interesting world indeed. Its unusual nature at opposition, although the planet will show
became apparent when it was discovered by for a month before that exact date, getting
the Italian astronomer Giovanni Cassini in gradually higher each night. On the actual
1671. He fi rst saw the moon on the western day of opposition, Saturn will be in the sky
side of Saturn but found it missing on a later all night moving up towards its highest
search, when it should have been on the point in the sky, which is due south.
eastern side. The planet will still be visible for about a
It wasn’t until 34 years later, when month after its opposition date too,
telescopes had improved, that Cassini although its height in the sky decreases
fi nally saw Iapetus to the east, because gradually until it sinks below the horizon
when it’s here it’s almost two magnitudes and is no longer visible.
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