Page 97 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO SEE

THE GALILEAN MOONS

IO
Diameter: 3,640km
The tremendous gravitational pull of Jupiter on this innermost of the
four Galilean moons, together with its closeness to the planet,
means Io whizzes round Jupiter in just 1.75 Earth days. This
fast orbital speed is easily seen in a small telescope: it
visibly shifts position in just a few hours. Physically, Io is
the most volcanic place in the entire Solar System. The
whole world is covered in sulphurous lava fl ows and
volcanoes erupting in plumes more than 500km high.

GANYMEDE
Diameter: 5,260km
The third major moon out from the
planet is not only Jupiter’s biggest, but
it is also the largest moon in the entire
Solar System. This is a world with a
cold ice surface, a large warm ice
(possibly water) mantle, a rocky interior
and a liquid iron core. It measures a
tremendous 5,260km across, which
is bigger than Mercury. Indeed, if
Ganymede was released into space, it
EUROPA would be classed as a planet.
Diameter: 3,140km CALLISTO
The second Galilean moon out from Jupiter, Europa, should Diameter: 4,820km
theoretically be visible with the naked eye since it shines The last of the four giant Galilean satellites is Callisto. It is the third largest
at mag +5.3. But Jupiter’s overwhelming brightness makes of the Solar System, after Titan, the biggest of Saturn’s moons. Callisto
it diffi cult to separate the moon from the planet. Europa’s ranks as one of the most cratered worlds known – its entire icy, ancient
brightness is due to its surface being smooth and icy. surface is covered with impact craters that date right back to the time of
Scientists suspect that underneath is a liquid water ocean, the early Solar System, when the moon formed. Like Europa, it is thought
leaving open the possibility that life may lurk in the depths. that beneath the surface may lie a watery ocean.

Moons will appear brighter and fi ll more zones, but features within the gaseous The next time Jupiter’s at opposition will
of the fi eld of view. atmosphere as well. At this level of detail, be 3 December 2012. The best views will be
Don’t necessarily expect to see all four, observers can also see the occasional dark when the planet reaches its highest point in
however: as the moons travel around spot caused by the moons casting the sky, due south around midnight. It will
the planet they may be behind their shadows onto Jupiter’s stand out near the constellation of Taurus.
or in front of Jupiter when atmosphere. The joy of After that, Jupiter will fi nd its way back to
you’re looking. Jupiter is that whatever opposition on 5 January 2014 (when it’ll be
It’s by using a larger your level of equipment, visible around 2.30am in Gemini) and on
scope with a front there’s always 6 February 2015, near midnight.
lens over 6 inches in something to see. Happy observing, and remember
diameter that you really The best times to – what seems like an easy amateur target
start to see detail on the see the planet is when today was once a hugely signifi cant sight
planet itself: not only the it’s at opposition. This is that made history at the start of the 17th
darker belts and lighter when Jupiter is positioned century. When Galileo fi rst saw Jupiter’s
directly opposite the Sun in moons, it proved scientifi cally that Earth
With larger scopes you can see the sky from our point of view, was not unique and wasn’t at the centre of
shadows of the moons on Jupiter’s disc and so it’s really bright. the Universe.

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