Star Spy: Eye of the bull

21:46, Jan 15 2013
Galilean Moons
Galilean Moons
Jupiter and Galilean Moons
Jupiter and Galilean Moons


Jupiter shines brilliantly in the north, and can be seen in the blue sky within an hour of sunset. It takes longer, perhaps another hour, for the bright star Aldebaran to appear nearby. 

Aldebaran is also known as 'The Eye of the Bull', being located in the upside down head of Taurus, Latin for bull. 

Once the sky is dark, looking just above and to the right of Jupiter, you will spot the slightly orange-tinted star. 

It is the bottom right-hand star, and also the brightest, of an upside down V-shape made up of 5 stars.  

Its orange tint is due to the fact that it has cooled substantially as it swells up and nears the end of its life. 


The star’s name is actually from Arabic and means The Follower. What it is following nightly across our summer sky is Matariki, the glowing open cluster of stars also known as the Pleiades or Seven Sisters. 

Taurus is one of the earliest identified constellations. A depiction of Taurus has been found in a cave painting in France dating back to 15,000 BC.


The fourth Galilean moon out from Jupiter is called Callisto. Different from the other three, Calisto’s surface is an ancient and somewhat uniform mix of water-ice and rock.

As with Jupiter’s other two icy Galilean moons, an underground liquid ocean is possible, but would be very deep beneath the rock and ice.

It is believed that a kind of tidal action between the moons and massive Jupiter with its strong gravitational field, may be responsible for heating the interiors of the moons.  

Io, being the closest of the four large moons, is the most affected, constantly renewing its surface through its many active volcanoes, distributing molten material from deep within the moon. 

Europa, the second one out, also has a relatively young surface, but of ice, probably frequently renewed by the liquid sea beneath which breaks through the ice, filling craters and then freezing into a smooth solid surface.

Ganymede is next, and as you might expect, is thought to have a much thicker layer of ice than Europa, and appears to be a mix of ancient and newer terrain. 

While still pock-marked with many craters, many also appear to have disappeared beneath the ice. 

In contrast, Callisto’s surface still retains remnants of its earliest craters, some of which may have originated in collisions during the formation of the Solar System over four billion years ago.

Some or all of these four moons, depending on their orbital positions at any given time, are easily visible with a decent pair of binoculars off to either side of the planet. 

Through a good telescope it is sometimes possible to see a moon or its shadow crossing the face of Jupiter.

The four also share the common feature, that, Like Earth’s moon, they are tidally locked with their planet, meaning that each moon turns just one face inward to Jupiter.  

Jupiter is the largest of the Sun’s planets, about one 10th of the Sun’s diameter.  

Earth’s diameter is a little less than one 10th of Jupiter’s, and more than 1000 Earths would fit inside it.

Such an imposing planet merits a special gallery of moons.  There is much to be learned about Jupiter and its systems of rings and moons. 

Callisto is large - Earth’s moon is less than three fourths its diameter - and is far enough from Jupiter to be much less affected than the other Galilean moons by the harmful radiation emitted by the planet. 

For these reasons Callisto is considered to be a potentially good place for a human base for exploration of the Jovian system.