Star Spy: Galileo's discovery

FREIDL HALE - TEKAPO STARLIGHT
Last updated 12:58 28/11/2012

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Pizza moon

In 1609 it was accepted by most that Earth was at the centre of the universe, the spheres in the sky circling unchanging around us. But that was about to change.

Galileo had made improvements to the new instrument, the telescope, and in January of 1610 he pointed it at Jupiter. 

What Galileo saw was similar to the image above, but with only "three fixed stars” aligned on the planet and “totally invisible by their smallness" to the naked eye.  

Within a week he had discovered a fourth tiny light, and, by observing them on different nights he discovered that their arrangement was always changing, some even disappearing and reappearing. He concluded that they were, in fact, orbiting Jupiter.

This explanation was entirely unacceptable to the Church, with the result that the philosopher-scientist eventually recanted and spent the remainder of his life under house arrest. But the burgeoning scientific revolution had been given a significant boost. 

We refer to these four as Jupiter’s Galilean Moons, in honour of their discoverer.

In order of their appearance from left to right in Fraser’s image above, they are named Io, Europa, Ganymede and Callisto, and are the largest of the 67 natural satellites of Jupiter. 

In fact, each of the four is larger than dwarf planet Pluto.

It might seem logical that all of Jupiter’s moons would resemble each other, sharing the same or similar composition and appearance. 

It wasn’t until the late 1800s, early 1900s, that our technology advanced sufficiently for telescopes to just begin to resolve any of the moons’ surface details.  

What we have discovered in the century since then has come as a great surprise. The Galilean Moons are very different from one another. 

Io, looking like a spotty pizza, with cheese and tomato sauce bubbling and flowing around darker bits of olive and other toppings, was perhaps the most unexpected. 

That visual image has some basis in reality, as the surface of Io is dotted with more than 400 active volcanoes spewing bright yellow sulphur and other minerals from deep inside the moon’s molten core, to as high as 500km above it’s surface.

In the past we easily held to the comforting view that the universe out there was unchanging, with the occasional terrifying exception of a visiting comet or a solar eclipse, or the portent of a suddenly very bright star.   

We didn’t know that our Moon is moving slowly away from Earth. 

We didn’t guess that we would witness active volcanoes erupting on the moon of another planet, nor that we would one day capture on camera raging dust storms, or landslides and avalanches, as they occurred, on our neighbour Mars. 

Such revelations are expanding our minds and extending our science to explore and search for, and even to expect to find, an amazing and growing diversity of dynamic worlds; worlds some of us may have imagined, and worlds we could not even begin to imagine.

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Speaking of Eclipses

A minor lunar eclipse will occur during Full Moon Wednesday night/ Thursday morning this week. All but a sliver of the Full Moon will enter the partial shadow – penumbra - of the Earth.

No part of the Moon will go dark and red in the deep umbra of Earth’s shadow, but at maximum eclipse, around 2:30, it may appear slightly dim, especially the small area of its edge closest to the umbra.

- Stuff

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