Galilean Moons

Jupiter and Galilean Moons - From top to bottom: Europa, Io, Callisto and Ganymede

I am not some one who is very sentimental, but I can't stop going back to the times that I was about 13-14 years old when I look at this picture.

Back in around 1994-95 I constructed a telescope using nothing more than a lens of dad's old spectacles, an eye piece from a toy binocular and PVC tubes. It was a very crude construction but worked pretty well. If I could remember it had a magnification factor of about 20x. The first celestial object I pointed it at was the Moon and I was pretty overwhelmed to see the craters of the moon. That was the first time I saw them "for real". I am pretty sure I would've pointed it at a few other objects, probably at the Orion Nebula as well, but what I can distinctly remember is seeing the moons of Jupiter.

For the past month or two, Jupiter has been high in the sky during the evenings and being quite a bright object, have been a good target for my astrophotography. Sometime last week I figured I should probably reconstruct that image I had carved in my head on a photography plate (well, more on a CCD I guess). 

In the image you can see above, Jupiter is the brightest blob. The four smaller dots are the Galilean Moons; Ganymede, Callisto, Io and Europa. These are the biggest four of Jupiter's numerous moons (67 in total) and were discovered by Galileo Galilei in 1610 with his telescope. This would have been a turning point in astronomy as these were a good example of objects rotating around another object than Earth and seeing them would have undoubtedly played a role in changing the way we thing about the universe. Up until then the popular theory was that the everything rotated around the Earth. There were even explanations for the retrograde motion of the planets (when two objects are rotating around another object such as Earth and any other planet rotates around the Sun, an observer in one object will see that the other object suddenly "goes in reverse". This is because either the observer has over took the other object or the other object has done a complete circle and going in the other direction - will explain more about this later :) ) using a concept of epicycles.

The discovery of Moons that rotate around Jupiter questioned the belief of everything rotating around the Earth and proved the possibility that objects might be rotating around other objects. By extension, Earth was not really special, Earth was probably like any other planet and perhaps everything didn't rotate around Earth. This and many other ideas paved way to an idea what eventually turned out to be correct; the Heliocentric model of the Solar System.

So yeah, that's Galilean Moons and that's they did to us :)