Galileo discovered moons rotating around Jupiter and some were none too pleased

Galileo Galilei

On January 7, 1610 at around 1am, using a 20x telescope he designed and called a spyglass (although he didn't invent it), Galileo Galilei discovered three moons rotating around the planet Jupiter. Over the ensuing days, the Italian astronomer would discover a fourth moon and would name the objects “The Medicean Stars” (after Cosimo deMedici and the deMedici brothers for whom he was seeking patronage--he had originally considered calling the moons The Cosmian Stars).

Galileo's telescope

On March 12, 1610, Galileo wrote Sidereus Nuncius (“The Starry Messenger”), which opened by telling the reader that:

"...with the aid of a spyglass lately invented by him, In the surface of the Moon, in innumerable Fixed Stars, in Nebulae, and above all in Four Planets swiftly revolving about Jupiter at differing distances and periods, and known to no one before the Author recently perceived them and decided that they should be named The Medicean Stars” (emphasis mine).

The importance of Galileo’s discovery cannot be underestimated. First, Galileo demonstrated the power of the "spyglass" (the term telescope was not coined until 1611) to observe distant objects not visible to the naked eye.  More importantly however, Galileo's discovery would ignite a storm that would last for centuries.

In The Starry Messenger, Galileo would write:

Sidereus Nuncius,
“The Starry Messenger”

"On the seventh day of January in this present year 1610, at the first hour of night, when I was viewing the heavenly bodies with a spyglass, Jupiter presented itself to me; and because I had prepared a very excellent instrument for myself I perceived (as I had not before, on account of the weakness of my previous instrument) that beside the planet there were three starlets, small indeed, but very bright. Though I believed them to be among the host of fixed stars, they aroused my curiosity somewhat by appearing to lie in an exact straight line parallel to the ecliptic, and by their being more splendid than others of their size. Their arrangement with respect to Jupiter and each other was the following:

Galileo's sketch

that is, there were two stars on the eastern side and one to the west. The most easterly star and the western one appeared larger than the other. I paid no attention to the distances between them and Jupiter, for at the outset I thought them to be fixed stars, as I have said. But returning to the same investigation on January eighth led by what, I do not know I found a very different arrangement. The three starlets were now all to the west of Jupiter, closer together, and at equal intervals from one another..."

Galileo's sketch of his observations.  It is interesting how Galileo noted that it was cloudy on January
9 and 14, 1610; and that it looks like he discovered the fourth moon on January 13, 1610.

Galileo's notes on his observations

Recall that until this time, the world adhered to the Ptolemic world view that held that the Earth is the center of the universe and that everything revolved around the Earth.

Of course this view was supported scripturally which, among other things, held that:

1. "Thou, Lord, in the beginning didst lay the foundation of the earth." (Heb. 1:10)

2. The sun, moon, and stars were created after the firm "foundation of the earth" was laid. (Gen. 1:9-18)

3. "He established the earth upon its foundations, so that it will not totter, forever and ever." (Ps. 104:5)

4. "And the sun rises and sets and returns to its place." (Eccl. 1:5)

5. "The world is firmly established, it will not be moved." (Ps. 93:1 & 1 Chron. 16:30)

6. "For the pillars of the earth are the Lord's, and he set the world on them." (I Sam. 2:8)

7. "It is I who have firmly set its pillars." (Ps. 75:3)

8. "Who stretched out the heavens...and established the world." (Jer. 10:12)

Copernicus' view of the universe

Also recall that in 1543 Nicolaus Copernicus published De Revolutionibus Orbium Coelestium, (“The Revolutions of the Heavenly Spheres”) wherein he espoused the idea that it was the Sun that was the center of the Universe and that the Earth rotated around the Sun.

For what it’s worth, Copernicus was simply advancing the teachings of Aristarchos from the 3C. BCE who proposed the first heliocentric view of the universe—a position that was widely viewed until, well, you know…

Interestingly, in his introduction to De Revolutionibus Orbium Coelestium, Copernicus would write to Pope Paul II:

Copernicus' De Revolutionibus Orbium Coelestium,
“The Revolutions of the Heavenly Spheres”

“Those who know that the consensus of many centuries has sanctioned the conception that the earth remains at rest in the middle of the heaven as its center would, I reflected, regard it as an insane pronouncement if I made the opposite assertion that the earth moves. Therefore I debated with myself for a long time whether to publish the volume which I wrote to prove the earth’s motion or rather to follow the example of the Pythagoreans and certain others, who used to transmit philosophy’s secrets only to kinsmen and friends, not in writing but by word of mouth…”

Here’s a link to an English translation of Nicolaus Copernicus’ famous book: De Revolutionibus (On the Revolutions), 1543 C.E.

And here's a link to an enjoyable November 8, 2011 NPR story about Copernicus and his dangerous idea: For Copernicus, A 'Perfect Heaven' Put Sun At Center

Anyhow, back to Galileo. Galileo was a supporter of the Copernican model of the universe and it would be his writings and defense of the heliocentric view of the universe that would see him write Dialogo sopra i due massimi sistemi del mondo (“Dialogue Concerning the Two Chief World Systems”), and famously find himself condemned for heresy by the Catholic Church on June 22, 1623 based on: 

“…the two propositions of the stability of the Sun and the motion of the Earth were by the theological Qualifiers qualified as follows:

The proposition that the Sun is the center of the world and does not move from its place is absurd and false philosophically and formally heretical, because it is expressly contrary to Holy Scripture.

The proposition that the Earth is not the center of the world and immovable but that it moves, and also with a diurnal motion, is equally absurd and false philosophically and theologically considered at least erroneous in faith.”

Here's a link to Galileo's Dialogue:The Dialogue Concerning the Two Chief World Systems

Here’s a link to the Papal condemnation: Papal Condemnation (Sentence) of Galileo

More to the point, here’s a link to Galileo’s The Starry Messenger, which pretty much started the whole conundrum: The Starry Messenger

Ironically, some speculate that in 365 BCE, Gan De, a Chinese astronomer may have discovered Ganymede...

In any event, today’s a very important date in scientific discovery…