Galilean Satellites 

One of the most significant contributions made in science is the discovery of the four main moons in orbit with Jupiter. The fact that there exists an orbiting system of bodies other than Earth with the rest of the planets was a major support for the Copernican theory. On January 7, 1610, Galileo Galilei observed what appeared to be three stars in the vicinity of Jupiter. Not realizing that what he saw were actually the moons of Jupiter, he dismissed them to be just regular stars. However, the following evening he observed that the three stars seen the previous night had shifted position and moved the direction opposite of what he anticipated. Striking his interest, Galileo continued to observe these stars for another week. Four days later, he saw a fourth star that later was confirmed to be Ganymede. The conclusion of his observations was that the four stars he observed seemed to be in orbit with the planet Jupiter, making the five planetary bodies a system of moons and a planet. The following is a direct excerpt from Galileo's notebook.

Galileo published his observations in Sidereus Nuncius in March 1610:

"I should disclose and publish to the world the occasion of discovering and observing four Planets, never seen from the beginning of the world up to our own times, their positions, and the observations made during the last two months about their movements and their changes of magnitude; and I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day . . . On the 7th day of January in the present year, 1610, in the first hour of the following night, when I was viewing the constellations of the heavons through a telescope, the planet Jupiter presented itself to my view, and as I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, namely that three little stars, small but very bright, were near the planet; and although I believed them to belong to a number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic, and to be brighter than the rest of the stars, equal to them in magnitude . . . When on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for there were three little stars all west of Jupiter, and nearer together than on the previous night."

"I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun; which was at length established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions around Jupiter."

Even though Galileo was credited toward the discovery of the moons of Jupiter, a man named Simon Marius could have beat Galileo to it. He claimed to have seen Jupiter's moon in November 1609, which is about one month before Galileo. However, Marius did not publish his findings and so was not credited toward the discovery. But he still named the moons of Jupiter in 1614 with suggestions from Johannes Kepler. As written in his findings:

"Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River, Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell . . . I think, therefore, that I shall not have done amiss if the first is calledby me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, theFourth Callisto . . ."

"This fancy, and the particular names given, were suggested to me by Kepler, Imperial Astronomer, when we met at Ratisbon fair in October 1613. So if, as a jest, and in memory of our friendship then begun, I hail him as joint father of these four stars, again I shall not be doing wrong."

Since these names were not officially adopted until the mid-1800's , Galileo's nomenclature stuck for several centuries. He called the moons the "Medicean planets", named after the Medici family and identified each one by a Roman numeral from I to IV.