GANYMEDE, CALLISTO, IO & EUROPA; GALILEAN MOONS

Today, The Grandma has received the wonderful visit of her closer friend Joseph de Ca'th Lon, who is a fan of History, Science and Astronomy. They kave been talking about the Galilean moons, that were firstly observed by Galileo Galilei on a day like today in 1610.

The Galilean moons, or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto.

They are the most readily visible Solar System objects after the unaided visible Saturn, the dimmest of the classical planets, allowing observation with common binoculars, even under night sky conditions of high light pollution.

The invention of the telescope enabled the discovery of the moons in 1610. Through this they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit a planet other than the Earth.

They are planetary-mass moons and among the largest objects in the Solar System; Titan and Triton, together with the Moon, are larger than any of the Solar System's dwarf planets. 

The largest of the four is Ganymede, which is the largest moon in the Solar System, and Callisto, both of which are either larger or almost as large as the planet Mercury, though not nearly as massive. The smaller ones, Io and Europa, are about the size of the Moon. 

The three inner moons -Io, Europa, Ganymede- are in a 4:2:1 orbital resonance with each other. While the Galilean moons are spherical, all of Jupiter's remaining moons have irregular forms because of their weaker self-gravitation, in addition to being much smaller.

The Galilean moons are named after Galileo Galilei, who observed them in either December 1609 or January 1610, and recognized them as satellites of Jupiter in March 1610; they remained the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter Amalthea in 1892.

Galileo initially named his discovery the Cosmica Sidera (Cosimo's stars) or Medicean Stars, but the names that eventually prevailed were chosen by Simon Marius. 

Marius discovered the moons independently at nearly the same time as Galileo, 8 January 1610, and gave them their present individual names, after mythological characters that Zeus seduced or abducted, which were suggested by Johannes Kepler in his Mundus Jovialis, published in 1614.

Their discovery showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted Ptolemaic world system, a geocentric theory in which everything orbits around Earth.

As a result of improvements Galileo Galilei made to the telescope, with a magnifying capability of 20×, he was able to see celestial bodies more distinctly than was previously possible. This allowed Galileo to observe in either December 1609 or January 1610 what came to be known as the Galilean moons.

More information: The Planetary Society

On 7 January 1610, Galileo wrote a letter containing the first mention of Jupiter's moons. At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs from 8 January to 2 March 1610. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.

Galileo's discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than Earth dealt a blow to the then-accepted Ptolemaic world system, which held that Earth was at the center of the universe and all other celestial bodies revolved around it.

Galileo's 13 March 1610, Sidereus Nuncius (Starry Messenger), which announced celestial observations through his telescope, does not explicitly mention Copernican heliocentrism, a theory that placed the Sun at the center of the universe. Nevertheless, Galileo accepted the Copernican theory.

A Chinese historian of astronomy, Xi Zezong, has claimed that a small reddish star observed near Jupiter in 364 BCE by Chinese astronomer Gan De may have been Ganymede. If true, this might predate Galileo's discovery by around two millennia.

The observations of Simon Marius are another noted example of observation, and he later reported observing the moons in 1609. However, because he did not publish these findings until after Galileo, there is a degree of uncertainty around his records.

The names that eventually prevailed were chosen by Simon Marius, who discovered the moons independently at the same time as Galileo: he named them at the suggestion of Johannes Kepler after lovers of the god Zeus (the Greek equivalent of Jupiter), in his Mundus Jovialis, published in 1614:

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 called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth 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.

More information: NASA

We cannot teach people anything;
we can only help them discover it
within themselves.

Galileo Galilei