“…three fixed stars, totally invisible.” —Galileo Galilei, from Sidereus Nuncius, 1610

On a January night in 1610, just over 400 years ago, the Italian mathematician Galileo Galilei pointed a carefully made telescope toward Jupiter. We can imagine him peering through the objective of the tube-shaped refractor and gazing at the distant planet. Known across millennia, Jupiter was one of the original “wanderers” of primitive astronomy. In a sense, for Galileo, Jupiter was old news.

However on the night of January 7th 1610, Galileo spotted something new. Through his telescope he saw two tiny “stars” to the east of Jupiter, and one to the west. These, bright, tiny specks formed three points on an imaginary line that ran across Jupiter’s equator.

Galileo quickly sketched what he saw and then turned to other heavenly bodies. The odd chance of seeing three stars lining up so neatly next to Jupiter likely puzzled him, and the next night he looked again toward Jupiter. On January 8th the inconceivable had seemed to happen as now the tiny stars had switched positions! As if to befuddle him all three of the new stars had moved to line up on the west side of Jupiter. His observations continued on January 9th, 10th, and 11th. All during these nights the little stars skipped around the parent body.

Galileo continued to observe Jupiter for several weeks. Nightly he tracked and recorded the changes in position of the three stars. However, within a week of his initial observation he noted a fourth, slightly smaller speck and began to record its position as well. Over the course of the observations he began to surmise that these new stars were not fixed against the background but were moving around the planet. This was a logical step for the astronomer since it was known that the Moon revolved about the Earth.

Galileo soon worked with the publisher Thomas Baglioni of Padua to print these and other observations in his Sidereus Nuncius, or the Sidereal Messenger. The little book is sometimes described as a treatise or pamphlet. The observations are quite detailed given the basic instrument Galileo used. The observational drawings, particularly those of the Moon, are quite striking.

The little book was groundbreaking, however. At the time the Ptolemaic geocentric system held that all other objects in the Cosmos revolved around the Earth. Galileo’s little stars, circling not Earth but Jupiter,  showed that this widely accepted system was flawed. Although an amazing discovery, Galileo also had more worldly matters to attend to, and in the Sidereus Nuncius he referred to Jupiter’s newly discovered moons as the “Medicean or Cosimo Stars” in order to pay respect to Cosimo II, Duke of Tuscany and the Medici family, who were his patrons.

Today, these four satellites of Jupiter are called the Galilean satellites. In order of distance from Jupiter they are known as Io, Europa , Ganymede, and Callisto. The names were based on a suggestion by Johannes Kepler but were applied for the first time by Simon Marius of Germany, who actually discovered the moons independently of Galileo in late 1609.

The four satellites have been observed and studied for many years. Today any amateur astronomy magazine will offer a monthly sidereal table of the moons’ positions around Jupiter. Space probes such as the Pioneers and Voyagers have flown past Jupiter and imaged the Galilean satellites. These moons are worlds unto themselves and the Voyager probes provided dramatic imagery, particularly of Io with its volcanoes.

The Galilean satellites are worlds of fire and ice. Io’s volcanoes of sulfur and heated volatiles seem to be turning the moon inside out. Io was the first world other than Earth where active volcanoes were seen erupting. Color imagery from the Voyager spacecraft and later the aptly named Galileo probe revealed Io to have a hellish but fascinating landscape.

Seemingly nearby but at another planetary extreme is Europa. Europa is a frozen ball of ices with curious geyser-like plumes and vast sheets of knotted surface ice. Intriguing fracture lines make this world look like a cosmic cue-ball. Tidal flexing as this moon races around mighty Jupiter likely causes the kilometers-thick layers of ices to expand and contract. Across Europa, Jupiter drives forces that seem akin to a form of plate tectonics. The tidal flexing of the ice sheets likely releases its energy as heat. This suggests that somewhere far below its surface of ice Europa may have an ocean of liquid water. And where there is water could there also be life?

Ganymede is a mix of topographies and landscapes. As with its nearer siblings, Ganymede’s surface features are wrenched over millennia by the slow tectonic upheavals caused by Jupiter’s patient yet powerful gravity. The world is a splotchy mix of rock and ice and has a worn, weathered and in places sinuous appearance.

Further out and away from Jupiter sits Callisto. This world is a place seemingly trapped and frozen like a snapshot in time. The tidal flexing that impacts the three inner moons is less powerful here. And very little heat lurks below Callisto’s surface. Thus, not much changes on its mottled dark and light surface. Indeed, it is believed that Callisto’s features have changed very little across billions of years. The biggest upheavals come with the occasional asteroid or meteor impact. Otherwise, Callisto’s topography is subject to the slow, patient gnawing of crater walls and rilles by the process of sublimation. Ices and other volatiles slowly out-gas, leaving only rock or the occasional slump of gravel and dust. Callisto is a tomb, but astronomers use it as a baseline for what other moons eventually become. It is a starting point in the study of planetary formation, and an important one.

And all this started when somebody turned a small telescope skyward.

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