Cassini Solstice Mission: History of Saturn Observation

History of Saturn Observation

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History of Saturn Observation

Early Days

Sketch of Saturn by Galileo in 1610

The oldest written records documenting Saturn are attributed to the Assyrians. Around 700 B.C., they described the ringed planet as a sparkle in the night and named it "Star of Ninib."

The Assyrians, who occupied what is now Iraq, prospered between 1400 and 620 B.C. They incorporated the culture of the Babylonians, a civilization with a keen eye for astronomy. Historians believe that as early as 3000 B.C. the Babylonians recognized major constellations and eventually developed a calendar of astronomical events.

A few centuries later, in 400 B.C. the ancient Greeks named what they thought was a wandering star in honor of Kronos, the god of agriculture. Kronos (sometimes spelled Cronos

Sketch of Saturn by Huygens in 1655

or Cronus) was the Titan ruler and the father of Jupiter.

Then the Romans, who adapted much of their culture from the Greeks, changed the name of the ringed planet from Kronos to Saturnus, the root of Saturn's English name. Saturnus was the son of Uranus and Gaia and the father of Zeus (Jupiter). Like the Greeks, in Roman mythology Saturn was also the god of agriculture. It was in his honor that in December the Saturnalia festival was celebrated, a seven-day affair that became ancient Rome's most popular festivity.

Throughout the next millennium, our knowledge of Saturn didn't change much, and the planet was still considered to

Cassini's Sketch of Saturn in 1676 Showing the Gap in the Rings

be a wandering star until the invention of the telescope. This new tool, invented in the early 17th century by a Dutch optician, quickly revolutionized astronomy.

Told about the new instrument, in 1609 the Italian scientist Galileo Galilei built his own homemade version of a telescope and pointed it at the heavens. Using what we would call today a meager telescope -- it had only magnified images by 20 times -- he noticed there was something special about the ringed planet. Galileo couldn't quite grasp what was "wrong" with the planet and he could only guess wrong answers.

These Hubble telescope images, captured from 1996 to 2000, show Saturn's rings open up from just past edge-on to nearly fully open as it moves around the Sun.

At first he assumed Saturn was a group of three close-knit planets, with two smaller ones on each side of a bigger planet. Two years later, however, changes in Saturn's appearance baffled him. The two smaller planets had vanished and Saturn was now all by itself. Galileo wrote that he was "astonished" by this phenomenon. We know now that the rings seem to disappear as our view of the ring plane shifts. When they are seen at an edge-on angle, the rings are virtually invisible. A couple of years later, Galileo's observations became even more confusing when the rings reappeared, in their places next to Saturn.

"I do not know what to say in a case so surprising," he wrote in despair. He eventually suggested that Saturn must have had arms or "cup handles" that mysteriously grew and disappeared periodically. Galileo died without knowing that through his homemade telescope he was looking at Saturn's rings.

Huygens's theory of Systema Saturnium, 1659

Confusion reigned until Christiaan Huygens, a Dutch astronomer, developed the concept of a planetary ring system in 1659. Using an improved telescope - one that could magnify images by 50 times -- Huygens theorized the rings to be solid, thin and flat. The new idea provided a model for astronomers of the day, who were then able to understand what they were seeing. As the quality of telescopes continued to improve, features became easier to identify. In 1676, Giovanni Cassini, an Italian astronomer who eventually became a French citizen, was able to see the biggest gap within the ring system, now known as the Cassini Division or the Cassini Gap. Cassini and Huygens also discovered moons around the ringed planet, and the known number of satellites orbiting Saturn has been growing ever since.

Visit "Your Guide to Historical Celestial Observations" for a brief recap of historical observations of Saturn, the solar system and beyond.

Jet Propulsion Laboratory