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The 400th Anniversary of Galileo's First Saturn Observations
What's Up: Brought to you by Jane Houston Jones

On July 15, 1610, four months after the publication of Sidereus Nuncius, Galileo first aimed his 30 power telescope at Saturn.

Sketch of Saturn by Galileo in 1610
From Huygens' Systema Saturnium, this represents a sketch of Saturn by Galileo in 1610.
He was surprised at what he saw and wrote, that "the star of Saturn is not a single star, but is a composite of three, which almost touch each other, never change or move relative to each other, and are arranged in a row along the zodiac, the middle one being three times larger than the two lateral ones, and they are situated in this form o O o.

He quickly alerted his patrons but wanted to keep his observation secret until publication. He sent this anagram to fellow scientists in late July 1610:


In November he sent the solution to his friends, "The letters...combined in their true sense, say thus: Altissimum planetam tergeminum observavi," which means, "I have seen the highest planet [Saturn] triple bodied."

In July 1610, when Galileo observed Saturn, the rings were open to a narrow inclination or tilt. In his modest 30 power telescope, it appeared as three objects. In one of his less powerful telescopes, Saturn looked oval shaped.

Saturn 1616
Saturn 1616
He wrote again that year, "If one looks at them with a perspective which is not of very great multiplication, they will not appear as three distinct stars, but it will appear that Saturn is a long star in the shape of an olive, like this (). But when a perspective which multiplies one thousand times in surface detail is used, three globes will be seen distinctly, and [it will be seen] that they almost touch, no greater separation appearing between them than a thin dark line."

Galileo observed Saturn from July 1610 to May 1612 and did not see any change in the triple-bodied appearance. But in December 1610, he made this observation: "... Having ceased to observe him for more than two months, as one who does not doubt his constancy, ...I found him solitary without the assistance of supporting stars, and, in sum, perfectly round and clearly defined as Jupiter."

A view of Saturn from earth
A view of Saturn from earth. Image credit: Anthony_Wesley.
In 1616, Galileo once again observed Saturn. He was expecting to see the triple bodies, but, he wrote of Saturn "whose two companions are no longer two small, perfectly round globes as they were before, but are at present much larger bodies, and no longer round, as seen in the adjoined figure, that is, two half ellipses with two dark little triangles in the middle of the figures, and contiguous to the middle globe of Saturn, which is seen, as always, perfectly round."

On the 400th anniversary of Galileo's first sighting of Saturn, you can see Saturn for yourself and you don't even need a telescope. A trio of planets are visible in the western sky just after sunset. Lowest and brightest is Venus. Above Venus is Mars, fainter in magnitude and reddish in color.

Finally, above both Venus and Mars, you'll find the beautiful outer planet Saturn, appearing golden and brighter than Mars.

Through binoculars, Saturn will appear round or slightly oval, and a telescope will reveal a thin nearly edge-on ring.

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About: Jane Houston Jones
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Jane is an astronomer, NASA science podcast developer, writer, educational and outreach artisan and social media enthusiast. She and her husband have an asteroid (22338 Janemojo) named after them.
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Last Updated: 20 Jan 2011