Professor Hanington's Speaking of Science: More moons for Saturn
Professor Hanington’s Speaking of Science

Professor Hanington's Speaking of Science: More moons for Saturn

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They just announced the discovery of 20 more moons for the planet Saturn this week and you can even help name them if you want!

A team led by Carnegie Institute’s Scott S. Sheppard found the moons using the 8.2-meter Subaru telescope atop Mauna Kea in Hawaii. The observing team included astronomers Sheppard, David Jewitt of UCLA, and Jan Kleyna of the University of Hawaii.

This brings the ringed planet’s total number of moons to 82, surpassing Jupiter, which has only 79. The discovery was announced Monday by the International Astronomical Union’s Minor Planet Center.

Each of the newly discovered moons is about three miles in diameter and may be asteroids that the planet’s gravitational field captured long ago or debris that came from collisions between earlier moons. Seventeen of them orbit the planet in a retrograde direction, or clockwise when viewed from above our own North Pole. This is completely opposite of Saturn’s rotation around its axis.

Because astronomers believe our Solar System originated from a huge disc of spinning matter, all planets circle the Sun and spin counter-clockwise (except Venus and Uranus) due to the angular momentum left over from the start. Even our own Moon travels around the Earth in that way. When a moon travels against this direction it usually indicates they joined the pack much later on.

“Studying the orbits of these moons can reveal their origins, as well as information about the conditions surrounding Saturn at the time of its formation,” Sheppard explained.

These outer moons of Saturn appear to be grouped into three clusters in terms of the inclinations of the angles at which they are orbiting around the planet. Two of the newly discovered moons fit into a group with inclinations of about 46 degrees and these may have once comprised a larger moon that was broken apart in the distant past.

Almost all of the newly discovered objects take two to three years to circle the planet, making them the farthest out from the planet. By comparison, the closest moon of Saturn, Pan, is 83,000 miles out and takes only a half day to circle the planet. Because Saturn has so many moons it is interesting to trace their discovery.

When Galileo became the first person to observe Saturn’s rings in 1610, he missed the opportunity of discovering any moons for that planet. Many historians think this odd because he had just discovered the four moons of Jupiter a few weeks before.

It wasn’t until 45 years later that Christiaan Huygens, the great Dutch astronomer, picked out the bright Titan and pronounced it to be a moon of that planet. As the years rolled on, astronomers such as Cassini in the 1670s found four moons and Herschel a hundred years later found a few more.

In the late 1800s the use of long-exposure photographic plates made possible the discovery of additional moons to the list. The first to be discovered in this manner, Phoebe, was found by Pickering working at Percival Lowell’s Flagstaff Observatory in 1899.

By 1966 the tenth satellite of Saturn was discovered by Audouin Dollfus when the rings were observed edge-on near an equinox, clearing the way for an unobstructed view. This moon was named Janus in honor of the Roman god of doorways and passages.

But, only a few years later it was realized that all observations of 1966 could only be explained if another satellite had been present and that it had an orbit similar to that of Janus. This object is now known as Epimetheus, the eleventh moon of Saturn. It shares the same orbit with Janus — the only known example of co-orbitals in our Solar System. By 1980, three additional Saturnian moons were discovered from the ground using very long time exposures.

The study of the outer planets has since been revolutionized by the use of unmanned space probes. The arrival of the Voyager spacecraft to Saturn in 1981 resulted in the discovery of three additional moons and with the Cassini mission in 2004, the count really jumped.

“Using some of the largest telescopes in the world, we are now completing the inventory of small moons around the giant planets,” says Scott Sheppard. “They play a crucial role in helping us determine how our Solar System’s planets formed and evolved.” Last year, Sheppard discovered 12 new moons orbiting Jupiter and Carnegie hosted an online contest to name five of them. They are doing this again with a contest end date of December 6, 2019. But there are some rules:

“The moons must be named after giants from Norse, Gallic, or Inuit mythology,” says Sheppard. If you want to take part, just Tweet your suggested moon name to @SaturnLunacy and tell them why you picked it. Also, don’t forget to include the hashtag #NameSaturnsMoons. They also say to make sure your proposed name is not already in use and current names can be checked out online at the International Astronomical Union’s Minor Planet Center.

Gary Hanington is Professor Emeritus of physical science at Great Basin College and chief scientist at AHV. He can be reached at or


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