Saturn is one of five ringed planets

Afew years ago, at one of our observatory’s Open Houses, we were lucky enough to be loaned a replica of one of the telescopes that, back in the 17th Century, Galileo first trained on the sky.

Having a chance to look through this replica telescope was a privilege, but the main thing we obtained from the experience was how much telescopes have improved since then, and how meticulous an observer Galileo must have been.

Long before Galileo, five starlike objects in the sky were known to be different. They did not twinkle and they wandered to and fro along a particular track among the “fixed stars.” These became known as “planets” — wanderers. The five wanderers were Mercury, Venus, Mars, Jupiter and Saturn.They were logical targets for Galileo and his telescope.

The other planets, such as Jupiter, looked like discs in his telescope. However, when in 1610 Galileo pointed his telescope at Saturn, he got a surprise. The expected disc had some sort of bright blob on either side. Then, to his further puzzlement, when he looked again in 1612, the blobs had vanished.

Christopher Wren, although widely known as an architect, was an astronomer. He was probably the first, when in 1652 he suggested that Saturn has a ring round it, but did not comment as to whether he thought the ring was attached to the planet.

However, around the same time Christiaan Huygens, making use of the rapidly improving science of telescope design, described Saturn as being surrounded by a ring system that nowhere touched the surface of the planet.

Soon after, with even better telescopes, Giovani Cassini observed the ring was really a set of many concentric rings with gaps between them. The largest gap is known as Cassini’s Division. The ring system is inclined to the plane in which the Earth moves around the Sun, so we get to see them from above, and below. In 1612 the Earth was in the plane of the rings, so we were seeing them edge on, which explains why

Galileo, with his primitive telescope, thought they had vanished. There are other planets in our Solar System have that rings, but they are not as spectacular as Saturn’s.

Saturn is a large planet, with a diameter of about 120,000 km; almost ten times the diameter of the Earth. The ring system starts about 7,000 km above Saturn’s equator and extends out to about 80,000 km. The rings all lie in the same plane and are incredibly thin, just a few kilometres, and consist mainly of small chunks of ice.

The most likely explanation is that when the Solar System formed, and the planets and their moons grew from the coagulation of discs of dust and other material, the disc of material around Saturn was inhibited from growing into larger bodies by the planet’s strong gravity. Over time, bits of material passing through the rings got captured by collisions, until all the material had evolved into a thin disc, all moving in the same direction.

Close observations by spacecraft, such as Cassini, have revealed the rings to be even more complex and intriguing. Small “moonlets” a few kilometres across have cleared gaps between the rings, and the complex interplay of gravitational forces between Saturn and its large moons have broken the ring system into a multitude of concentric rings. Saturn is a truly fascinating planet.

There are five ringed planets:- the four gas giant planets: Jupiter, Saturn, Uranus and Neptune, although only Saturn has really spectacular ones. The other ringed planet, unique in the Solar System, is the Earth. It has a ring around 38,000 kilometres out in space above the equator, and we made it. It is made up of a large number of broadcasting, communication and other satellites.

--Venus shines brightly in the west after sunset. Left to right, Mars, Saturn and Jupiter lie close together low in the southeast before dawn; The Moon will reach First Quarter on the 30th.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory in Penticton. This column appears Fridays.