Introduction

This month's article is targeted at two widely-separated groups of amateur astronomers. Beginners: the next few months are your opportunity to get out and observe Saturn in good conditions. Your first view of Saturn through a telescope is an experience you will never forget. Experienced Observers: the next few months are the perfect time to use Saturn as the showpiece object in public outreach. Relive the thrill of seeing it for the first time by giving someone else their first view.

Late February marks the return of Saturn to the early-evening sky, after it is fully dark but before it is very late at night.

An experienced observer may tell you that when it is first rising is not the ideal time to view a planet; ideally you should wait until it is high in the sky, so you are looking through less atmosphere. However, we've been waiting many months for Saturn's return and, in fact, there is really no bad time to view this stunning object. More important, early evening in February and March is arguably the best time to display the planet to non-astronomers. It's prime time; people are outside walking or shopping; kids are still awake.

This mix of convenient timing, a spectacular observing target, and the approaching end of winter, marks the beginning of the public outreach season for those of us who love that aspect of this hobby. Now is the time to arrange astronomy demonstrations for schools, community groups such as scouts and guides, and for sidewalk astronomy -- simply setting up your scope somewhere that passers-by will say "whatcha lookin at?" and letting them see.

Significant Dates

This year, 2008, the planet is visible in the dark shortly after sunset from late February until late June. It is at opposition (closest to Earth in this year's orbit) on February 24.

Saturn is high in the sky -- and thus visible through the least amount of distorting atmosphere -- at about 11:00 PM in late March, getting earlier to about in April. After April, it won't be dark in time to catch it high in the sky.

Eastern Quadrature, when our line of sight to Saturn is at right angles from our line of sight from the Sun, is May 23 this year. This position gives the planet its most attractive "3-dimensional" appearance because this is when the shadow the planet casts on its rings is most prominent.

Finding Saturn

Beginners are sometimes puzzled that planets are not shown in long-term star charts like published "star atlases". This is because, unlike the stars, they move a significant distance in the sky from year to year. (This motion is what made ancient observers realize something special was going on with those bright points of light, and it made developing early models of how the Earth, Stars, and Sun moved in relation to each other very challenging.)

So, to find planets on star charts, you need charts published for specific dates. The outer gas giants such as Saturn and Jupiter move slowly enough that annual charts are good enough, while the inner planets such as Mercury move so much that you will need charts published for specific months.

This year Saturn is easy to find. It is the brightest object in its general area of the sky, and shines with a non-twinkling yellow light. In the late evening, look south to find Orion (obvious by his distinctive 3-star belt, shoulders, and knees). Move left about twice Orion's height to the very bright star Procyon, and a little more than that far left again. The bright yellow "star" in that region of the sky is Saturn.

Image from Starry Night

If you know the constellations, and have darker skies, look East-SouthEast for Leo the Lion, easy to recognize by the distinctive C-shaped "sickle" that forms his head. The bottom star under the sickle, at his front shoulder, is Regulus. Saturn is just below and to the left of this star.

Image from Starry Night

Observing Saturn

While Saturn is beautiful under any conditions, the best observation of planets is on nights of good "seeing", meaning nights when there is limited thermal turbulence in the upper atmosphere. The Clear Sky Clock predictions for Seeing will help you locate those rare perfect evenings.

To see Saturn as a ringed planet, you will need some magnification. At about 20x magnification, you will see that it is not a star, and that it is also not a simple round disk. Somewhere between there and 100x you will first see the rings; at 100x the sphere and rings are tiny, but will be sparkling clear and sharp, and this view is something you should include in your observations. If your telescope permits, however, you should try for 150x to 200x, which will give a stunning view of the planet, the distinct ring system, and the pitch black gap between the rings and the planet. Observers with access to larger apertures will enjoy using 300x or more on nights of good seeing.

With good seeing and careful focus you will see the space between the rings and the planet, the shadow of the planet on the rings or of the rings on the planet, and possibly the gaps inside the rings. The largest gap, visible in most telescopes, is called the Cassini Division. The fainter Encke Division is a challenge target for mid-sized amateur scopes. You may also be able to make out the subtle banding in the atmosphere, caused by the high winds and rapid rotation of the planet, but much less distinctive than the equivalent markings on Jupiter. Finally, you should be able to see several of Saturn's dozens of moons arranged around the planet, in line with the plane of the rings.

Public Outreach

February through April are the perfect time to show off Saturn to friends or passers-by. It is visible in prime time, is bright enough to observe even from poor locations such as parking lots, and is the best-known astronomical object after our own Moon. People will tell you they will never forget their first view of Saturn, and there is a real thrill in providing that opportunity.

Another delightful aspect of such outreach is the comments you will receive: exclamations, and even disbelief. (I have had visitors flatly refuse to believe they were looking at something "real", and look into the objective end of the telescope for a fake image.) And another special feeling comes when a visitor returns a second time, having gone home to fetch spouse, parents, children, or neighbors.

Some Information about Saturn

Your observation of Saturn may be more enjoyable if you are armed with a few interesting scientific facts about the planet, and they can certainly make a conversation with a visitor more interesting and stimulating. New information about Saturn is coming in rapidly with the exploratory spacecraft in that region.

Rings

Saturn's rings are, of course, its most obvious and spectacular feature. But did you know it is not the only ringed planet? In fact, all 4 of the gas giant planets (Jupiter, Saturn, Uranus, and Neptune) have rings, but only Saturn's are visible from Earth with amateur telescopes.

Saturn's rings are generally believed to be a "failed moon", that is, material collected during the formation of the planet but which was unable to coalesce into a solid body because it is inside Saturn's Roche Limit - the distance inside which a planet's tidal forces overwhelm a satellite's own gravitational forces, preventing it from holding together. (Small rocky satellites, held together by chemical cohesion, can exist inside the Roche limit but no large satellites exist inside their parent planet's Roche limit. On the other hand, every gas giant's rings are inside their planet's Roche limit.)

The sharp, crisp boundaries of the rings are believed to be defined by the effect of "shepherding satellites" -- moons orbiting in the region whose gravity and tidal forces act to clear particles from the orbit outside specific regions. The rings do not dissipate over time because the particles are being replenished by material swept off small inner satellites by friction and tidal forces.

You will notice you are not looking at Saturn's rings exactly edge-on. That's good, because they would be very hard to see. The planet's rings are tilted 27 degrees to its orbit, so our angle of view to them changes as saturn goes through its 29-Earth-year orbit around the Sun. We will be looking at the rings edge-on in 2009, then again in 2025. That means this year, 2008, will be the last chance to see them well for a couple of years.

Moons

Saturn has so many moons that any published count is probably incorrect. As I write this about 50 have been discovered, of which 35 have been officially named.

The most interesting moon to contemplate or to point out to visitors is Titan. It is the only moon known, with certainty, to have liquid on its surface, because a probe was successfully landed there in 2005. The temperature is far too low for liquid water, and the liquid is believed to be Methane.

Planet

The planet itself has an interior rocky core surrounded by a very thick atmosphere of mostly Hydrogen and Helium. There is so much gas that, on average, the planet's density is less than that of water. In other words, if you had a big enough puddle, Saturn would float.

Like the Earth and all the gas giant planets, Saturn has a magnetic field. However, unlike any other planet, the magnetic axis is almost precisely aligned with the rotation axis. In other words, the magnetic North Pole is at the real North Pole. (On Earth, by comparison, the magnetic pole is in northern Canada, quite far from the North Pole.)