Few people are not awe struck by their first look at Saturn through a telescope, and with good reason. Saturn and its ring system are simply spectacular! Jupiter may be the king of the planets, but Saturn may be the most beautiful. Saturn is the sixth planet from the Sun and the second largest in the solar system after Jupiter. It was named after the Roman god Saturn, who was the god of agriculture and the harvest. Like Jupiter and the other planets in the outer solar system, Saturn is a gas giant that does not have a visible solid surface. Saturn is made up mostly of hydrogen gas with a small amount of helium. It probably has a rocky core, surrounded by liquid metallic hydrogen.
Saturn's equatorial diameter is 120,536 kilometers (about 74,897 miles), but is squashed a bit at the poles because of its rapid rotation. It is about 9.5 times the Earth's diameter of 12,742 km (7,917 miles). It takes 29.4571 Earth years for Saturn to orbit once around the Sun. It's brightness can vary from magnitude +1.47 to -0.24, and it's size can vary from 14.5 to 20.1 arcseconds for the ball of the planet, not including the rings. Saturn is largest and brightest when it is closest to Earth at opposition, which occurs about every year and two weeks (378.09 days). Because Saturn's axis of rotation is tilted 27 degrees to the plane of its orbit around the Sun, we see the rings tilted towards us at different angles during its 29 year orbital period. Sometimes we see them tilted towards us, and sometimes we see them edge-on at "ring-plane crossing", when they almost disappear because they are so thin.
Plus numbers in the table at right for the ring plane tilt indicate that Saturn's northern hemisphere is tilted towards us. Minus number indicate that we are seeing the bottom of the rings and that Saturn's southern hemisphere is tilted towards us. Ring plane crossings occur once every 14 to 15 years when the rings are tilted 0 degrees towards us and we see them edge on. The next time Saturn's rings are at maximum tilt towards us occurs in June of 2017. The next ring plane crossing, when the rings are edge on, occurs in March 2025, but it happens when Saturn is only about 8.75° from the Sun, so it will not be easily visible. Features on Saturn
Like Jupiter, Saturn has belts and zones of clouds in its atmosphere that are visible on the ball of the planet. Saturn also has an amazing set of rings surrounding it. Saturn's Rings Saturn's rings are its most distinctive and amazing feature. They are not solid, but rather are made of water ice particles which range from microns to several meters in size. The rings are actually amazingly thin. They are only about ten to twenty meters (about 33 to 66 feet) thick! We see Saturn's rings because of sunlight reflected off the ice particles. There are two main theories as to how Saturn's rings formed. One is that they are left over from the original material that formed Saturn when the solar system itself formed. The other is that the rings formed from ice stripped from a moon that was destroyed by tidal forces. Jupiter, Neptune and Uranus all have ring systems, but nothing even remotely resembling Saturn's. Those planet's rings are basically invisible in amateur equipment from Earth. Saturn's rings, on the other hand, are stunning. Saturn's three main rings are the A, B and C rings, named in order with the A ring being farthest from the planet, and the C, or Crepe, ring being closest, with the B ring in between. Cassini's Division and Encke's Gap Galileo Galilei was the first person to observe Saturn's rings in 1610. Christian Huygens first described them as a disk in 1655. Giovanni Domenico Cassini first observed the gaps in the rings, with the largest named "Cassini's Division" after him. The rings are broken up by a series of divisions, gaps and minima. Cassini's division is the most famous and is easily visible in a small telescope. To resolve Cassini's division in an image requires a larger telescope however. Cassini's division, which separates the A and B rings, is 4,800 kilometers (2,982 miles) wide. From Earth at opposition, Cassini's division subtends an angle of about only 0.8 arcseconds. While Cassini's division can be detected with a small telescope (it was discovered with just a 2.5-inch refractor), it really takes about a 10-inch telescope to resolve the division. The Holy Grail of world-class, high-resolution planetary imagers is to record Encke's gap, a tiny gap located about 1/5th of the width of the A ring in from its outer edge. Encke's gap is only 325-kilometers wide (about 202 miles). At an opposition average of 1.2 billion kilometers distance from Earth, Encke's gap subtends an apparent size of only 0.06 arcseconds, or six hundredths of an arcsecond, an incredibly small angle.
This size angle can not be resolved with any amateur-sized instrument. It can, however, be detected and recorded with large amateur instruments with excellent optics by astrophotographers with top-notch skills under periods of outstanding seeing. See our previous discussion of the difference between resolving and detecting detail in Chapter 1. Most amateurs who believe they have visually observed or photographed Encke's gap in moderately small-sized telescopes are simply mistaken. Many visual observers confuse Encke's minima, in an apparent darkening in the middle of the A ring, for Encke's gap, which is really located in the outer portion of the A ring. Many photographers mistake processing artifacts such as The Ghost of Encke for Encke's gap. Confusingly, Encke's gap was never observed by Johann Franz Encke. What he saw is generally considered to be what is now called Encke's minima, an apparent wide darkening in the middle of the A ring. Encke's gap was possibly first observed visually by Father Francesco De Vico in 1838 and William Lassell and the Rev. William R. Dawes in 1843. NASA, however, credits the first definitive observation of this gap to James Keeler in 1888. The International Astronomical Union (IAU), which is responsible for the official names of planetary features, has chosen to call the gap near the outside of the A ring "Encke's Gap" to "avoid confusion", because this is the name most commonly used for the feature. To make matters even more confusing, if that is possible, the IAU has also named another gap in the extreme outermost portion of the A ring after Keeler. "Keeler's Gap" is only 42 km (26 miles) wide. It was never observed by Keeler because it is not visible from Earth. It was discovered by the Voyager space probe. It was named after Keeler by the IAU in honor of his observations of Saturn. So there are really two features that are visible from Earth with the name of Encke in the A ring. The unofficially named "Encke's Minima", the apparent broad darkening in the middle of the A ring, and the officially named "Encke's Gap", the tiny gap almost at the outside edge of the A ring. The minima is not an official IAU name for a feature of Saturn's rings. To attempt to observe or photograph Encke's gap, the planet should be high in the sky with the ring plane tilted as wide open as possible towards the Earth. Use high magnification on a night of excellent seeing. Observe or photograph through a large (12 inch or larger) perfectly collimated telescope with excellent optics and a small secondary obstruction. The Opposition Surge
The image at right was taken on April 14, 2012, one day before Saturn reached opposition. It shows the effects of the brightening of Saturn's rings caused by the opposition surge, or Seeliger effect. Hold your mouse cursor over the image to see an image taken on March 15th, 2012, one month before opposition. It is easy to see the tremendous difference in the brightness of Saturn's rings. Opposition is when a planet is directly opposite the Sun in the sky. The opposition surge, also known as the opposition effect or Seeliger effect, is caused by several different factors including shadow hiding and coherent backscatter. The effect is the brightening of Saturn's rings compared to the ball of the planet. It is most prominent about a week before and after opposition when the Sun directly illuminates the particles in Saturn's rings from behind our viewing position on Earth. Also note the changes in the shadow of the planet on the rings at the top back right where the rings meet the ball of the planet, increased visibility of the Crepe ring, and the reduced shadow of the rings on the front of the ball of the planet. Saturn's Cloud Bands
Saturn's atmosphere is made of Hydrogen (about 96%) and Helium (about 3%). The rest is a mix of methane, ammonia, water and ammonium hydrosulfide. The ammonium hydrosulfide gives some of Saturn's cloud bands a yellowish color. Like Jupiter, Saturn also has cloud bands on the ball of the planet, but the colors are much more subtle. The clouds are driven by winds which can peak at 1,800 km per hour (1,118 mph). Bands are differentiated by winds of different speeds. Saturn's upper clouds are made of mostly ammonia ice crystals. The middle level clouds are made of ammonium hydrosulfide and water. The lower layers of clouds are comprised of water with ammonia in liquid solution. Chemical reactions caused by the Sun give the cloud bands their colors. Saturn's atmosphere can display ovals and features similar to Jupiter's atmosphere. Sometimes a storm can break out in one of these bands and the outbreak can stretch all the way around the planet. Like the Sun and Jupiter, Saturn's atmosphere rotates at different rates near the equator and the poles. Rotation rates are an important consideration if you want to capture details in the clouds on the planet, or incredibly fine detail in the rings, such as the famous "spokes". If there are details visible in the cloud bands of Saturn, such as a storm outbreak, that you want to capture in a video, you will be limited in the total time that you can shoot before Saturn's rotation blurs these details. If these details aren't present, you can shoot for a much longer time. See Chapter 1, Section 8 - Planetary Rotation and Detail Smearing. Saturn's Moons Saturn has 53 officially named moons and many more moonlets. Eight are bright enough to be seen and recorded in amateur instruments. They are, in order of distance from Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion and Iapetus. Titan is Saturn's largest moon, and the second largest moon in the solar system after Ganymede. Even though Titan is a moon, it is larger than the planet Mercury. Titan has a thick atmosphere where complex organic chemistry is thought to take place. Titan also has lakes made of hydrocarbons. Enceladus is another fascinating moon which may have oceans of water under a crust of water ice. Water plumes containing hydrocarbons also vent from the surface. These may be the source of material that forms Saturn's E ring. There is evidence that Enceladus is volcanically active, and may be one of the most promising locations in the solar system for life outside of the Earth.
The inner large moons are Mimas, Enceladus, Tethys and Dione. The outer large moons are Rhea, Titan, Iapetus and Hyperion. The large moons of Saturn and their magnitudes: Saturn has several other moons that are brighter than 19th magnitude. They are: Photographing Saturn's moons is a problem because of their faintness in comparison to Saturn's brightness. Exposures long enough to record the fainter moons will greatly overexpose Saturn. If you want to try to photograph Saturn's faint moons in a single exposure, you can just use a long exposure and let Saturn overexpose. Any moons that are close to Saturn will probably be lost in the glare of the planet however. You can shoot a short exposure that is correct for Saturn, and then a longer exposure for the faint moons and composite the two images together. You can also shoot a series of long and short exposures and put them together as a high-dynamic range image. With my C11 with 11 inches of aperture, at f/10, I recorded Enceladus at magnitude 11.7 with a 1 second exposure at ISO 1600. To record Hyperion, at magnitude 14.2, I used a 15 second exposure. The problem is that the correct exposure for Saturn was only 1/60th of a second. This is about 10 stops difference. Sky & Telescope's Saturn's Moons Calculator will show the locations of Saturn's moons. Tips for Photographing Saturn
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