Choosing A Telescope
Telescope Aperture Considerations
Introduction to Planetary Sketching
Refractor Dobsonian Style Mounts
The Bonds: Pioneers of American Astronomy
Saturn's Encke Minima and Encke Division
Nature and Travel Photography
Recommended Astronomy Books
In the above sketch the SPR = South Polar Region, STrZ = South Tropical Zone, SEB = South Equatorial Belt, EZs = Equatorial Zone, south, EZn = Equatorial Zone, north, NEB = North Equatorial Belt, NTrZ = North Tropical Zone, NPR = North Polar Region.
Following and Preceding - when observing Saturn with a telescope with an even number of reflections, such as a Newtonian reflector, or refractor used without a star diagonal, south is at the top. Features on the disk appear to rotate from the right hand side to the left hand side of the disk (when using a telescope of different configuration, note that north may be at the top, and the planet may appear to rotate from left to right).
The right hand side of the disk (as defined by the Central Meridian or CM) is the following side of the disk; the left hand side is the preceding side. Using an accurate watch, pencil, and notepad it is possible to make CM timings. When a feature, such as the GRS, approaches the CM, note the time to the nearest minute the time its preceding edge transits the CM. Do the same for its center and following edge.
Note that the North Polar region will be tilted towards the Earth until the spring of 2025.
Features in Saturn's Atmosphere
Saturn's atmosphere is thought to be composed mostly of hydrogen and helium (almost 99%), with traces of ammonia, phosphine, water vapor, and hydrocarbons. The white and pale yellow colored zones are caused by the upwelling of clouds that rise into the cool upper atmosphere of the planet, while the darker yellowish and tan belts are caused by descending air into the lower and warmer portion of the atmosphere. Note that there is a haze layer in Saturn's upper atmosphere that makes Saturn appear much less colorful than Jupiter's atmosphere.
As with Jupiter, Saturn's alternating bright zones and dark belts are designated in order from the pole to the equator as: polar, temperate, tropical, and equatorial. Below is a list of each of the belts and zones, its abbreviation, and the system it resides in. Note that belts and zones can sometimes have a north, center, and south component to them, and not all belts and zones are visible during each opposition.
*Since Saturn does not rotate on its axis at a uniform rate, two rotation periods have been assigned to it: System I is 10 hours 14 minutes and 00 seconds, encompassing the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 hours 45 minutes and 45 seconds, which is System II.
Saturn has a number of rings, designated as A, B, C, D, E, F, and G. The three rings that can be observed from earth are A, B, and C. The rings are most likely composed of ice and rocky material that range from sub-millimeter in size to meters across in size. Recently cameras on the Cassini spacecraft recorded plumes of fine, icy particles and water vapor emanating from the warm, geologically region around the south pole of Enceladus. This supplies Saturn's E-ring with snow particles. Brightness measurements from the Cassini spacecraft have indicated also that the rings of Saturn are only about one kilometer (0.6214 miles) thick.
Galileo was the first to observe Saturn through a telescope in 1610, and during subsequent observations it appeared to him that the planet had handles (or "ansae") on each side. In 1675 Giovanni Cassini discovered that the A-Ring is separated from the B-Ring by a division that now bears his name, the Cassini Division. The C-Ring or Crepe Ring was co-discovered The Bonds, and William Dawes in 1850, although there is evidence to suggest that other observers saw the Crepe Ring before this without realizing they were seeing a separate ring. It was William Lassell who gave the name "Crepe" Ring as it reminded him of a Crepe veil in front of Saturn's globe.
Of the A-Ring, B-Ring, and C-Ring, the B-Ring is the brightest and widest. It is also the densest ring, as noted during an occultation of a star by Saturn . The A-Ring is dimmer than the B-Ring, while the C-Ring is dimmer than both the A-Ring and B-Ring and easier to see when the rings are wide open. Within the A-Ring are the Encke Minima and Encke Division.
As seen from Earth the rings vary from 0 degrees to -27 degrees when the southern portion of the globe is visible, then from 0 degrees to +27 when the northern portion of the globe is visible. There is evidence to suggest that this change in ring angle causes seasonal color changes of Saturn's atmosphere. For example, when one hemisphere has been in eclipse by the rings for a number of years, the color of that hemisphere has more of a gray, gray-green, or blue-gray color to it. In my first sketch above the color of the southern portion of the globe, after it was coming out of eclipse for a number of years, had more of a gray-green color to it. In the second sketch, made after the southern hemisphere had been exposed to the sun for a longer period of time, the colors appear more yellow and yellow-brown in appearance.
This graphic was created by a Curt Renz and used with his permission.
Moons of Saturn
As of March 2012, Saturn has 53 moons. Only a hand full are bright enough to be visible in backyard telescopes. This includes Titan (pronounced "TI-ton", magnitude 8.2), Rhea (pronounced "REE-a", magnitude 9.6), Tethys (pronounced "TEE-thiss", magnitude 10.1), Dione (pronounced "dy-OH-nee", magnitude 10.3), Iapetus (pronounced "eye-AP-eh-tuss", magnitude 10.2 to 11.9), Enceladus (pronounced "en-SELL-ah-dus", magnitude 11.6), Mimas (pronounced "MY-mass", magnitude 12.8), and Hyperion (pronounced "hi-PEER-ee-en", magnitude 14.1).
Titan is the easiest moon of Saturn to observe. It was discovered by Christiaan Huygens using a refractor telescope with an aperture of 57mm on March 25, 1655. It is the largest moon of Saturn and the second largest moon in the solar system, as only Jupiter's moon Ganymede is larger with a diameter of only 62 miles (112 kilometers) larger. Titan is also larger than the planet Mercury. Titan is similar to the Earth in some ways, as it has a dense atmosphere (composed mostly of nitrogen and methane), but unlike the Earth its atmosphere does not contain oxygen. It appears also that Titan's atmosphere may have lightning, and it is cold enough ( -293° Fahrenheit or -180° Celsius) that methane may fall as rain. While there is ice on the surface of Titan, there is no liquid water because it is too cold, so life would not exist because water is a prerequisite for life. However if there is liquid water in the interior of Titan than life may exist there. When observing Saturn with a 7" aperture telescope with steady seeing conditions Titan is resolved as a disk and a light reddish-orange color. In a 5" aperture telescope with steady seeing conditions Titan sometimes has a light reddish-orange color to it but it is not resolved as a disk.
Rhea, Dione, Tethys, and Iapetus are the next easiest moons to observe. Giovanni Cassini, using a refractor telescope with an aperture of 108mm, discovered Iapetus in 1671, Rhea in 1672, Dione and Tethys in 1684. Rhea, Dione, and Tethys are composed mostly of water ice and are similar to each other. Rhea is the largest airless satellite of Saturn, while Dione is the densest moon of Saturn other than Titan. This is because Dione may have rocky core making up one-third of the its mass, with the remainder being water-ice. The ice coverage on Dione is less than that of Tethys and Rhea. Iapetus is somewhat different as one half is dark, while the other half is four to five times brighter than the dark half. Hence its magnitude varies from 10.2 to 11.9. The dark half faces forward as the moon revolves around Saturn, so it may have swept up dark reddish-colored material orbiting Saturn that may have originated from the moon Phoebe (pronounced "FEE-bee").
Mimas and Enceladus are very similar to each other and composed mostly of ice. William Herschel discovered Mimas and Enceladus using an 18.5" reflector in 1789. Mimas and Enceladus can be somewhat challenging to observe. This is because they are among the innermost moons of Saturn, which makes them harder to see, particularly when Saturn's rings are wide open. However I have seen them when the rings are more closed up in a 7" aperture telescope. Recently cameras on the Cassini spacecraft recorded plumes of fine, icy particles and water vapor emanating from the warm, geologically region around the south pole of Enceladus. This supplies Saturn's E-ring with snow particles and may eventually coat other moons. The interior of Enceladus may be heated by a tidal friction due to Saturn's gravitational field and by the large neighboring satellites Tethys and Dione. This puts Enceladus in the class of geologically active moons with Jupiter's Io and Neptune's Triton.
Hyperion was discovered by William Cranch Bond, his son George Phillips Bond using a 15" refractor, and William Lassell, using a 24" reflector, in 1848. Even though Hyperion is one of the smaller moons of Saturn, it is the largest irregularly shaped natural satellite ever observed. Its shape suggests that meteors may have hit it, and it may be the oldest surface in the Saturn system. The color of Hyperion is very similar to the dark reddish color on the dark material on Iapetus. Hyperion has an eccentric orbit and its rotational period to varies from one orbit to the next.
Spokes in Saturn's rings have been reported by observers at least as far back as the 1850's. Often however these accounts were dismissed as optical illusions as these features did not seem consistent. However, data from the Voyager spacecraft confirmed that these spokes, often appearing as radially-extended features, are indeed a real phenomena. The most likely explanation is that spokes are caused by clouds of submicron-size ice or dust particles that are levitated into Saturn's magnetic field lines and rotate at the same speed as the magnetic field, which has a rotation period of 10 hours 45 minutes and 45 seconds. Radial spokes have been reported in the A-Ring, B-Ring and C-Ring.
"I do not know what to say in a case so surprising, so unlooked for, and so novel."
Galileo, 1612, who after observing Saturn's rings when they were only narrowly open on in 1610, was unable to see them when he observed Saturn again in 1612 when the rings were near edge on. He correctly predicted however that the rings would become visble again.
"There is not perhaps another object in the heavens that presents us with such a variety of extraordinary phenomena as the planet Saturn: a magnificent globe, encompassed by a stupendous double ring: attended by seven satellites: ornamented with equatorial belts: compressed at the poles: turning upon its axis: mutually eclipsing its ring and satellites, and eclipsed by them: the most distant of the rings also turning upon its axis, and the same taking place with the farthest of the satellites: all the parts of the system of Saturn occasionally reflecting light to each other: the rings and moons illuminating the nights of the Saturnian: the globe and satellites enlightening the dark parts of the rings: and the planet and rings throwing back the sun's beams upon the moons, when they are deprived of them at the time of their conjunctions."
Sir William Herschel, 1805
Article © 2000 - 2013, Eric Jamison, All rights reserved.
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