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Agena AstroProducts Observing Guide: The Planet Saturn

By: Brian Ventrudo
July 20, 2017
 
Observing the planet Saturn
Figure 1 – The planet Saturn. Image credit: NASA/JPL.

1. Overview

Many casual observers get hooked on amateur astronomy after a first look at Saturn through a telescope. To most observers, Saturn is an incredibly beautiful sight. The color, the proportions, the apparent 3D perspective of this grand icy world make it arguably the finest sight accessible with a small telescope, and many stargazers look forward to its frequent oppositions when the planet makes its closest approach to Earth. This article helps you understand what to see on and around the planet Saturn, and it gives you some tips for choosing and using the best equipment to see this ringed planet.

 

2. Saturn – The Basics

Saturn is the sixth planet from the Sun and the second-largest planet in our solar system after Jupiter. The planet takes its name from the Roman god of agriculture and time, Saturn, whom the ancient Greeks called Cronus.

At its equator, Saturn has a diameter of about 120,600 km, about 9.5 times the diameter of the Earth, while at the poles it spans about 8.5 Earth diameters. The planet has a volume of 760 Earths and a mass of about 95 Earths. Its density is 30% less than that of water.

The planet averages about 9.5 astronomical units (AU) from the Sun, or about 1.4 billion km. At this distance, it revolves around the Sun every 29.5 years. So it makes a complete trip around the Sun a little more than twice during an average human lifespan.

Like Jupiter, Saturn rotates quickly, once every 10.57 hours. During the course of a night, you see an appreciable fraction of the entire planet. Also like Jupiter, the planet's non-solid nature means its quick rotation flattens it into an oblate spheroid, with a larger diameter at the equator than at the poles.

Saturn is a gas giant planet, with no solid surface. The planet's core consists mostly of iron, nickel and silicates, all squeezed by gravity to a blazing hot temperature of some 11,000 K. And like Jupiter, Saturn radiates about 2.5x more heat than it receives from the Sun.

Saturn's core is likely surrounded by a deep layer of liquid metallic hydrogen around which lies a layer of compressed liquid hydrogen and helium. Its icy outer gaseous layers also consist of about 96% hydrogen and 3% helium, with traces of heavier molecular gases like ethane, methane, acetylene, and ammonia.

The clouds of Saturn, the parts of the planet we see directly from Earth, have a pale sand color and are far less colorful than Jupiter's cloud layers. Their color is caused by frozen crystals of ammonia in the upper atmosphere and ammonium hydrosulfide in the lower layers.The temperature in the upper reaches of the clouds is as low at 100 K (-173 °C). The planet's faint bands have a nomenclature very similar to those of Jupiter (see image below).

Observing the planet Saturn
Figure 2 - The architecture of Saturn's rings and cloud bands. North on Saturn is the side that faces the north celestial pole as seen from Earth.Image credit: Robert English.

The wind speeds in the outer Saturnian atmosphere reach up to 1,800 km/h, faster than on Jupiter, and these winds stir up some interesting phenomena including many fascinating transient features and many puzzling permanent features. In 1990, amateur and professional astronomers watched a white oval well up from deep in the planet's atmosphere and float to the surface, then dissipate slowly over the course of many weeks. This short-lived storm seems to reoccur approximately once every Saturnian year when summer peaks in the planet's northern hemisphere. A similar storm was observed in 1903, 1933, and 1960. If the pattern holds, the next storm will occur around the year 2020. The cause of this phenomenon is unknown.

Another unrelated storm welled up in Saturn's atmosphere in 2010. It started as a small white spot, then spread out across the planet as it caught the high differential winds in the upper atmosphere. The storm was imaged by the Cassini spacecraft and by Earthbound professional and amateur observers for many months. It was even visible visually to observers with small telescopes and steady skies.

Saturn also has a very strange hexagonal feature in its atmosphere near its north pole. It was first discovered by the Voyager spacecraft, but it can now be seen in detailed images from amateur telescopes. The hexagon spans about 14,000 km on side, bigger than Earth, and seems to rotate with the planet's magnetic field.

 

3. The Rings of Saturn

If not for its rings, Saturn would be a paler, small, and fainter version of the planet Jupiter. But it does have rings, of course, that are big and bright enough to see with nearly any telescope. Jupiter and Uranus also have rings, but Saturn's system is by far the brightest and most intricate. The rings extend from about 7,000 km to 120,000 km above the equator, yet are only about 20 meters thick. They consist mostly of small particles of water ice and bits of frozen organic compounds. The particles range in size from fine dust-like grains to boulder-sized pieces no more than 10 meters in diameter.

The origin of Saturn's rings is unknown. They may be been formed by material left over from Saturn's formation some 4.5 billion years ago. Or they may be the remnants of a small moon destroyed by tidal forces when it wandered too close to the planet. If this is the case, the rings may be quite new.

Observing the planet Saturn
Figure 3 - Saturn as it appears at high magnification in a small telescope. Image credit: Pete Lawrence.

The ring system is amazingly complex. There are dozens of tiny "shepherd" moons among the rings which split the rings into bands and finer subrings that astronomers are still sorting out. But from our perspective as backyard stargazers, the rings are split into three main sections: the outer A-ring, the middle B-ring, and the darker inner C-ring. Between the A and B rings lies the dark gap called the Cassini division named after the 17th-century astronomer who first noticed it. This division is visible in small telescopes in reasonably steady skies. The smaller Encke division in the outer reaches of the A-ring is a much more challenging sight; you need rock-steady sky and a telescope of at least 6-inches to 8-inches aperture.

The apparent tilt of the rings, as seen from Earth, changes from year to year in a cycle that lasts 13-16 years. In 2002-2003, the rings were tilted at a maximum of 26-27°. In 2009, the rings were edge-on for a time and briefly became invisible from Earth. Their angle of inclination has been increasing since then and in late 2017 the rings reach their maximum tilt once again of about 27°. When they're near maximum tilt, you may be able to trace the outer rings all the way around the planet, even partially around the far side of the planet.

More than most planets, when seen in steady sky in a good telescope, Saturn displays a striking 3-D effect caused by the darkened edges of the disk and, when you can see them before and after opposition, the shadows cast by the rings on the planet. Also in the days around opposition, when the planet lies exactly opposite the Sun in the sky, you may see the rings shine a little brighter than in the weeks before and after opposition. This is a consequence of the Seeliger effect, the temporary disappearance from our point of view of the shadows of the tiny ice particles that make up the rings.

 

4. Saturn's Moons

Saturn has more than 200 moons, 62 of which have well-determined orbits. The remainder are small bodies... moonlets... that bob and weave through the planet's elaborate ring system. Of this large collection of moons, nine were discovered with telescopes before the age of spaceflight. They are, in increasing distance from Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Phoebe. The moons were named after the Titans of ancient Greek legend. Just seven of Saturn's moons are large enough to pull themselves into a spherical shape.

Observing the planet Saturn
Figure 4 – A simulated view using Stellarium of Saturn's brightest moons.Image credit:Stellarium.

Titan is by far the largest of Saturn's moons, making up nearly 96% of the mass of all bodies orbiting Saturn, including the rings. Titan is the second-largest moon in the solar system after Jupiter's Ganymede, with twice the mass of Earths' moon, and it's larger than the planet Mercury. Titan is also the only moon with a substantial atmosphere, one made mostly of nitrogen along with traces of methane and other organic molecules.

You can easily see 8th-magnitude Titan in binoculars or small telescope. Larger scopes resolve the moon into a disk and reveal the orange color of the moon's dense atmosphere. With a telescope of 4-inch aperture or larger, and dark sky, you can also find the moons Iapetus, Rhea, Dione, and Tethys, all of which are approximately magnitude 10-11. It's hard to tell one from another. Here's a link for an online javascript tool at Sky and Telescope to help you find which moon is which from night to night:

http://www.skyandtelescope.com/observing/celestial-objects-to-watch/saturns-moons-javascript-utility

Simply enter the date and it will render a small diagram to help you find which moon is in which position around the planet. You may need to register a free account at Sky and Telescope to access this tool.

 

5. Saturn Observing Tips

The best time to observe Saturn is when the planet is near or at opposition when it's opposite the Sun in the sky and rises as the Sun sets. At opposition, the Earth and Saturn are on the same side of the Sun and the planet is closest to Earth. Oppositions of Saturn occur at approximately 12.5 month intervals. Some dates of recent and upcoming Saturnian oppositions are: June 15, 2017,June 27, 2018, July 9, 2019, July 20, 2020, and August 1, 2021. At or within a couple of months of opposition, Saturn shines at about magnitude 0.0 and has an apparent diameter, including rings, of 44" to 47". That's about the same apparent diameter as the entire disk of Jupiter at opposition.

In most amateur telescopes, the visual images of Saturn and other planets, even at high magnification, are quite small. Beginners may find this disappointing at first. But you can still see quite a bit of detail, especially when observing on nights of good seeing when the air is steady. You can get a good idea if seeing is good by looking for twinkling of bright stars. If stars exhibit very little twinkling, the air is reasonably steady and you can expect to see sharp images of Saturn. If the air is very turbulent, even the best telescope will show the planet as an extended featureless blob.But even on good nights, you must be patient and wait for moments of very steady air when the planet's disk appears to grow sharp and starkly defined.

To get the best view, wait until Saturn is near the meridian, its highest point in the sky for the night. Just after it rises or just before it sets, the planet will lie low in the sky and its light will pass through thick, dusty, and unsteady layers of the atmosphere. As the planet rises, you will have to look through less of the Earth's atmosphere.

And make sure you give your telescope at least 15-30 minutes to come to the ambient temperature of your surroundings after bringing it outside.

More magnification will always give you a bigger image, but it will not necessarily give you a better image. The steadiness of the atmosphere and the quality of your telescope and eyepiece will influence the optimum magnification you can use on a particular object on a particular evening. You must experiment with magnification each night to get the best view. In any case, don't expect a Hubble-like image. Visually, in an amateur telescope, you will never see Saturn quite as well as you want to!

 

6. The Best Telescopes for Observing Saturn

Binoculars of 10-12x will show Saturn as a tiny, slightly non-circular disk its largest Moon Titan as a tiny point. But that's about it. To clearly see the rings of Saturn, the Cassini and Encke divisions within the rings, the yellow-orange cloud bands, and the planet's fainter moons, you will need a telescope.

Which telescope is best for seeing Saturn?

With a lack of a central obstruction, refracting telescopes generally offer the best contrast for visual planetary observation. Achromatic refractors with long focal ratios of f/12 or f/15 were once the gold standard for amateur planetary observers. But these long-tube refractors have largely been replaced by shorter focal length apochromatic or semi-apochromatic refractors that give sharper views with much less false color from the refractive properties of the lenses. Semi-apochromatic apochromatic refractors with doublet objective lenses are a particularly good value for visual observation of Saturn and other planets. These scopes are sometimes called "ED" or extra-low dispersion refractors. The f/9 100mm ED refractor and f/7.5 120mm ED refractor from Sky-Watcher are good choices as they offer reasonably large apertures of slightly longer focal ratios than other refractors and come at an affordable price.

Observing the planet Saturn
Figure 5 – A small, portable refractor like the TeleVue 85 is an excellent choice for observing Saturn and other planets.Image credit: Tele Vue Optics

Larger-aperture scopes give higher resolution of fine detail, but smaller high-quality apochromatic refractors can be pushed to provide higher magnifications on nights of good seeing. In smaller scopes, good quality optics is a big help. The 85 mm TV-85 refractor from Tele Vue Optics is renowned for its sharp images. This telescope can provide magnifications of 200x or more, plenty high enough to give a pleasing image of Saturn in good sky. The relatively small aperture of this scope, however, renders the image somewhat dim. Vixen Optics also has an 80mm apochromatic refractor with good optics for viewing Saturn at moderate magnifications.

Achromatic refractors of 6" aperture or less can also be used for planetary observation, but their inherent chromatic aberration can make for fuzzier images and a distracting purple glow around bright images of Saturn and other bright planets.

Observing the planet Saturn
Figure 6 – The long focal lengths of telescopes such as the CPC800 Celestron 8" Schmidt-Cassegrain give larger images of Saturn and other planets.Image credit: Celestron.

While not offering the same high contrast as a refractor, long-focal length compound reflecting telescopes such as Schmidt-Cassegrains and Maksutov-Cassegrains are a popular choice for many planetary observers and imagers. These scopes pack long focal lengths into a small optical tube, and modern optical anti-reflection coatings help boost image contrast and brightness. The larger apertures of 6", 8", 9.25" or more also offer inherently higher resolution than smaller refractors for revealing fine detail in good seeing conditions. They also give brighter images at high magnification than smaller-aperture scopes. The large mirrors, lenses, and tubes, however, take longer to cool down to ambient temperature.

For visual planetary observers who want the larger aperture of a compound scope, Celestron's NexStar and CPC telescope systems are a good choice. They offer good optics and long focal lengths in compact packages as well ascomputerized alt-azimuth mounts.

Meade's LX-90 systems and the new ETX125 also give good visual views of the planets, as do the more advanced Meade LX200 systems.

For even longer focal ratios and focal lengths, and therefore larger images, the razor-sharp optics of a good Maksutov-Cassegrain are a good choice for serious planetary and lunar observers. Unlike Schmidt-Cassegrains, these scopes rarely require alignment and have smaller secondary mirrors for higher image contrast. The 127mm, 150mm, and 180mm Mak-Cass scopes from Sky-Watcher are excellent planetary performers. Like Schmidt-Cassegrain scopes, they also take a longer time to cool down because of the large thermal mass of the mirror, tube, and corrector lens.

With whatever telescope you have at your disposal, use as much magnification as it will handle when you look at the planet. This depends on the quality of your optics and on the seeing. Generally, a magnification of 30-50 times the aperture of your telescope (in inches) works well on nights of average seeing. For example, if you have a 4-inch telescope, try 120x to 200x. If you have an 8" scope, try 240x to 400x. Again, experiment to get the best view each night. It changes according to the conditions of the atmosphere.

 

7. Eyepieces and Accessories for Observing Saturn

A good eyepiece is a critical component for visual planetary observation, especially if you have a well-made telescope with good optics or large aperture.

Any good eyepiece will show you detail on Saturn, but some eyepieces are designed especially with lunar and planetary observing in mind. A planetary eyepiece does not need to give wide "space-walk" views because the images of planets are small and usually held near the center of the field of view. To give maximum contrast, planetary eyepieces tend to forgo wide fields to minimize the number of glass elements and surfaces. So dedicated planetary eyepieces often have an apparent field of view (AFOV) of less than 60° compared to wide-angle eyepieces with apparent fields of view of 68°, 82°, or larger.

Many manufacturers make a series of eyepieces for planetary observing. Some good choices include:

Plossls. A tried-and-tested design, Plossls are a reliable performer for planetary and lunar observing. They are also a good value for observers on a budget. Many manufacturers make a Plossl design including GSO and Tele Vue Optics. Agena's Starguider eyepieces are also an economical and well-regarded choice.

Brandons. A long-established design that dates back to the 1940s, Brandon eyepieces are favored by planetary observers for their high-contrast and bright views of the planets. They work best with telescopes of focal ratio f/7 or greater.

Orthoscopics. Also a long-time favorite of planetary observers,Orthtoscopic eyepieces from Kokusai Kohki, Baader Planetarium, and other manufacturers give excellent contrast in a small form factor.

Tele VueDelites. A premium eyepiece from Tele Vue Optics, the relatively new Tele Vue Delites have replaced the company's Radian eyepiece line which was popular among planetary observers. With a 62-degree AFOV, Delites are also a good general purpose eyepiece and they have a long eye relief.

Zoom Eyepieces. A high-quality zoom eyepiece is ideal for getting a wide range of magnifications without the bother of actually changing the ocular. The Baader Hyperion Mark IV eyepiece is excellent for planets and for all around use. The 3-6mm Nager zoom eyepiece is intended for use in small refractors to give high-contrast planetary views and relatively high magnifications.

Observing the planet Saturn
Figure 7 – GSO Plossl eyepieces are an economical choice for planetary observation

Eyepieces are critical components for serious planetary observers. But if you use a refractor or compound reflecting telescope, don't forget your star diagonal. Lower priced or stock diagonals with many telescopes do not have optimized surfaces or coatings and can somewhat degrade images of the planets and other celestial objects. A higher-quality diagonal is an investment, but it need not be prohibitively expensive. 1.25" diagonals from GSO and William Optics are affordable. And if you want a premium unit, Baader makes an excellent prism-based 1.25" diagonal that minimizes scatter and maximizes image contrast.

Finally, a good 2x or 3x Barlow lens comes in handy for extending the focal length of your telescope and increasing image size with your set of eyepieces.

While Saturn is not nearly as colorful as Jupiter and Mars, color filters can help you see a little more detail on the face of the planet. These filters, which are numbered according to the traditional Wratten numbering system used for photographic filters, are mounted in 1.25" cells that thread into the barrel of an eyepiece. Color planetary filters pass some colors and block others to help improve contrast and visibility. Some good choices for filters for observing Saturn include:

#11 Yellow-Green – Good for improving image contrast when observing the Cassini division

#21 Orange – Good for seeing the bands on the disk of the planet as well as detail in the polar regions

#80A Blue – Very good for seeing the bands on the disk of the planet as well as detail in the polar regions

#47 Violet – Good for see ring detail and structure

You can get a more detailed guide to choosing color filters for observing Saturn and other planets at this page:

https://agenaastro.com/choosing-a-color-planetary-filter.html

Observing the planet Saturn
Figure 8 – A #21 Orange filter works well for improving the view of the bands on Saturn's disk
 

8. Summary

Saturn is one of the most beautiful sights you can see in a small telescope, and it's an excellent target to show newbie stargazers at outreach events.The planet itself is pretty enough, but its ring system is particularly stunning in nearly any telescope. With this guide, you have about what to look for on and around Saturn through visual observation with a small telescope, and you got some suggestionsfor goodoptical tools and accessories to help you get the best view of the planet. The rest is up to you! So head out on the next clear night to see this beautiful planet for yourself.

Brian Ventrudo
About the Author

Brian Ventrudo is a writer, scientist, and astronomy educator. He received his first telescope at the age of 5 and completed his first university course in astronomy at the age of 12, eventually receiving a master's degree in the subject. He also holds a Ph.D. in engineering physics from McMaster University. During a twenty-year scientific career, he developed laser systems to detect molecules found in interstellar space and planetary atmospheres, and leveraged his expertise to create laser technology for optical communications networks. Since 2008, Brian has taught astronomy to tens of thousands of stargazers through his websites OneMinuteAstronomer.com and CosmicPursuits.com.