First thought to be gods and goddesses, later believed to be inhabited by alien life forms, today the planets are well known as strange and marvelous worlds. Each has its own set of characteristics and personality; each is a wonder to behold through a telescope.
Most telescopes of any aperture will give pleasing views of the planets, at least of Saturn and Jupiter, because they're so big, bright, and distinctive. The telescope must have clean, high-quality optics, a sturdy mount, and good eyepieces. High-quality refractors have the edge for studying the planets because of their "clear apertures." Any time something blocks a portion of a telescope's aperture (such as a secondary mirror), some image sharpness and contrast are lost. While reflector and catadioptric telescopes need small, secondary mirrors to redirect light to their eyepieces, refractors have a clear light path straight to their eyepieces, keeping contrast at its greatest. Reflectors and catadioptric telescopes work very well on the planets, it's just that a larger aperture is needed to give the same view.
Regardless of the type of telescope, always use your best eyepieces to get the sharpest views. This is especially critical with the planets. Most telescopes come with one or two eyepieces that are fine to start off with, but usually cannot reveal a telescope's full potential. The same instrument will show much more planetary detail just by switching eyepieces. It's almost as good as getting a whole new telescope!
Use Reasonable Power, Not More
A common mistake made by beginners is to think that high power eyepieces are best for viewing the planets. Often, the exact opposite is true. A telescope's ability to gather light from a distant target is based on its aperture (the diameter of its main mirror or lens). Low magnification concentrates that light into a small, bright image, while high magnification spreads that light over a larger area, producing a dimmer image. Too much magnification will cause the image to become so faint that it becomes impossible to focus, or even see.
So, what magnification is best for viewing the planets? That's hard to say, but a rule often mentioned is the "sixty power per inch" rule. Never use an eyepiece that will magnify more than 60 times for every inch of aperture. In other words, 240x is the highest you should use in a 4-inch telescope, while 480x is the highest in an 8-inch telescope. But this also assumes that the telescope and eyepiece are optically perfect and that "seeing" — the measure of how steady the Earth's atmosphere appears — is calm. Many nights may only let you use 30x per inch, or even 20x. The best advice is to increase magnification until the image brightness and sharpness begin to deteriorate.
Good "Seeing" is a Must
"Seeing" is just as important as optical quality. Even the best telescope will show only fuzzy planetary blurs if the Earth's atmosphere is turbulent. A good way to judge seeing conditions is to check the stars. If the stars appear to be twinkling, which is caused by a turbulent atmosphere, then conditions are poor for planet gazing. Frequently, the steadiest nights appear slightly hazy, when our atmosphere is more tranquil and seeing is enhanced.
Even with steady seeing, however, a telescope still won't do the planets justice unless its optics are cooled to the outdoor temperature. Depending on telescope aperture and the change in temperature, acclimatization may take anywhere from ten minutes to more than an hour. To keep this time as short as possible, store your telescope in an unheated, but protected, place.
Many observers also recommend using color filters to enhance subtle features on some of the planets. While filters do help, their benefit is best appreciated by experienced observers. Filters are not a substitute for quality optics and steady skies.
Finally, the greatest tool for viewing the planets cannot be purchased anywhere: a trained eye. Most first-time observers only see small, round blurs when viewing the planets. That's normal, so don't be discouraged! Stick with it and observe the planets as often as possible. Don't merely take a quick look. Switch eyepieces (magnifications) until you find that perfect view and study it intently. As time goes on, your eye and brain will become accustomed to seeing fainter, more tenuous detail. Skill as an observer will only be gained by going out night after night and observing.
What Can You Expect to See?
Jupiter: Cloud Bands and Dancing Moons
Of all our planets, none is more impressive through amateur telescopes than Jupiter. This largest planet of our solar system shows off its impenetrable atmosphere of parallel light and dark bands through even the smallest astronomical instruments. The bright equatorial zone dominates the center of the planet, framed on either side by the dark north and south equatorial belts. To either side of these lie the north and south temperate zones, and finally, the darker north and south polar regions. Four-inch and larger telescopes also reveal subtle markings and details within these broad regions. Most notable is the Great Red Spot, a cyclonic storm located on the outer edge of the south equatorial belt. The name is a bit misleading, as the spot usually appears more of a pale pink or orange.
The smallest telescopes will show four of Jupiter's satellites. They make a pretty sight: four small dots flanking the planet's large disk. All were discovered by Galileo, and are collectively known as the Galilean satellites. Which ones are in view depends on timing. Sometimes, two are visible on both sides of the planet, while at other times, three might be seen on one side and one on the other. If one or more is missing, it may be either behind Jupiter, hidden in its shadow, or passing in front of the planet's bright disk. Trying to identify each Galilean satellite by name through a telescope is a fun activity for observers. Astronomical periodicals such as Astronomy and Sky & Telescope publish charts that show satellite orientation during the given month.
Saturn: The Ringed Wonder
Jupiter may be the most impressive planet, but the stunning ring system surrounding Saturn casts it as the solar system's most breathtaking sight. Most people are amazed by the view. Best of all, Saturn's rings are visible with telescopes as small as 2-inches aperture and at magnifications as low as 25-power. Naturally, larger telescopes will show greater detail, including Cassini's Division, a tenuous band of opaque material that divides the outer A ring from the middle, broader B ring. The faint C ring lies along the inner edge of the B ring, but is usually very difficult to make out. Look for its subtle silhouette against the planet's brighter disk.
While Saturn's atmosphere is chemically similar to Jupiter's, it lacks the same prominent banding. Instead, most amateurs can only make out a whitish-beige equatorial region, slightly darker temperate zone, and a still darker polar region.
Saturn is encircled by 18 satellites, with the largest, Titan, visible through small telescopes. Six-inch instruments will show an additional four to five satellites, although all are considerably fainter than 8th-magnitude Titan.
Venus: Goes Through Phases
Venus is the brightest planet in the sky and is often the closest to Earth, yet it shows very little detail through telescopes. Its surface is forever hidden beneath a thick layer of carbon-dioxide clouds. Although this frustrates both amateur and professional astronomers, we can watch Venus go through phases like our Moon, from a thin crescent to a broad gibbous.
Nearly any telescope will show these phases easily. When nearest to Earth, just to one side or the other of the Sun in our sky, Venus shows a slender crescent phase that can actually be seen through 7-power binoculars. As it continues in its orbit, Venus pulls away from Earth, shrinking in apparent size, but broadening to a half phase. Finally, Venus widens to a gibbous phase before moving behind the Sun from our vantage point. As Venus pops out from the other side and moves farther away from the Sun in our sky, the order of phases reverses.
Mars: The Red Planet
Mars has always attracted wide attention among stargazers. Its appearance through a telescope changes dramatically depending on where it is in its orbit compared to Earth. When far away, its small apparent size makes it nearly impossible to see any surface details, except for perhaps a hint of its white polar caps. But when Mars is closest to Earth (at opposition), the Red Planet displays some interesting dark surface features. These dark markings were thought by some in the late 19th century to be man-made "canals," but that was merely an optical illusion.
Even when Mars is closest, 200x or more will be needed to see fine details, demanding a high-quality telescope and eyepieces. During an opposition, 4-inch and larger telescopes can certainly show Mars' two white polar caps as well as some amorphous surface features, but near-perfect seeing conditions are a must for the best view. The easiest dark marking to spot on Mars is called Syrtis Major, which looks like a triangular wedge extending North and South. A second, smaller feature is called Meridiani Sinus, which looks a little like a claw. Finally, take a look for the "Eye of Mars," Solis Lacus, a bright, circular area surrounding a dark middle.
Mercury: Low Rider
Mercury is the most difficult of the five naked-eye planets to see, since it is only visible very low in the west right after sunset, or low in the east just before sunrise. The best time to hunt for Mercury is when it is near greatest elongation from the Sun, when the distance between the two in our sky is farthest. Since Mercury takes only 88 days to orbit the Sun, greatest elongations occur often. Due to the tilt of the Earth compared to the plane of our orbit, the spring and early summer prove to be the best times of year for observers in the Northern Hemisphere to spot Mercury during evening elongations, while the fall and early winter are best for seeing Mercury in the early morning. The exact opposite is true from the Southern Hemisphere. Even then, seeing Mercury with the naked eye is difficult because of the bright, twilight sky. Binoculars and finder scopes can help isolate the planet.
In a telescope, Mercury appears as a tiny gray disk. Like Venus, it goes through different phases; the apparent size of its disk varies with phase. Because Mercury hugs so close to the horizon, the disk usually shimmers from atmospheric turbulence. A better image can be obtained when the planet is higher in the sky, during evening or morning twilight, even though the contrast between the planet and the sky won't be as good.
Uranus, Neptune, & Pluto: Challenging Quarry
Uranus and Neptune are challenges that can only be met using optical aid. The former is seen as a greenish, 6th-magnitude "star," while Neptune looks like an 8th-magnitude, turquoise point. Neither will show any detail through most amateur telescopes apart from their distinctive colors. Finally, Pluto shines faintly at about 13th magnitude. While Uranus and Neptune can be glimpsed through 7-power binoculars, Pluto demands at least a 6-inch telescope to be seen. All require a detailed star map to identify them from their surroundings.
Relish Those Fleeting Moments of Great Seeing!
The view of a planet is not continuously crisp and clear in the eyepiece. Usually the seeing conditions effectively "blur" the image to varying degrees, smearing out some of the finer detail. But there are brief moments when the planet's disk will appear very sharp, and these are the moments to relish. Under the right conditions you get enough of these moments of clarity to make the viewing highly worthwhile. Be patient, keep tweaking the focus to make sure it's right on, and adjust the magnification down enough so that your optics aren't blurring the image.
Each planet shows off a set of unique characteristics through telescopes both large and small. If one of the planets is visible in tonight's sky, take a moment to gaze its way and say hello to one of your celestial neighbors.