Follow the night sky throughout the course of a year, and you'll start to pick out some patterns. The stars all seem to march in lock step across the sky. They rise in the east and set in the west, moving with a little more alacrity than the Sun. As a consequence, if you observe at the same time each night, the stars seem to slide slowly westward. That's why the summer sky appears so different from the winter sky. During the course of one full year, the stars make a complete circuit.
Yet a few bright points of light break those rules. Even early civilizations recognized that a handful of celestial objects don't follow the same pattern but instead move relative to the stars. The ancients called these wandering objects "planets" and, along with the Sun and Moon, regarded them with special significance.
Early civilizations recognized only five planets — Mercury, Venus, Mars, Jupiter, and Saturn — the ones bright enough to show up easily to the naked eye. Astronomers using telescopes discovered three more during the past 225 years: Uranus, Neptune, and Pluto. Together, the planets make up the vast bulk of the material in the solar system outside the Sun.
The eight planets besides Earth fall into two observational classes. The inferior planets — Mercury and Venus — lie between the Sun and Earth, and the superior planets (the other six) lie beyond Earth's orbit. The two categories show very different observational properties.
Because inferior planets orbit closer to the Sun than Earth, they never stray far from their parent star. Innermost Mercury never appears farther than 28° from the Sun in our sky. This means it stays almost perpetually in twilight, glowing either in the west shortly after sunset or in the east shortly before sunrise. Despite shining brighter than all but the most prominent stars, Mercury remains an elusive object.
With a bigger orbit than Mercury, Venus can appear up to 47° from the Sun. Although it often lies in twilight, Venus occasionally climbs into a completely dark sky. But you don't need that help to spot it — Venus is by far the brightest object in the sky after the Sun and Moon. It shows up easily except during the fairly brief periods when it passes behind or in front of the Sun.
An inferior planet appears most conspicuous around the time of its greatest elongation from the Sun. If you envision an inferior planet's orbit relative to Earth's, the moment when it passes directly between Earth and the Sun is called inferior conjunction. Because an inferior planet moves faster than Earth, it quickly pulls ahead of the Sun in our sky and eventually becomes visible in the east before dawn. It stands highest in the morning sky around the time it reaches greatest western elongation, then it sinks back toward the Sun. The planet then passes on the far side of the Sun from Earth, a configuration known as superior conjunction, before climbing into the western evening sky. It appears highest around greatest eastern elongation and then heads back toward the Sun for its next inferior conjunction. It takes Mercury an average of about 116 days to complete one cycle; Venus requires 584 days.
Observers who own telescopes have to be content with tracking Mercury's and Venus's changing sizes and phases. Because Mercury presents such a small disk and turbulence near Earth's horizon distorts our view, there's really no hope of discerning any surface feature. And thick, highly reflective clouds permanently shroud the surface of Venus. Neither inferior planet varies much during the relatively long period it spends near superior conjunction. It lies farther from Earth then and so appears smaller, and its gibbous phase changes slowly. At greatest eastern elongation, the planet appears half-lit. The pace of change quickens between this elongation and inferior conjunction. The planet's size grows rapidly and the phase dwindles precipitously. For Venus, the apparent size grows from roughly 10" across near superior conjunction to 60" across at inferior conjunction. The size and phase changes play out in reverse as the planet moves from inferior to superior conjunction.
A superior planet shows a far different pattern. When it lies on the far side of the Sun as seen from Earth, astronomers call that conjunction. The planet then moves into the morning sky, where it climbs steadily higher. Eventually, it reaches the point in its orbit where it lies opposite the Sun in our sky, a configuration called opposition. Opposition marks the best time to view any superior planet. Because it's opposite the Sun, it remains visible all night. Opposition also brings the planet closest to Earth, so it appears largest through a telescope and shines at its brightest. After opposition, a superior planet moves into the evening sky and eventually sinks into the Sun's glare again. The closer a planet lies to Earth, the longer it takes to complete the cycle from one conjunction or opposition to the next. For Mars, it takes more than two years. Jupiter takes about a month longer than one year, Saturn two weeks longer than a year, and the outer planets just a few days longer than a year.
When viewed through a telescope, Mars shows the biggest changes. (As the closest superior planet, there's a greater percentage difference between its distance from Earth at opposition and conjunction.) Around the time of opposition, Mars looms quite large and shows intriguing details. In 2003 (its closest approach in nearly 60,000 years), the Red Planet appeared 25" across. Mars typically spends only a few months at that size during each 26-month apparition, so good views are rather fleeting.
For backyard observers, Jupiter always offers a bigger target than Mars. That's partly because it's the largest planet in the solar system and partly because its distance from Earth doesn't vary as much. Its apparent diameter at opposition ranges from 44" to 50", but, even at conjunction (when it lies behind the Sun and can't be seen), it never drops below 30". Larger scopes reveal an entire series of alternating dark belts and bright zones. Look for swirling eddies in the turbulent border regions between the belts and zones. The most famous feature of Jupiter's cloudtops is the Great Red Spot, a huge atmospheric feature more than twice the diameter of Earth. The spot's color now appears more of a faint salmon than a bright red, so it's not easy to see. Look for it on the southern edge of the south equatorial belt. If you can't spot it, it may be on the far side of the planet's disk. Wait a couple hours — Jupiter takes less than 10 hours to rotate once — and it should be on the Earth-facing hemisphere.
Any telescope also shows four bright dots arrayed on either side of Jupiter's disk. These are the Galilean moons, discovered by Galileo in 1610, when he first turned his telescope toward the planet. Watch them dance around the planet from one night to the next. Spacecraft revealed the four moons to be worlds in their own right. Innermost Io ranks as the most volcanically active object known. Bright, smooth Europa apparently harbors a vast subsurface ocean of liquid water. Giant Ganymede is the largest moon in the solar system — and bigger than the planets Mercury and Pluto. And outermost Callisto sports the solar system's most heavily cratered surface.
No object in the solar system fascinates observers more than Saturn. Even a small scope shows the system of rings surrounding the planet's pale-yellow globe. It looks just like people expect it to — it's a rarity in the world of backyard observing. Three main rings can be seen through a telescope. Most prominent are the outer A Ring and, just inside it, the brighter B Ring. The dark Cassini Division separates these two. The dusky, innermost C Ring appears with difficulty to those with large scopes. Saturn is smaller than Jupiter and lies farther away, so it never looms as large as its giant brother. Saturn's ring system stretches wide enough, however, that it spans a greater diameter than Jupiter most of the time.
Saturn also retains a large retinue of moons. Although none appears as bright as Jupiter's Galilean moons, backyard observers can spot several. Easiest to find is 8th-magnitude Titan, the second largest moon in the solar system and the only one with a substantial atmosphere. A 4- to 6-inch scope reveals 10th-magnitude Tethys, Dione, and Rhea. Oddest of all is Iapetus. When it lies west of Saturn, it glows at 10th magnitude — two magnitudes brighter than when it's east of the planet.
The outer gas-giant planets — Uranus and Neptune — offer less to backyard observers. Uranus glows at 6th magnitude and can be glimpsed with the naked eye from a dark site. Unfortunately, a telescope doesn't reveal much detail. Around the time of its opposition, Uranus shows a distinctly blue-green disk that measures a bit less than 4" across. Neptune glows at 8th magnitude and, through a telescope around opposition, appears blue-gray and slightly more than 2" across.
Distant Pluto shows absolutely no detail at all. You'll need an 8-inch telescope and a detailed star chart to have a decent chance of spotting this 14th-magnitude glimmer of light. The reward for spying Pluto comes not from viewing any detail but from the mere accomplishment of locating the plutoid.
As great as observing a planet or moon can be, many skywatchers place the interaction of three objects at the top of their solar system observing list. When the Sun, Moon, and Earth line up, observers flock to see an eclipse. When Earth lies between the Sun and Moon, our planet's shadow falls on the Moon and we see a lunar eclipse. When the Moon comes between the Sun and Earth, the Moon blocks all or part of the Sun from view, creating a solar eclipse. Eclipses occur at either Full Moon (lunar) or New Moon (solar). Because the Moon's orbit around Earth tilts with respect to Earth's orbit around the Sun, we don't get eclipses at every Full and New Moon. Instead, they come at roughly six-month intervals when the Moon crosses the plane of Earth's orbit at the right phase.
During a lunar eclipse, Earth's shadow gradually creeps across the Moon's bright face. In a penumbral eclipse, the Moon remains in the outer, lighter part of Earth's shadow (the penumbra) and many people are hard-pressed to see the Moon darken at all. In a partial eclipse, the Moon enters the inner, darker part of Earth's shadow (the umbra) and the Moon appears to have a bite taken from it.
Few sky events can rival the majesty of a total lunar eclipse, when the entire Moon plunges through Earth's umbra. These eclipses start as penumbral ones and progress through partial phases until the Moon lies totally within the umbra. You might think the Moon would disappear during totality because, to the eyes of a hypothetical observer on the Moon, Earth blocks the whole Sun from view. Yet the Moon normally takes on a reddish color. The culprit — Earth's atmosphere. If Earth were an airless planet, the shadow would be pitch black and the eclipsed Moon would vanish. But our atmosphere acts like a filtered lens, bending red sunlight into the shadow and scattering out blue light. It's the same reason sunrises and sunsets appear reddish. In fact, the ruddy light hitting the Moon during totality is the glow from all of our planet's sunrises and sunsets.
Lunar eclipses seem fairly common because they can be seen from the entire nightside of Earth if the weather cooperates. Solar eclipses seem rare in comparison because they produce noticeable effects over a limited geographic area. During a partial solar eclipse, the Moon covers a fraction of the Sun that can range from a nick up to near totality. Because the Sun appears so bright, however, more than half the Sun needs to be blocked before any discernible effect can be seen on the ground. The Sun's brilliant surface is also why you never should view a partial eclipse directly without a proper solar filter.
The best eclipses occur when the Moon passes centrally across the Sun's disk. Because the Moon and Sun have nearly the same angular diameter, a central eclipse can be seen only from a narrow path on Earth's surface. If the Moon lies relatively far from Earth, it doesn't block the whole Sun from view and a ring of sunlight remains visible. This is called an annular eclipse.
But the most spectacular eclipse of all is a total solar one. In this case, the Moon lies close enough to Earth that it blocks the Sun's entire disk from view. With the brilliant photosphere hidden, you can view totality with the naked eye or optical aid without a solar filter. During totality, the Sun's faint outer atmosphere — the corona — appears front and center. This gauzy, pearly-white light typically extends two or three times the diameter of the uneclipsed Sun. Also look for fiery prominences, hot tongues of reddish gas that arch above the Sun's limb and come into view with the photosphere blocked. Total solar eclipses appear so impressive that many observers travel the world to see as many as possible.