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Observing Planets

by John Shibley and Deborah Byrd

Planets have been in the news lately. They appear to orbit nearby stars and—here in our own solar system—they’ve been known to gobble up hapless spacecraft. But here’s a newsbreak. You don’t need fancy telescopes or spacecraft to glimpse a planet or two tonight! All you need is a current sky map and a good place from which to view.

Chances are you’ve seen one of the planets in our solar system without even realizing it. Bright planets look like bright stars, except where stars twinkle, planets generally shine with a steady light. What’s more, planets are often brighter than any star. Five planets routinely outshine the brightest stars in our sky: these planets are Mercury, Venus, Mars, Jupiter, and Saturn. All five have been known since ancient times. Because they shine so steady and brightly and appear to move independently of everything else in the heavens, these planets were assigned the names of gods by many ancient cultures. The names of planets in use today are drawn from Greek and Roman legends.

The planets orbit at different distances from the sun, and they come in a variety of sizes. What’s more, some planets reflect more sunlight than others. For all of these reasons, each planet is unique among sky objects. If you take the time to learn to identify the bright planets and watch them with some regularity for perhaps a year, they will come to seem as individual as your closest friends.

Mercury

MercuryBy far the most elusive bright planet is Mercury. It’s the closest planet to the sun in our solar system—a rocky, airless world barely larger than our moon. As it swings around the sun in just 88 days, Mercury alternately goes from being an evening “star” to morning “star” visible in Earth’s sky. In either location, this planet is never found more than 28 degrees from the sun. It moves quickly! You have to look in just the right part of the sky, at an appropriate time, to see Mercury.

You will always locate Mercury above a flat, unobstructed horizon within an hour or so before sunrise or after sunset, depending on where the planet is in its orbit. This planet has three evening appearances and three morning appearances each year, but not all of these are favorable for viewing. The best time of year to locate Mercury in the evening is springtime, when the angle of the ecliptic—or path of the sun, moon and planets—is inclined almost perpendicularly to the horizon. When days are lengthening and the world at your feet seems born anew, you will often find Mercury shining gaily in the western sky, above the place where the sun set. Meanwhile, the best predawn appearances of Mercury always come in the autumn.

Some people think that Mercury is the faintest planet, but that’s not so. To the eye, this innermost world looks like a bright star. Within a few days of passing between Earth and the sun, Mercury does something fascinating for telescope users. It takes on the appearance of a tiny crescent through any telescope working at modest magnification.

Venus

VenusThe next world out from Mercury, and the next one in towards the sun from Earth, is Venus. Once you see her, there’s no mistaking Venus for anything else. This planet is the third most brilliant object in the sky, after the sun and moon. It’s so bright that you can sometimes see it in a blue daytime sky. Venus appears bright partly because it’s nearby—and also because it is wrapped in white clouds that reflect more than 65 percent of the sunlight striking them (for comparison, the moon reflects about 12 percent of incoming sunlight).

Like Mercury, Venus swings between morning and evening skies visible from Earth. Its location in our sky depends on where it is in its 244-day orbit around the sun. But because Venus orbits farther from the sun than Mercury, opportunities to see it stretch out for months rather than, as is Mercury’s case, days at a time. Venus can appear farther from the sun in our sky than Mercury, 48 degrees in contrast to Mercury’s 28 degrees.

Venus appears in the evening sky about six weeks after it has passed most directly behind the sun as seen from Earth. It starts low in the sky, but, evening by evening, grows brighter as it climbs higher in the west after sunset. At its highest above the sunset point, Venus is about halfway between the horizon and overhead. It is a lovely, conspicuous evening “star” that sets more than three hours after the sun.

After reaching its greatest height in the sky, the sun seems to reel Venus in, and the planet appears lower each day in the evening twilight sky. Because it is really coming near to us, during this time Venus is getting brighter. There is always a day of “greatest brilliancy” near the end of each evening apparition. Then reports of UFOs always increase as Venus looms eerily bright, low in the evening sky! Around this time, the view through a telescope reveals a waning crescent phase for Venus.

All too soon, Venus disappears into the sun’s glare as it passes between the Earth and sun—only to become visible again a week or two later as a brilliant light low in the eastern, predawn sky. There is another time of greatest brilliancy and then, as in the evening, Venus climbs away from the sun’s glare before dawn. About eight weeks after its reappearance before dawn, Venus reaches its greatest height in the eastern sky, at which time the planet rises more than three hours before the sun. Then Venus begins to sink toward the sun’s glare again, slowly, as the planet flees far ahead of Earth in orbit. After more than nine months as a morning “star,” Venus once again disappears from view for earthly stargazers, this time because the planet is so far ahead of us in orbit that the sun comes between us and it. There is a period of several months in which we cannot see Venus. Then one fair evening you will spot her low in the west again—surprisingly bright for being so low in the sky!

Mars

MarsThe next planet out from Earth’s orbit is Mars, which looks to the eye like a reddish “star.” Many cultures associated this planet’s red appearance to the unaided eye with war’s fixtures of blood and fire. In reality, Mars glows red because of oxidized soils that reflect the red portion of sunlight. Today we look to Mars as a possible abode for life because conditions on its surface most closely match our own planet’s, relative to other worlds in our solar system. This fascinating world holds a lot of interest for the naked-eye stargazer. Mars’ orbit around the sun takes only two years, in contrast to our single year. Thus Mars follows about a two-year cycle of observability in our sky.

The cycle begins when we first glimpse Mars on the opposite side of the solar system, perhaps 19 light-minutes away. At this time, Mars appears in our predawn sky—low in the east, near the sun’s glare—very faint and remote. Bright stars in their constellations rise earlier from one day to the next, and thus the stars sweep up in the east behind Mars. But Mars itself is destined to loom near the early morning twilight for many months. During this time, Earth’s faster motion in orbit—and smaller orbit around the sun—cause the distance between us and Mars to decrease. Thus Mars slowly gets brighter. Then there always comes a time when we suddenly seem hot on the heels of Mars in the race of the planets around the sun. Suddenly Mars is rising noticeably earlier each day. Before you know it, the planet can be seen rising at midnight, then at 10 p.m. All the while, Mars is getting much brighter—and brighter still. Soon it’s as bright as the sky’s brightest stars.

Then amateur astronomers begin talking about Mars because we’re near another martian “opposition,” the best time to observe Mars with a telescope. Opposition for Mars occurs about every two years, when Mars appears opposite the sun as seen from Earth. Now the planet rises in the east when the sun sets in the west and arcs across the sky—a brilliant reddish light—all night long. This happens, of course, around the time we on Earth are flying between Mars and the sun.

The distance of Mars at opposition varies over about a 15-year cycle, but at its closest Mars can be extremely close, only about three light-minutes away. At such times, lots of people are talking about Mars! At these very close oppositions, the planet appears brighter than the brightest stars and shines as red as flame. The last exceptionally close opposition of Mars was in 1988, and the next one will be in 2003. Between now and then there will be a perfectly enjoyable martian opposition in 2001.

Like Earth, Mars has seasons, polar ice caps, and a measurable atmosphere. Many of its geological features suggest a warmer and wetter past. No clue of Mars’ fascinating surface features can be glimpsed with the eye alone, but you can get a feel for the surface of Mars by looking at this planet through a telescope. Rough, rocky terrain looks like dark markings that stand out against lighter desert regions. Seasons spawn dust storms while polar caps swell and shrink. Mists and clouds burn off around huge volcanoes after martian sunrise, only to reform as evening approaches.

Jupiter

JupiterThe next planet out from Mars is Jupiter. After the sun, this is the largest object in our solar system. Jupiter is a mammoth world greater in volume (and more than double in mass) than all the other planets combined. Because Jupiter is some five times Earth’s distance from the sun, it orbits the sun much more slowly than we do. It has a much greater distance to cover, and its speed in orbit is less, about 8 miles per second in contrast to Earth’s orbital speed of 18 miles per second. Thus Jupiter takes 12 years to orbit the sun once, and it is Earth’s yearly orbit around the sun that dominates our view of this planet in the night sky.

We see Jupiter for much of every year. We do not see it only when the sun is between us and it. The planet emerges from the sun’s glare in the east before dawn—much as Mars does—but because Jupiter moves more slowly with respect to the stars, it follows the stars’ motion up and away from the dawn. It shifts into the midnight sky, and then the late evening sky, and then—like Mars—comes to opposition when it is rising in the east as the sun sets in the west, as we fly between Jupiter and the sun. But the opposition of Jupiter is yearly instead of every two years. What’s more, although Jupiter is indeed brighter at opposition, this planet is always so amazingly bright that the contrast in various parts of its orbit is not so great as that of Mars. Whenever it’s visible, Jupiter is a bold starlike object in our sky, brighter than any star—but not so bright as Venus!

It’s fun to watch Jupiter’s progress around the sun over the course of some years. Because this planet takes 12 years to orbit the sun, and because there are 12 constellations of the Zodiac, Jupiter is in a different zodiacal constellation every year. Thus this giant planet is like a literal giant, stepping around the sky—one Zodiac constellation after another—year by year.

A steadily held pair of 7×50 binoculars will reveal four bright moons whose positions around Jupiter change from night to night. Each of these moons, dubbed Galilean Satellites because Galileo first saw them in 1610, is larger than our moon. One, Ganymede, is even bigger than Mercury. All told, Jupiter has 16 known moons at the present time.

Saturn

SaturnThe last planet in our solar system known since antiquity—the last one visible to the eye alone—lies beyond Jupiter. Saturn is probably the solar system’s most showcased object for telescopic observers. To the unaided eye and through binoculars, however, it looks nothing more than a bright, butterscotch-hued star.

No one would use a word like “brilliant” or “bold” to describe Saturn, even though the planet is brighter than most stars. Saturn is the faintest of the bright planets, and it moves most slowly of these planets among the stars, taking 30 years to orbit the sun once and thus 30 years to complete a single circuit around our sky. That is why some early stargazers called Saturn a name that means “oldest of the old sheep.”

Saturn follows, more or less, the same pattern as Jupiter from one year to the next. We see it in the eastern, predawn sky—then at midnight—then in late evening—then rising in the east at sunset as the planet comes to its yearly opposition. Like Jupiter, Saturn is brightest to the eye at opposition, but the contrast in its brightness from one part of the year to the next is not great. Indeed, this world’s appeal lies not in its changeability and not in any dramatic movement of Saturn in front of the stars. The appeal of Saturn is in the gentle quality of its bright golden light, and in the fact that this planet returns reliably, year after year, to nearly the same part of the sky. If you begin watching Saturn in the evening in autumn one year, as the world at your feet is becoming golden and dry, and cold winds are beginning to blow, you can be sure that Saturn will grace your evening sky at nearly that same season the following year. It does shift with respect to the stars, as all the planets do, but it does so with an impressive slowness.

The view through a telescope at Saturn is nothing short of spectacular. Even a small telescope will reveal the planet’s ring system composed of rock and ice particles that never formed a moon. A telescope may also reveal reddish Titan, a smog-enshrouded moon that takes almost 16 days to complete a circuit of its home planet.

Other Planets

Our solar system’s three remaining planets were discovered relatively recently. Uranus, which at its brightest is barely visible as a greenish “star,” was discovered during a telescopic search in 1781. This planet takes 84 years—about the length of a human lifespan—to orbit the sun once. Backyard observers routinely spot Uranus through a pair of binoculars, provided they have a current finder chart.

Computational mathematics pulled Neptune in from the cold in 1846. This planet is a thousand million miles farther from the sun than Uranus. It requires 165 years to orbit the sun once. Neptune usually looks like a nondescript star through the eyepiece of a small telescope. Some people say its color is bluish. The challenge is spotting it among background stars, which is where a good finder map is necessary.

Faint Pluto was found in 1930 after an exhaustive photographic search. This planet orbits the sun once every 248 years. Only observers armed with telescopes that have mirrors at least ten inches in diameter go for this faint target under dark skies.

Backyard observers track down elusive outer planets with the same care and fervor as naturalists who scour a tropical rain forest for rare and wonderful animals. Many conduct expansive searches for other denizens of the solar system, such as brighter asteroids and comets. Some have even taken a cue from birdwatchers and started solar system life lists. If you feel inspired to follow the same lead, here’s a suggestion to get you started: Terra Firma, or Earth!

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