Astronomy Under Light-Polluted Skies: What's to See
If you live where the glow of city lights washes out the starry sky, you may think that astronomical observing with a telescope would be about as exciting as snorkeling in a muddy river. What could you possibly see? As it turns out, plenty. Sure, your best views of deep space galaxies and nebulas will be had under a dark sky away from major population centers, streetlamps, and neighborhood house lights. But there are some brighter deep-sky gems that you can see, as well as interesting stars, planets, and our magnificent Moon, which shine brightly and make for pleasurable gazing from even heavily light polluted locales. With a little perseverance, a clear night, and the tips provided here, there's no reason you can't enjoy astronomy under skies that are, shall we say, less than stellar.
What's the Best Telescope for Urban Observing?
The "best" telescope for exploring the heavens from a light-compromised site is the same one you'd choose for a dark sky. That is, a scope that fits your budget, that is easy to set up and move when you're ready to use it, and that you have room to store in between viewing sessions. As far as aperture goes, larger optics collect more starlight, and that applies whether your sky is light polluted or pitch black. You'll be able to see more objects and resolve fainter detail with an 8" telescope than with a 4" one. So get the biggest telescope you can comfortably afford, but one that's not too unwieldy to conveniently set up and use on a frequent basis, or too bulky to stash when it is idle.
For city astronomy you might consider getting a telescope with a computerized, or Go-To, mount. It will make pinpointing objects to view much faster than star hopping the old-fashioned way with a star chart, since fewer stars are visible in a bright sky to guide your way. A Go-To mount works equally well in any sky conditions.
Where Should You Set Up?
Pavement and rooftops absorb heat during the day and radiate it back off during the night. The resulting turbulent air can distort the image through your telescope. So for best results, set up your telescope on dirt or grass, which absorb much less heat, and avoid aiming directly over nearby buildings. Of course, try to find a spot that isn't in the direct line of sight of a streetlight or a neighbor's porch light. Not only can such light be distracting, but it also can diminish the contrast of the images you see in the eyepiece. But if there's no escaping one or more such fixtures, try draping your head and the eyepiece area of the telescope with an opaque black cloth to block out the light.
There are a couple of other points worth mentioning here. First, the best time to view an object is when it is high in the sky. You're looking through less sooty air pollution than when viewing an object near the horizon, so the view will be clearer. Also, there's less skyglow higher up. Second, the amount of light pollution usually decreases late at night, as downtown businesses close and households turn off some of their outdoor lights. Take advantage of the darker skies by stargazing near midnight or during the "wee hours" of the morning when possible.
What Objects Will You Be Able to See?
Now that you've got your telescope set up, preferably in a spot shaded from direct street and house lights, what celestial objects can you expect to see? We'll leave the Sun out of this discussion and concentrate just on nighttime objects. (The Sun makes a fascinating target for daytime viewing. Just make sure the front of your telescope is capped with a proper solar filter to protect your eyes!)
Let's start with the biggest and brightest object first ? the Moon. Earth's crater-pocked satellite makes a spectacular telescopic target even from the city. Different surface features are highlighted every night along the terminator, the border between the lit and unlit portions of the Moon's face, as the phase changes. Low-grazing rays from the Sun cast long shadows from mountains and craggy crater walls. You'll see hundreds of impact craters of all sizes, plus valleys, rilles, rounded domes ? all served up in sharp relief. Indeed, the Moon can be so bright that it helps to tone down the glare with a neutral density or variable polarizing Moon filter, which threads into the eyepiece barrel or diagonal.
Four of Earth's fellow planets shine brightly enough to provide particularly striking views through the telescope for urban astronomers. Getting a crisp view requires a steady atmosphere, i.e., good "seeing," and patience at the eyepiece to wait for moments of maximum clarity, when the surface details pop sharply into focus. Use our monthly star chart to locate the planets, as their positions change relative to the stars throughout the year.
Our closest planetary neighbor, Venus, is the third brightest orb in the sky after the Sun and the Moon. Because it lies between the Earth and the Sun, it is only partially illuminated from our vantage point and displays Moon-like phases, from crescent to gibbous. In its crescent phase Venus is closer to Earth and thus appears much bigger than it does in its gibbous phase, when Venus is farther away. Its surface is perpetually shrouded by atmospheric clouds, so you won't see much detail, but it's a cool sight nonetheless.
Mars is a small and challenging telescopic object, for sure, but one whose subtle surface features are definitely worth checking out. In unsteady air, the "red planet" ? more a light salmon color ? will look like little more than a blurry, "boiling" ball. But on a night of excellent "seeing," when atmospheric turbulence is low, a high-quality telescope at high magnification may reveal one or possibly two polar ice caps and distinctive dark markings, including the famous Syrtis Major feature. Sometimes a Martian dust storm can alter the shapes of the dark regions, or temporarily mask them altogether.
The best times for viewing Mars are when it is at opposition, which occurs about every 26 months. That's when the planet passes closest to the Earth and, thus, appears larger in size.
Our solar system's biggest planet, Jupiter, rewards city dwellers and anyone else who aims a telescope at it with fascinating and dynamically changing detail. Parallel cloud bands of varying hue and the famous "Great Red Spot" ? a swirling anti-cyclone that spans three Earth diameters ? are easily seen on its large disk (though the Spot is more of a light tan color). Large optics will also reveal whorls, small light and dark spots, and grayish festoons within the major bands. A new, smaller red spot, dubbed Red Spot Jr., appeared in 2006 and remains visible today. See if you can make it out!
Jupiter's four Galilean moons, named after Galileo Galilei who discovered them in 1610, are another reason to train your telescope on the gas giant. Io, Europa, Ganymede, and Callisto can be easily seen in small telescopes ? and even binoculars. The satellites orbit Jupiter at different rates, from two days to two weeks, so their positions change from night to night. Sometimes you will see all four ? tiny dots lined up "in a row" on one side or flanking both sides of the planet's disk. On other nights, you may see only two or three of them, because one or two are hidden behind the planet, or one could be passing in front and is washed out by the glare. On occasion you may see a tiny, distinct black dot on the planet's disk. That's the shadow cast by a moon in the foreground!
There is no more dazzling sight in the solar system than that of the ringed planet Saturn. City lights can't diminish the beauty of this tiny but picturesque crowd pleaser! Anecdotes abound about non-astronomers who catch their first glimpse of Saturn through a telescope, only to check the front of the scope in disbelief to see if there isn't a picture of the ringed planet dangling there. For how could an object so far away in space appear so perfectly clear and sharp? Such is the utter coolness of Saturn.
You can easily see the rings with any size telescope and 40x power or more. With a medium-sized or large telescope, you can often detect some subtle variations in hue between the center and the poles of the ball, and maybe even the thin black shadow cast onto the "surface" by the rings. On a night of good seeing you should be able to distinguish two rings ? the outer gray "A" ring and an inner white "B" ring, separated by a thin black gap known as the Cassini Division. Closer inspection might even reveal a third, dark-gray "C" ring inside of the B ring.
As you observe Saturn, you will notice that the tilt of its rings varies from edge-on to our line of sight to a maximum of 26 degrees over about a nearly 15-year period. When they are edge-on the rings actually disappear from view for a time.
Saturn has many moons that are also visible in amateur telescopes in close proximity to the planet. They are sometimes hard to distinguish from stars, but you should have no trouble identifying Titan, the biggest of the bunch.
The planets Mercury, Uranus, and Neptune can also be spotted in backyard telescopes from light-polluted sites, if you know where to look. But their tiny disks look little different from stars. The thrill is in finding them and saying you've seen them, more so than in discerning any detail in them, which you won?t. Pluto you can forget about seeing from downtown. Besides, it isn't even a "planet" anymore!
Double and Variable Stars
Double and variable stars cut through urban skyglow and thus make easy and interesting targets for urban stargazers. Most double and multiple stars are gravitationally bound, orbiting systems (visual binaries). Others only appear close together in our line of sight, but really aren't physically associated at all (optical doubles). Doubles are fun to view because, like snowflakes, every one is different. The component stars may or may not differ in brightness and/or color. They might be very tightly paired, such that you need high power to resolve them, or more widely separated.
One of the most beautiful doubles is Albireo in the constellation Cygnus. Its primary star is a golden 3rd magnitude gem that glows in striking contrast to its sapphire blue 5th-magnitude companion. A spectacular sight in even the smallest telescope! Another fine example is the "Double Double" in Lyra. In a low-power eyepiece you see two widely separated white stars of roughly equal magnitude. But swap in an eyepiece that provides 100x magnification and suddenly both stars resolve into separate binaries themselves ? a quadruple star system!
Variable star observing requires more patience. There are different types of variables ? eclipsing binaries, pulsating stars, and eruptive variables ? each of which exhibits a change in brightness over some length of time, from just hours to many months. You can estimate the magnitude of the variable at any given time by comparing its brightness to stars of known, fixed magnitude that lie close to it. Keep monitoring the star and recording your observations, and over time you will see how its brightness has changed.
There are some variable stars whose brightness swings can even be perceived with the naked eye, though they are not dramatic. Algol in Perseus is one. Its magnitude varies from 2.1 to 3.4 every 2.87 days. If you're up for a challenge, see if you can detect the change.
You can find lists and descriptions of interesting double and variable stars in many astronomical observing guidebooks, including Nightwatch by Terence Dickinison.
So-called deep-sky objects, most of which are dim in modest-sized telescopes even from rural locations, have a tough time peeking through urban skyglow. The faintest types ? galaxies and nebulas ? fare the worst, while open and globular star clusters pierce through the muck better. But don't despair, because some specimens of all classes of deep-sky object can beam their photons through to your eyepiece.
Again, the higher the object is in the sky and the later at night you look, the better. Globular clusters and tightly packed open star clusters concentrate a lot of light into a small radius, which makes them easier to see than objects whose light is spread out more, such as galaxies and nebulas. The Double Cluster in Perseus, the Hercules Cluster (M13), the Wild Duck Cluster in Scutum, the Pleiades in Taurus, M44 in Cancer, M52 in Cassiopeia, M4 and M6 in Scorpius, and M22 in Sagittarius are a few examples of bright star clusters worth taking a gander at.
Planetary nebulas are small but their disk-shaped shells of ejected gas display high surface brightness. Two good planetaries to shoot for with a backyard telescope are the Ring Nebula (M57) in Lyra and the Dumbbell Nebula (M27) in Vulpecula.
For galaxies and nebulas, the selection is rather slim if you're observing in heavy light pollution. You'll enjoy a longer roster of candidates in more modestly tainted suburbs. Start with low power and try occasionally tapping the telescope tube, as the eye is more sensitive to motion and more likely to discern the object when it is vibrating slightly in the field of view. Using averted vision can also help.
Some "faint fuzzies" that you should have success seeing are the Andromeda Galaxy (M31), Whirlpool Galaxy (M51), and M81, a spiral galaxy in Ursa Major. For diffuse nebulas, the Orion Nebula (M42) is the easiest pickings, and the Lagoon Nebula (M8) and Swan Nebula (M17) in Sagittarius are possibilities. You might try using a narrowband light-pollution or "nebula" filter to improve the contrast between the nebulas and the background sky.
I hope this article has convinced you that there is plenty to see with an amateur telescope even in a light-polluted night sky. Escape to a dark-sky observing site whenever you can. But for the rest of the time, you needn't keep your telescope locked away in a closet. Get it out, set it up, gather a few friends or family members around, and enjoy a sampling of the many bright celestial showpieces that still manage to out-glow the skyglow.