From the August 2004 issue

Perseids prime the pump

August 2004: The view of the Milky Way in a star-studded sky highlights August nights, although the best sights this month should be of meteoric debris flaring to incandescence.
By and | Published: August 1, 2004
Summer Milky Way
The summer Milky Way dominates the August evening sky — as long as you get well away from the lights of the city.
John Chumack
The grandeur of a star-filled sky, viewed on a balmy summer night under a dark country sky, leaves an indelible impression on anyone who sees it. Not coincidentally, our first wishes upon shooting stars often occurred on such back-country sojourns. From late July to late August, Earth sweeps up innumerable interplanetary dust particles and the atmosphere burns them into oblivion, creating silent streaks that slice across the sky.

The best time this year to seek such a grand vista is Wednesday night, August 11/12, the peak of the annual Perseid meteor shower. To see dozens — or perhaps hundreds — of meteors, your best strategy is to rest early, set an alarm for 1 a.m., then watch until dawn. Advances in understanding meteor dust trails have boosted confidence that an additional profusion of Perseids should be visible this year for Eastern Hemisphere observers.

Under a dark sky, the stars sometimes seem close enough to reach out and touch. But reality is far different — and much more interesting. Look toward the southeast on an August evening and you’ll see what appears to be a steam cloud floating in front of the stars. That glow comes from billions of stars so far away you can’t make them out individually. Like grains of sand on a beach, the stars combine into a smooth radiance when seen from a distance. The dark lane that courses through the middle — the “Great Rift” — does not represent a chasm empty of stars. It’s actually an absorbing curtain of dark dust much closer to us than the glowing river we call the Milky Way.

The best nights to enjoy such views fall within a week or so of the August 15/16 New Moon, which conveniently includes the peak of the Perseid shower. The Moon begins the month just a day past full phase, rising around sunset and progressively later each succeeding night. By the night of the Perseid peak, the Moon rises after 2 a.m. local daylight time and appears only 12 percent lit, so it won’t hinder the view much at all.

The week of the Perseids should be perfect as well for following two modestly bright comets in the evening sky: C/2001 Q4 (NEAT) and C/2003 K4 (LINEAR). Using binoculars, you will have an easy time finding Q4 near the bowl of the Big Dipper. We provide a finder chart on page 67 for the slightly brighter K4.

As luck would have it, the five planets bright enough to see easily with the naked eye have all but disappeared from the evening sky. The splendid spring lineup has slipped behind the Sun, leaving a lone “wanderer,” Jupiter, dipping into the sunset glow. It masquerades as a distant plane’s landing light during the first half of August. A low, difficult-to-see pairing with the waxing crescent Moon occurs on the evening of the 17th.

The two outer gas giants, Uranus and Neptune, rise near sunset and appear highest in the south around midnight. They both reach opposition in August, lying opposite the Sun in our sky and remaining visible all night. The blue-green glow of Uranus is easy to spot in binoculars, but you’ll need a telescope to spy the subtle blue-gray shading of Neptune.

The deep sky

There’s nothing quite like lying back and staring up at the center of the Milky Way Galaxy arching overhead. For dedicated deep-sky observers, it’s nearly as impressive as a total solar eclipse. Unfortunately, you need to travel south of the equator to make this dream come true. Yet even from mid-northern latitudes, the sight of Sagittarius the Archer and Scorpius the Scorpion dazzles.

The bright stars of the scorpion’s tail twinkle near the horizon. If you scan this region with binoculars, a bright triangular grouping of stars quickly stops your sweep. This visual gem includes Zeta1 (ζ1) and Zeta2 (ζ2) Scorpii. The brighter component (Zeta2) appears distinctly yellow, while Zeta1‘s white hue typically is tinged with some atmospheric color.

Immediately above this pair you’ll see a bright knot, which a telescope turns into a magnificent, compact star cluster that ranks among most observers’ top-ten clusters. Truly a sight to savor, NGC 6231 boasts ten stars brighter than magnitude 7.0 and appears more densely packed than the core of the Beehive cluster. Discovered by French observer Nicolas Louis de Lacaille around 1750, NGC 6231 is one cluster that you can keep pouring on the magnification. The higher the power, the more tight doubles and triples you’ll see.

Then go back to a lower power and sweep slightly north to land on the sprawling splash of stars cataloged as Harvard 12. At first it may surprise you that both William and John Herschel missed H12 in their systematic searches of the sky. Their 18-inch telescopes had one characteristic very different from modern scopes — a small field of view, only 15′ across, equivalent these days to about 250x with a wide-angle eyepiece. This part of the Milky Way is packed so densely with stars that the Herschels literally swept right across H12 without being able to see it in a wide-field context.

Finder chart for Scorpius objects
Bug Nebula
The Bug Nebula sports a distinctive reddish glow from its perch near the stinger of Scorpius.
Ken and Matt Blankenship
The nebulous glow of IC 4628 surrounding H12 stands out when seen through an OIII filter. This gas cloud proves a real challenge for northerners compared with the easy find in 8×50 binoculars from south of the equator. Slide north again at low power and you’ll run into NGC 6242, a lovely star cluster having the shape of the small Greek letter lambda (λ).

On any color image of the Scorpion’s tail, a conspicuous red “star” appears to the right of the stinger stars. Even a small telescope will reveal something quite odd: a squashed blob nicknamed the Bug Nebula (NGC 6302). Like many other planetary nebulae, the Bug can take a lot of power, and that’s the only way to see its details. A bright knot of gas nearly hides the central star. A darker lane just west of center breaks the Bug’s elongated shape. An OIII filter will mask the central star and show fainter extensions rising out of the background.

Although the Lagoon and Trifid nebulae to the north in Sagittarius certainly appear more spectacular, if you like hunting for rarer treasures, spend some time on the Cat’s Paw Nebula. Plotted as NGC 6334 on most atlases, this group of five nebulae is easier to see on photographs than visually. The farther south you live, the better the view. You’ll need a high-contrast filter to pull the fainter wreaths of nebulosity through the glow of our atmospheric shield.

The planets

The sky before dawn hosts a couple bright planets this month: Venus ascends to the peak of its outstanding morning apparition, and Saturn returns to view after a brief sojourn behind the Sun. Meanwhile, two giant yet distant planets reach opposition in August — both Uranus and Neptune appear at their best around midnight.

The sky after sunset appears much more barren. Lonely Jupiter can be viewed for a brief period in the darkening twilight throughout August, although by the end of the month, it sets soon after the Sun. On the 1st, Jupiter sets nearly 2 hours after the Sun, a value that drops to 45 minutes by month’s end. Through a telescope, the planet’s low-contrast atmospheric features still show up during twilight and offer an attractive subject against a deep-blue sky. The four bright Galilean moons also prove to be easy targets.

On the evening of August 17, look for Jupiter 4° to the left of the waxing crescent Moon about a half hour after sunset. Bright twilight will hinder the view, but you should be able to see them before they set an hour after the Sun.

Mercury proves a tougher object to find, appearing even lower in the west after sunset in early August. You can spy the inner planet in the bright twilight about 13° to the lower right of Jupiter. Don’t confuse Mercury with Regulus, Leo’s brightest star, which lies 7° to its right. The planet shines at magnitude 0.7, noticeably brighter than the star. Mercury disappears from view by the end of the first week in August and passes between the Sun and Earth on the 23rd. It reappears in the morning sky in early September.

This past spring, all five planets known since antiquity were on view during the evening hours, spanning the sky from east to west. Uranus and Neptune glow much fainter and were unknown until their discoveries in 1781 and 1846, respectively. Both planets come to opposition this month and can be spotted through binoculars if you know where to look.

Neptune reaches opposition on August 5/6 near the 4.1-magnitude star Theta (θ) Capricorni. As August opens, the gas-giant planet conveniently lies one Moon-width above Theta. This makes the eighth planet an easy target. For the best views, wait until midnight approaches, when this region lies due south and is at its highest point above the horizon. Viewing over two or three consecutive nights easily reveals the planet’s motion and confirms its identity.

Neptune also glows its brightest of the year, magnitude 7.8, at opposition. Several 9th-magnitude stars surround it, helping to aid in the planet’s identification. During August, Neptune tracks westward against these background stars, ending the month northwest of Theta and forming a triangle with it and the 6th-magnitude star 21 Capricorni. Neptune’s retrograde motion continues until late October. At opposition, the planet lies about 29 astronomical units away from Earth, or 2.70 billion miles. (One astronomical unit, or AU, equals the average distance between Earth and the Sun, or about 93 million miles.)

Farther to the east, you can find Uranus wandering among the stars of Aquarius. This position places Uranus at opposition on August 27. On the 1st of the month, Uranus lies one Moon-width north of the 4.8-magnitude star Sigma (σ) Aquarii. This region, centrally located in the faint constellation, contains few bright stars. On the same night, the Full Moon lies 6° south of the planet. Despite the Moon’s glare, Uranus should be visible through binoculars. The planet glows at magnitude 5.7, about one magnitude fainter than Sigma. Uranus tracks to about 1° west of Sigma by August 31. At opposition, Uranus lies 19 AU away from Earth, or 1.77 billion miles. It takes light more than 2.6 hours to cover this distance. Point a telescope at the planet and you should resolve its 3.7″-diameter disk, which appears distinctly blue-green in color.

Venus appears conspicuous in the east before dawn during the entire month. Shining at magnitude -4.3, it’s brighter than any other point of light in the sky, and it reaches the peak elevation for its current apparition. In early August, Venus lies south of the southern tip of Taurus the Bull’s horns. By 4 a.m. local daylight time, the planet has climbed well above the eastern horizon. Through a telescope, its disk spans 29″, a value that shrinks to 21″ by month’s end. Over the same period, its phase waxes from 40 percent to 57 percent lit. It crosses the half-lit threshold on August 17, when it reaches its greatest elongation west of the Sun (46°).

On August 1, Saturn rises in the east-northeast just as dawn breaks, 90 minutes before sunrise. Venus lies to Saturn’s far upper right in Taurus. By mid-August, Saturn stands 10° above the horizon as dawn breaks. Venus has moved into Gemini the Twins by then, having crossed the northern reaches of Orion between August 4 and 12. (Venus’s path takes it 5° south of the bright star cluster M35 on August 9.)

Finder chart for Saturn and Venus, August 31, 2004
Venus keeps approaching Saturn during the second half of August in preparation for a superb conjunction at month’s end. Amid the stunning backdrop of the winter constellations, Saturn and Venus stand just 2° apart on the 31st. The pair then rises nearly 4 hours before the Sun. Their close conjunction is a line-of-sight effect only. In reality, a giant chasm of more than 800 million miles separates the two planets.

Since Saturn disappeared in the Sun’s glow in June, it has been joined by the Cassini spacecraft. Our view of Saturn and its retinue of moons will change forever as the Cassini mission continues to orbit the ringed planet for the next several years. In the quiet, early morning hours, grab your scope and take a look at magnificent Saturn. It currently shines at magnitude 0.2, and its disk measures 17″ across. The rings span 38″, more than double the planet’s size.

Also look for Titan, Saturn’s largest moon, which glows at magnitude 8.3. It’s the target of the European probe Huygens, which is scheduled to parachute through the moon’s thick atmosphere and land on its surface this coming January. Titan shows up easily in a small telescope. It lies due east of Saturn on August 4 and 20 and due west of the planet on August 12 and 28. Saturn’s other moons prove more difficult to find because they glow significantly dimmer and lie closer to the bright planet. The easiest ones to glimpse are the three 10th-magnitude satellites: Tethys, Dione, and Rhea.

Meteors and moons
Perseid meteor
A Perseid streaks toward the horizon in this view of the 1994 shower. Astronomers think we may be in for a spectacular show this year.
Dan Smiley
Summer’s grand show
As meteor showers go, the Perseids rank among the most reliable. This year’s shower promises to be one of the best thanks to a couple factors. First, the Moon barely interferes at all, and second, the potential exists for significantly increased rates on the nights of August 10/11 and 11/12.

Unlike last year, the Moon will be a minor nuisance at worst in 2004. A 19-percent-illuminated crescent rises around 2:30 a.m. local daylight time on August 11. By the next morning, the Moon has waned to just 12 percent lit and doesn’t rise until 2:30 a.m. So the Moon has no effect for most of both nights and interferes only negligibly in the immediate predawn hours.

Some astronomers predict a sharp peak in activity this year at 21h UT on August 11; unfortunately, that’s during daylight in North and South America. The prediction comes from a detailed study of debris ejected by Comet 109P/Swift-Tuttle, the parent comet of the Perseids. Meteors result when Earth runs into the debris left behind by comets as they orbit the Sun.

In the case of Swift-Tuttle, enhanced meteor rates were seen in the early 1990s, near the time it last reached perihelion, the closest approach to the Sun during its 130-year orbit. But streams deposited during previous returns to the Sun’s vicinity also can enhance rates — if Earth runs into them. Debris trails tend to dissipate over time due to the combined gravitational influence of the Sun and planets. More recent trails tend to be narrow, producing sharp but brief peaks of activity.

This year, Earth should run into a narrow debris trail left behind by Swift-Tuttle during its 1862 return to the inner solar system. Esko Lyytinen, the Finnish researcher who made accurate predictions for the past several Leonid meteor showers, predicts an outburst of up to a few hundred meteors per hour at 20h54m UT on August 11. This peak might last only 15 minutes. The timing of this event favors observers in Europe and Asia. Those in North America may see enhanced rates on the mornings of August 11 and 12.

The traditional peak of the Perseid shower arrives at 11h UT (7 a.m. EDT) on August 12. Although this tends to favor observers in eastern Asia, the peak is broad and should offer good viewing opportunities for everyone in North America during the early morning hours of the 12th.

Lunar crater Eratosthenes
The crater Eratosthenes, with its terraces and central peak, lies near the western end of the lunar Apennines. This image comes from The Consolidated Lunar Atlas.
End of the Apennines
The well-known crater Eratosthenes lies at the southwestern end of the lunar Apennine Mountains, not far northeast of Copernicus. Eratosthenes appears dramatic as the Sun rises over it when the Moon is nine days old. The crater measures 36 miles across and exhibits a nice central peak and mountainous ridges across its floor. The most impressive mountain ridge runs north to south inside the deep crater.

The terrain surrounding Eratosthenes is quite chaotic. The rugged Apennines connect to the crater’s northern wall, while patterns of hills and minor craters dot the surrounding ramparts. The day after the Sun rises over Eratosthenes, the terraced inner walls become illuminated. By Full Moon, the crater appears washed out, as do most lunar craters under full illumination. This month, you can watch the Sun rise over Eratosthenes on August 24, and then, follow its changing appearance each successive night.

Comets and asteroids
Finder chart for Comet C/2003 K4, August 2004
Backyard comets
Snowball season remains in effect for one of the warmest, and most popular, observing months of the year. Naturally, the best place for you to view a comet is from a site far away from the city’s veil of light pollution — particularly if you want to see the full extent of the comet’s tail. But if you can’t make it, both comets C/2001 Q4 (NEAT) and C/2003 K4 (LINEAR) should stay within small-scope range from a suburban backyard.

C/2001 Q4 patrols the sky fairly low in the north, above the bowl of the Big Dipper asterism in Ursa Major. At about 7th magnitude, the comet glows about as bright as the M81/M82 pair of galaxies off to the side. On Friday evening, August 20, with the Moon out of the way, Q4 floats between two 11th-magnitude galaxies, NGC 4036 and NGC 4041. A 6-inch scope will have little trouble showing the trio.

Don’t be afraid to push the magnification up a bit. The field becomes darker, but once you adapt and throw a hood over your head to cut out background light, you’ll be able to see more. Take the time to study the different shapes and brightness profiles of the three objects; the comet should have a much more noticeable core, and the galaxies will appear quite elongated.

Sixth-magnitude C/2003 K4 sets before midnight, so set your sights on it first. This month, it tracks from Boötes through Coma Berenices and into Virgo, a corner of the sky that also contains globular cluster M53. The comet and cluster should look pretty similar, but this is only an illusion. K4 lies millions of miles away in our solar system, while M53 shines across 60,000 light-years. This great distance reduces the cluster’s brightest stars to an apparent magnitude of a mere 13.8. With an 8-inch scope on a dark, steady night, you’ll be able to see these bright stars causing granulation against the glow of the fainter swarm.

Another nice conjunction occurs on August 22 and 23, when 11th-magnitude Comet C/2003 T3 (Tabur) slides just south of the 9th-magnitude galaxy NGC 2841 in Ursa Major. The observing challenge here is that the pair lies just 10° or so above the northern horizon as darkness falls.

Finder chart for Vesta and Metis, August 2004
Vesta and Metis part
With no bright asteroids to check out during the early evening, we need to wait until midnight for a peek at Vesta and Metis. These space rocks lie together south of the Pisces Circlet, near Iota (ι) Ceti, just below the waning gibbous Moon on August 3. You’ll have to wait a few days to let the bright Moon move out of the way before you can see the asteroids well.

Vesta, the fourth of the “Big Four” asteroids discovered in the first decade of the 19th century, will be easy to spot. At magnitude 6.5, it ranks among the brightest objects within a 2° circle (about twice the field of a low-power eyepiece). On the night of August 16, watch Vesta separate from a very close encounter with the 5th-magnitude star 3 Ceti.

Metis proves trickier to find because it glows three magnitudes fainter than Vesta. Only 1 percent farther away than Vesta, Metis appears much dimmer because it has a darker surface and a diameter of just 105 miles, one-third of Vesta’s size, so it reflects a lot less sunlight in our direction.

Martin Ratcliffe is former president of the International Planetarium Society. Alister Ling is a meteorologist for Environment Canada.