August: the August sky menu includes three brilliant planets, the year's best meteor shower, and a total lunar eclipse.
August evenings are not usually the best time for observing the planets, though. Only the southern half of the ecliptic is available at nightfall. Telescopic views of low-lying planets suffer from increased haze and turbulence. Even so, there's plenty of planetary activity this August.
Planets One, Two, Three
If you're waiting for Venus, you're too late--and too early. After months of splendor Venus has become a nighttime no-show as it reaches inferior conjunction with the Sun on the 16th. After that, it shifts quickly into the morning sky. Toward the end of the month, look due east at dawn to find Venus as it rises 1 1/2 hours before the Sun. At that time Venus will be magnitude -4.3 and appear in telescopes as an ultrathin crescent measuring 53 arcseconds from tip to tip--noticeably larger than any other planet.
Jupiter, the only planet to appear on our evening all-sky chart, is no slouch either. The giant world shines at magnitude -2.3 and presents an impressive, 39 arcsecond-wide disk in telescopes. As August opens, Jupiter is in southern Ophiuchus fairly low in the south at dusk and setting an hour after midnight. The big planet has been inching westward toward 1st-magnitude Antares since early April. It finishes its long retrograde trek on August 6th, when it stands about 5[degrees] north of the star.
During the last week of August, Mars grazes the V-shaped Hyades cluster. The alignment between planet and cluster is especially pleasing on the 23rd and 24th, when Mars extends the upper side of the "V" northeastward. Having brightened to magnitude 0.3, Mars clearly outshines 0.8-magnitude Aldebaran, the leader of the Hyades pack. These red rivals eye each other during the predawn hours all month long.
Sampling the August Deep Sky
The glowing band of the Milky Way is the summer night sky's most sublime feature. Although not usually visible from suburban backyards, the Milky Way materializes in all its glory from a dark country observing site. On clear August evenings that glow stretches from horizon to horizon, giving us a feel for the Milky Way Galaxy--the immense stellar metropolis in which we live.
Notice how the Milky Way broadens in Scorpius and Sagittarius near the southern horizon. There the bulging Sagittarius Star Cloud lies in the direction of the galactic hub, nearly 30,000 light-years away. From Sagittarius, the Milky Way stretches across the sky through Aquila and Cygnus. The vibrant Cygnus Star Cloud is part of the same spiral arm that our Sun inhabits. From Cygnus, the Milky Way falls toward the northeast horizon via Cepheus, Cassiopeia, and Perseus. When we gaze at the narrow Perseus sector of the Milky Way, we're looking toward the edge of the galaxy some 20,000 light-years away.
Several of the finest emission nebulae for mid-northern observers are located in the Sagittarius Milky Way. None is more remarkable than the Lagoon Nebula, M8. This misty cloud of hydrogen is about 5,700 light-years away (four times farther than M42 in Orion) yet it's visible to the naked eye as a 5th-magnitude patch just above the bright cloud of "steam" issuing from the spout of the Teapot.
A 4-inch telescope will show that M8 is an object of spectacular complexity. The readily visible portion of the nebula is about 45 by 30 arc minutes in angular size and elongated east-west. It is divided into two unequal parts by a curving lane of dust--the dark "lagoon" that gives the nebula its name. The nebula's lacy eastern portion envelops a loose, 5th-magnitude star cluster called NGC 6530. The more luminous western part surrounds a 6th-magnitude star (9 Sagittarii) that energizes the entire cloud. Larger telescopes reveal tenuous filaments of nebulosity east, west, and north of the main nebula, making M8 an even bigger target.
The Lagoon lies at declination of -24[degrees]. As a result, telescopic views of M8 are often compromised by atmospheric turbulence and haze. Light pollution is no help either. Thankfully, you can boost the contrast of any emission nebula with the aid of a nebula filter--especially a narrowband or O III type. These filters block unwanted light from the sky while transmitting the emission of the nebula itself. Such filters help the Lagoon even when it is viewed from a dark site.
The same is true for the Trifid Nebula, M20, which truly blossoms when viewed with a nebula filter at medium to high magnification. The Trifid is found only 1 1/2 [degrees] north-northwest of M8 and is about 30 by 20 arc minutes in extent. M20 features two different types of nebula located side by side in a north-south orientation. The southern component (the actual Trifid) is a trilobed emission nebula illuminated by a 7th-magnitude triple star. This delicate mist, complete with its network of trisecting dust lanes, is visible in a 4-inch telescope.
The northern portion of M20 is a smaller, fainter reflection nebula enveloping a 7.5-magnitude star. Because this part of the cloud is reflecting light, not emitting it, a nebula filter won't help--a dark country sky is your best bet for a good view. While you're at it, follow a curving chain of stars from the Trifid about 3/4 [degrees] northeast to the 6th-magnitude open cluster M21. This 15-arc minute-wide group is sparse but plainly visible in the same low-power field as M20. The graceful Swan Nebula, M17, fl oats in a Sagittarian sea of stars 9[degrees] north-northwest of 3rd-magnitude Lambda ([lambda]) Sagittarii. In city-bound telescopes at low power M17 is but a 6th-magnitude streak of nebulosity, about 15 arc minutes long, and oriented roughly east-west. Increased magnification reveals dark rifts cutting diagonally across it, plus a hook of nebulosity protruding from the streak's western end. The hazy hook, which reaches upward in the inverted south-up) field of my 8-inch reflector, defines the bird's curving neck and head. The bright streak is its body, and wedge of very faint nebulosity above the body suggests a raised wing. At the nebula's eastern end, a wispy extension curling northward is a leg dragging in the water. These tenuous bits look best in larger apertures, but I'm amazed at how well they show up in small, filter-equipped scopes under a dark sky.
If you desire especially elusive prey, set your sights on the Eagle Nebula, M16, located 2 1/2 [degrees] north-northwest of M17 in southernmost Serpens Cauda. This complex emission nebula is best known from its famous Hubble Space Telescope portrait, and because of that it is usually disappointing sight in small telescopes. M16's fan-shaped mass is large (about 35 by 28 arc minutes) but ill-defined. Look for a modest star cluster with extremely faint nebulosity surrounding it. A nebula filter and a dark sky are crucial for sighting the Eagle.
A Copper Moon
The year's second total lunar eclipse is a predawn spectacular that takes place on the 28th. This eclipse is visible in its entirety across the western half of North America, while viewers on the East Coast may see only the very beginning of totality in the brightening morning sky. Located in Aquarius, the Moon will be nearly due south when the faint penumbral darkening begins at 12:52 a.m. Pacific Daylight Time. The Moon enters the Earth's umbral shadow an hour later, and totality begins at 2:52 a.m. and lasts 91 minutes. The umbral phases will end at 5:24 a.m., just before moonset on the West Coast.
A small, red world awaits you in the northeast after midnight. Mars is still several months away from being a decent telescopic target, but it's already an attention-grabbing object to the naked eye. Early in the month, Mars shines at magnitude 0.5 as it swings south of the Pleiades cluster in western Taurus. Adding to the spectacle on the 7th is a waning crescent Moon, which appears 2[degrees] east of the cluster shortly after moonrise for viewers on the East Coast of North America. At that hour the Moon, the Pleiades, and Mars will form a right-angle triangle, and by the time dawn breaks on the West Coast, the Moon will have traveled eastward to make it an equilateral triangle. Beautiful!
AUG 3 Moon at perigee (368,891 km).
AUG 7 Moon passes through the Pleiades (Europe).
AUG 13 Perseid meteor shower peaks.
AUG 13 Neptune at opposition.
AUG 18 Moon at apogee (404,618 km)
AUG 21 Saturn in conjunction with the Sun.
AUG 28 Lunar eclipse (totality begins at 2:42 a.m. PDT).
AUG 31 Moon at perigee (364,171 km).
The Perseid meteor shower is the most popular meteor display of the year. You'll have no interference from moonlight when the Perseids come to maximum strength around the time of new Moon on the night of August 12th-13th. Peak activity will occur in the predawn hours of the 13th when the radiant is highest, and upwards of 60 meteors per hour can be counted by a single observer in ideal sky conditions. Get out that reclining chair and put on the coffee; it's always worth doing an all-nighter for the Perseids.
THE MILKY WAY
One of the most spectacular deep-sky objects is our own Milky Way Galaxy. August nights are among the best for exploring the rich swath of stars that runs from Sagittarius in the south up north through Cygnus.
BINOCULAR HIGHLIGHT The Lagoon Nebula
This is the best time of year to be a binocular astronomer. The bright core of the Milky Way, with all its attendant treasures, is crossing the meridian, and for most readers the nights are as mild and pleasant as they come. The combination provides the perfect inducement to a night of relaxed viewing from the comfort of a chaise longue. And one of the must-see objects of this time of year is the Lagoon Nebula, M8.
Although one can methodically star-hop to M8 from the spout of the Sagittarius Teapot, the richness of this part of the sky lends itself to a more casual, sweeping approach. You'll know you've reached your destination when you come across a small but conspicuous misty patch of light flanked by a chain of stars aligned east to west.
In his book The Messier Objects, Stephen James O'Meara evocatively describes the Lagoon as a "large curdle of galactic vapor." How much of this emission-nebula "vapor" you see will depend, as always, on sky conditions. That said, one April I viewed M8 from my light-polluted suburban backyard while twilight began to brighten the sky, and I had no trouble seeing the Lagoon in my little 10 ??30 binoculars. I'm sure the view from your chaise longue will be quite a bit better.
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|Date:||Jan 1, 2007|
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