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Mercury in the spotlight

March 2011: Mercury begins the month lost in the Sun's glare but quickly emerges for its finest evening appearance of 2011.
A crescent Mercury looked stunning when MESSENGER flew by in 2008. The spacecraft starts orbiting the inner planet this month. NASA/JHUAPL/CIW
For many of us in the Northern Hemisphere, March means that spring is in the air. (Or, at the very least, that thoughts of spring start dancing in our minds.) The evening sky mirrors this season of rebirth. Mercury begins the month lost in the Sun’s glare but quickly emerges for its finest evening appearance of 2011.

Brighter Jupiter points a spotlight on Mercury in mid-March, when the two planets pass just 2° from each other. Around the same time, NASA’s MESSENGER spacecraft will start orbiting Mercury, heralding a rebirth in scientific study of the innermost planet.

As Mercury and Jupiter sink toward the western horizon, Saturn appears low in the east. Although the ringed planet won’t reach peak visibility until April, the view in March proves equally stunning. Saturn remains on view the rest of the night. As it dips in the west with dawn’s approach, brilliant Venus pops above the eastern horizon.

During the first week of March, Jupiter dominates the western evening sky. The giant planet gleams at magnitude –2.1. It appears noticeably brighter than Sirius, at mag­nitude –1.5 the night sky’s brightest star, which stands in the south after darkness falls.

But after March’s first week, Jupiter gains a companion. Mercury comes into view around March 8, when it lies low in the west and almost directly beneath Jupiter. Look for a bright point of light some 3° above the horizon 30 minutes after sunset. Mercury shines at magnitude –1.3, bright enough to pierce the still-bright sky. Jupiter stands some 10° higher and pops out of the twilight glow sooner. If you can’t spot Mercury with your naked eyes, sweep along the horizon with binoculars.

If you view at the same time relative to sunset each evening, Mercury climbs nearly a degree per day. By March 14, the inner planet lies 2° to Jupiter’s lower right and the pair stands some 9° high. The two appear equally close the following two evenings.
Mercury’s best evening performance of 2011 occurs in March’s second half, although it’s easier to find near Jupiter at midmonth. Astronomy: Roen Kelly
Although Jupiter quickly falls from view during the ­following week, Mercury continues to climb. Its reaches its greatest angular separation from the Sun March 22, when it lies 19° east of our star. That evening, the innermost planet lies 12° high 30 minutes after sunset and sets an hour after that. It then shines at mag­nitude –0.3, and, with Jupiter all but gone, it’s the most conspicuous object in the western sky. Mercury fades quickly during the final week of March, dropping to 2nd ­mag­nitude by the 31st.

When viewed through a telescope, Mercury presents a disk that changes in size and phase but shows no other detail. On March 8, the planet appears 5" across and 91 percent lit. At greatest elongation March 22, its disk spans 7" and is just under half-lit. ­Mercury’s size continues to grow while its phase wanes as March winds down.

Just because a telescope won’t deliver sharp views of Mercury doesn’t mean you can’t see surface features. On March 17/18, NASA’s MESSENGER probe will enter orbit around the inner planet. MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, began its journey from Cape Canaveral in August 2004.

During its nearly 7 years in space, it has flown past Earth once, Venus twice, and Mercury three times. The Mercury flybys revealed remarkable details about this still-enigmatic planet, but the yearlong orbital mission that commences this month promises many more discoveries. As you observe Mercury this month from approximately 100 million miles away, take a minute to consider the probe performing a similar job from a distance of 125 miles.

In early March, Saturn rises an hour after twilight fades away. But by month’s close, it pokes above the eastern horizon just as the Sun sets. The ringed planet grows more conspicuous as it climbs higher in the eastern sky.

Two bright stars accompany Saturn this month. Spica lies 10° directly below the planet, and Arcturus appears 30° to the planet’s left. Saturn shines at magnitude 0.4, which makes it slightly brighter than blue-white Spica and a bit fainter than yellow-orange Arcturus. On March 20, a nearly Full Moon passes by and outshines everything.

Saturn lies in the constel­lation Virgo the Maiden, approximately halfway between 3rd-magnitude Gamma (γ) Virginis and 4th-magnitude Theta (θ) Virginis. By late spring, Saturn’s westward motion will carry it to within a degree of Gamma.
Track down Enceladus the night of March 12/13, when this dim moon passes near Tethys, which glows 1.5 magnitudes brighter. Astronomy: Roen Kelly
Although Saturn looks nice with naked eyes or through binoculars, you really should target it with a telescope. Only a scope reveals the gossamer-thin rings encircling the yellowish planet. In mid-March, the disk appears 19" across and the rings span 43".

For the past few years, the ring system has appeared nearly edge-on and revealed little structure. But the rings now tilt 9° to our line of sight. That’s enough to show clearly the dark Cassini Division, which separates the outermost A ring from the brighter B ring. On nights of steady seeing, you might glimpse the dusky, innermost C ring.

Another subtle feature to look for is Saturn’s shadow cast onto the rings behind. Search for a dark wedge just off the planet’s western limb where the rings should be. The shadow should be noticeable in early March but will disappear by month’s end because, from our earthly ­perspective, the Sun then illuminates Saturn from behind.

Any telescope will show Saturn’s brightest moon, 8th-magnitude Titan. This haze-shrouded satellite has an orange hue through scopes with good optics. Titan orbits Saturn every 16 days. It passes due north of the planet the nights of March 8/9 and 24/25 and due south March 16/17.

Small telescopes also reveal a trio of 10th-magnitude moons: Tethys, Dione, and Rhea. They orbit Saturn much faster than Titan (in 1.9, 2.7, and 4.5 days, respectively) so they change positions relatively fast. Look for Tethys and Dione next to each other the evening of March 18, when they lie to Saturn’s east.

Enceladus proves to be a bigger challenge. Not only is it fainter (glowing at 12th magnitude), but it also lingers close to the bright rings’ edge. You’ll need a 10-inch or larger telescope and good timing to see it. Try the evening of March 12, when Enceladus lies just 3" from Tethys between 10 and 11 p.m. EST. The two quickly drift apart during the next few hours. Alternatively, try the morning of March 17, when Enceladus lies well clear of the rings’ eastern edge.
Venus Neptune finder chart
Venus passes just south of Neptune in late March, but you’ll have to look carefully to see the distant planet’s 8th-magnitude glow. Astronomy: Roen Kelly
Iapetus presents a different kind of challenge. This moon takes 79 days to orbit Saturn and so normally lies either far east or west of the planet. On March 3 and 4, however, Iapetus passes due south of Saturn and appears only 2' away. The moon then glows at 11th magnitude. As it heads west of the planet in the following few weeks, Iapetus grows brighter as its ice-covered trailing hemisphere points more in our direction. At greatest western elongation March 24, the moon glows at 10th magnitude but lies 10' from Saturn.

Venus continues to dominate the morning sky, but not for as long as it did this winter. It rises 2 hours before the Sun in early March but only about 75 minutes before sunrise at month’s end. The brilliant “morning star” remains unmistakable, shining at ­magnitude –4.0.

The rapid changes that characterized Venus’ telescopic appearance the past several months have now slowed dramatically. The planet’s apparent diameter shrinks slightly in March, from 16" to 13", while its ­gibbous phase fattens from
71 to 80 percent lit.

March opens and closes with elegant conjunctions of Venus with a crescent Moon. On the morning of March 1, a 10-percent-lit Moon sits some 5° to the planet’s left. And on the 31st, a 9-percent-lit crescent lies a similar distance to Venus’ upper left. Add in the subtle shadings of morning twilight, and these two objects will offer a beautiful and photogenic scene.

Venus points the way to Neptune in late March — if you live south of the equator. The bright planet passes just 0.2° south of the dim one March 26. From mid-northern latitudes, the two lie just 3° high an hour before sunrise, and 8th-magnitude Neptune will be lost in the twilight.

But from the Southern Hemisphere, it’s a different story. From 30° south latitude, the pair stands nearly 20° high when twilight begins. Binoculars or a telescope will reveal Neptune’s blue-gray glow just below Venus.

Neither Mars nor Uranus makes an appearance during March. Both lie on the far side of the Sun from Earth and remain invisible in our star’s glow. They will return to view before dawn in late April.

Earth’s vernal equinox occurs at 7:21 p.m. EDT March 20. This marks the instant when the Sun crosses the celestial equator traveling north and the start of Northern Hemisphere spring.
The glowing pyramid known as the zodiacal light climbs from the western horizon as seen from Chile’s La Silla Observatory. ESO/Y. Beletsky
When dust sets the zodiac aglow
March features no major or minor meteor showers because Earth doesn’t encounter a stream of dust particles, which would spark a shower when they burn up from friction with Earth’s atmosphere.

But if you know where and when to look under a dark sky, you can see a faint glow from billions of dust grains in the inner solar system as the Sun shines on these particles, creating the so-called zodiacal light. The best time to view it is on March evenings after twilight has faded completely, about 90 minutes after sunset.

The cone-shaped glow has a broad base that points nearly straight up from the western horizon. Under superb conditions, the glow extends into Taurus. It appears slightly fainter than the Milky Way, so you’ll need to view it under a clear moonless sky far from any artificial lights. The Moon is out of the evening sky during March’s first week and its final 10 days.
Lacus Spei sits near the Moon's northeastern limb. You can spy its dark lava plains as the Sun climbs higher in the lunar sky from around March 8 to 20. Consolidated Lunar Atlas/UA/LPL
Hope springs eternal on the lunar limb
It’s always a treat to watch a lunar feature transform from near invisibility to prominence as the earthly nights tick past. When you check out the resplendent crescent Moon March 8, pay particular attention to the northeastern quadrant. There you will find the striking pair of craters Hercules and Atlas. A line drawn between these two and extended eastward toward the limb brings you to the crater Mercurius. One crater diameter south of it lies Lacus Spei, the Lake of Hope.

The low Sun angle exaggerates the highs and lows of the lunar surface, camouflaging the flatness of this lava lake. As the Sun climbs higher in the Moon’s sky during the next week or so, the shadows disappear and the surrounding terrain brightens, highlighting the relative darkness of Lacus Spei.

At midmonth near lunar high noon, the lake stands out as a smooth patch of dark gray material while Mercurius has practically vanished. Long white rays also play across the scene. The most obvious ones belong to Tycho far to the south. A less-conspicuous one slices past Lacus Spei. If you follow this ray northward, you’ll come to its birthplace at the bright crater Hayn. This young crater is roughly the same age as Tycho. A mere day after the March 19 Full Moon, the returning shadows begin to swallow the Lake of Hope.
When to view the planets
Mercury (west) Saturn (southeast)
Venus (southeast)
Jupiter (west)
Saturn (southwest)

Neptune (east)
It’s not a comet — but it plays one when viewed through small scopes. Gas and dust in NGC 1931 mimic a comet’s fan-shaped tail; embedded stars resemble a comet’s head. Al and Andy Ferayorni/Adam Block/NOAO/AURA/NSF
Don't fall for this comet imposter
The night sky normally features at least one comet bright enough to view through a modest telescope. Alas, this month is not typical. Astronomers predict that the brightest of these visitors to the inner solar system will reach only 13th magnitude — a tough find even through a fairly big scope. So this month we call in the reinforcements and target the sky’s finest comet look-alike: deep-sky object NGC 1931 in Auriga. You can find it high in the west after twilight ends, just 1° west of the bright star cluster M36.

NGC 1931 shows up easily through a 2.4-inch scope at 40x under a dark sky. It even appears bright enough to see from a suburban backyard through an 8-inch instrument. At low power, this mas­querader looks like a comet: The glow begins with a bright head
and then fans out into a weak tail of nebulosity. In a bigger scope, boost the power to 200x or more to get a closer look. The “comet head” turns out to be four closely spaced stars embedded in a cocoon of pale light.

NGC 1931 is a small star cluster with a surrounding reflection nebula, a cloud of dust and cold gas that scatters light from a nearby star. In other words, the nebula behaves much like comet dust does when it reflects sunlight.

With dozens of reliable periodic comets roaming the inner solar system and random ones coming in from the Oort Cloud, we almost always have a modestly bright comet to follow. But unless we get a surprise, we’ll have to wait until May for Comet C/2009 P1 (Garradd) to crack 10th magnitude.
Juno finder chart
Both Juno and Massalia reach opposition in southern Leo this month, when they each glow at 9th magnitude. Astronomy: Roen Kelly
A space-rock duo dances with the Lion
Thanks to a nice coincidence, we can nab two asteroids cruising through the same part of the sky. These main-belt asteroids, 3 Juno and 20 Massalia, reach opposition within 2 days of each other in mid-March. Even better, they both glow at 9th magnitude (bright enough to see through a small scope) and lie no more than 4° apart.

Juno and Massalia begin March in western Virgo the Maiden but soon cross the border into southern Leo the Lion. In midevening, this region lies about one-third of the way to the zenith in the eastern sky.

Be careful not to mistake the asteroids for background stars. Make a sketch of the area and then return to the same field a night or two later. The asteroids will be the points of light that move. Juno cruises 0.5° south of 4th-magnitude Sigma (σ) Leonis March 22 and 23 while Massalia skims within 0.1° of 5th-magnitude Tau (τ) Leo on the 23rd.

Although Juno lies considerably farther away than Massalia, they appear equally bright because Juno is nearly twice the size of 90-mile-wide Massalia, which was the first asteroid whose name did not originate in mythology. It is the Greek spelling for the French city Marseilles, where co-discoverer Jean Chacornac first sighted it.

Martin Ratcliffe
provides professional planetarium development for Sky-Skan Inc. Alister Ling is a meteorologist for Environment Canada.


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