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The Sky This Week, 2013 March 26 - April 2

Stars of winter, stars of spring.

Comet C/2011 L4 (Pan-STARRS), 2013 March 23, 00:23 UT 
Imaged from Alexandria, VA

The bright Full Moon begins the week amid the rising stars of the springtime constellations, then edges her way southward along the ecliptic to grace the morning sky as summer’s star patterns gather in the east before sunrise. Last Quarter occurs on April 3rd at 12:37 am Eastern Daylight Time. Look for the Moon just to the west of the bright star Spica on the evening of the 27th. As the night transitions to early morning Luna slowly approaches the star, closing to within three degrees by dawn on the 28th. The Moon next draws a bead on Saturn, rising just west of the ringed planet during the late evening of the 28th and just east on the following night. If you’re up before dawn on the 31st, the Moon passes through the stars that form the "head" of Scorpius, the Scorpion, which should be a very attractive sight in binoculars.

The early spring sky bids farewell to one familiar constellation and welcomes another. The first is the mainstay of the winter sky, Orion, the Hunter. You’ll find him in the southwest once twilight has given way to darkness, and by midnight he slips below the horizon. In his wake are several other bright wintertime stars, most notably the brightest star in the night, Sirius. Normally a dazzling blue-white color, the Earth’s atmosphere causes this star to flicker through all the colors of the rainbow as it follows Orion’s lead. This effect is particularly noticeable on those clear nights that follow the passage of a spring storm when the air is often very turbulent. The closer Sirius gets to the horizon the more dramatic its fluctuations become. While Orion and his cohorts settle in the west, look to the northeast for the familiar outline of the Big Dipper as its seven stars rise to prominence in the later evening hours. The brightest stars in the Big Dipper are only as bright as those in Orion’s belt, but the pattern that they create, which resembles a large soup ladle, is one of the first asterisms that most of us learn when we embark on getting to know the night sky. The Dipper forms roughly half of the larger constellation of Ursa Major, the Great Bear, and in another month it will be nearly overhead for viewers in north temperate latitudes. And just as Orion is followed by the bright glimmer of Sirius, if you follow the "arc" of the Big Dipper’s handle to the south and east you’ll run into rose-tinted Arcturus, the brightest star in the northern sky. I have always welcomed this cheery star’s appearance in the evening sky as a sure harbinger of spring.

This is probably the last week that skywatchers in urban and suburban locations will be able to catch a view of Comet C/2011 L4 (Pan-STARRS). I have been able to follow it in binoculars as it has drifted to the north from its initial westerly bearing two weeks ago. It has faded to between second and third magnitude which will make it difficult to pick out under heavily light-polluted skies, but if you have a good view to the northwest and no streetlights in the area you should be able to pick it out with binoculars. Observers in darker locations should be able to catch a few fleeting glimpses of it with the naked eye. Try looking for it at around 8:30 pm, when it will be just under 10 degrees above the horizon as the week opens. It will gain a few degrees of altitude by week’s end, but it will be fading as it goes.

The bright glow of Jupiter pops into view high in the western sky shortly after sunset, and by the end of evening twilight Old Jove is halfway to the horizon. Observing the giant planet is now getting to be a bit more challenging than it was a few months ago when he was on the meridian at midnight. His apparent disc is now just 75 percent of its opposition diameter, and as he heels over to the west his light has to pass through more of our atmosphere to reach our telescopes. The same turbulence that turns Sirius into a 1960’s light show smears out fine detail on Jupiter’s surface and causes rainbow-like fringes of color around the edges of his disc. Fortunately, his four bright moons remain easily visible, so you can continue to enjoy their nightly antics around their hulking master.

Turbulence may also hinder the early view of Saturn, which now rises in the southeast just before 10:00 pm. Fortunately the longer you wait to look at him the higher he climbs, and usually by the early morning hours the air settles down to provide a crisp view of this cold and remote world. Saturn is almost twice as far from us as Jupiter, so it’s no surprise that his disc looks tiny after you’ve spent some time looking the brighter planet. That small disc is more than compensated by the planet’s bright rings, though. Consisting of billions of independently-orbiting chunks of ice, the rings are corralled by Saturn’s gravity to form the largest and flattest structure in the solar system. While their origin is hotly debated by planetary scientists, one thing we can all agree on is that they are one of the most fascinating sights you’ll ever see through a telescope.

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