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Enjoy our starry night skies.
Be a considerate neighbor: reduce nighttime glare.
Shield your outside lights downward.
Let the stars light up the night.

March 2013 Night Sky

—Charlie Christmann

Start brightness—magnitudes explained

Okay, I have been remiss—I have not done a great job recently of defining how brightness is defined in astronomy. Astronomers talk about brightness on a logarithmic scale called magnitude. The scale is a bit strange in that dim stars have large magnitude values and bright stars have values near zero or even negative values. Each whole number change represents a difference of twenty times in brightness. For example, a magnitude 1.0 star is twenty times brighter than a magnitude 2.0 star. The sun and moon can also be assigned a brightness value; the sun is -26 magnitude; the full moon is -13 magnitude. Venus gets as bright as -4 magnitude, while Sirius, the brightest star in our night sky is a -1.4 magnitude.

Under perfect sky conditions, away from all lights, clear skies, and no moon, and with good eye sight, a human should be able to see a seventh magnitude star. Unfortunately, most skies are not near perfect. There is light pollution all around from cities and unshielded light sources. In rural southern Sandoval County, away from towns, you might see a fifth magnitude star thanks to Albuquerque, Rio Rancho, Bernalillo, a couple of casinos, and your afraid-of-the-dark neighbors lighting up the night.

Harvard University has a great chart on the astronomical magnitude scale on the web at www.icq.eps.harvard.edu/MagScale.html.

We have the potential to see two spectacular sights this year, and the darker we can keep our skies, the easier it will be to enjoy them.

Naked eye comet

It is not often we get to see a comet with our naked eye. In the last ten years, there have been only two: Hyakutake in 1996 and Hale-Bopp in 1997. Most comets are small and dim, needing a decent-sized telescope to find them. Currently there are more than one hundred being tracked by telescopes worldwide.

The one we are interested in for March is called C/2011 L4 PanSTARRS, discovered in June of 2011 by the PanSTARRS 1 telescope in Hawaii. The comet is approaching the sun from the south, almost perpendicular to the orbit of the planets. Observers at the U.S. Naval Observatory believe that C/2011 L4 is making its first trip around the sun, having been ejected from the Oort cloud at the edge of our solar system.

The Oort Cloud is believed be a vast reservoir of comets left over from the formation of our solar system some four-billion years ago. Its outer edge is thought to be three light-years (18 trillion miles) from the sun. Long period comets and possibly higher-inclination intermediate comets originate in this cloud, having been yanked by gravity and collisions into orbit closer to the sun. Comets such as Halley and Swift-Tuttle are the best known examples.

C/2011 L4 has astronomers excited, as this should be a naked eye comet, visible in the late twilight by March 12th, having rounded the sun just two days earlier. Being a new comet, any estimate of brightness and tail length is an educated guess; but a brightness of around +3 magnitude is expected. There is also a possibility that, passing within 28 million miles of the sun, the comet could break apart and the show will never happen.

March 12 or 13 should be the best time for your comet-viewing, picture-taking party, because C/2011 L4 should be visible alongside a magnificent crescent moon in the western skies after sunset. For best viewing, find a dark spot, away from streetlights. Look in the direction of the sunset just after the sun has gone down. The comet never gets higher than about ten degrees (width of your fist held at arm’s length) above the horizon between March 8 and 20. Binoculars should allow you to follow the comet well into May. After mid-May, the comet becomes circumpolar and is up all night long.

Enjoy C/2011 L4 now. It is expected to next visit the inner solar system somewhere around the year 112,000 (yep, that’s a long time from now).

Naked eye comet—round two

If you miss the March spectacle (or if it fizzles out), fear not. While astronomers are excited about C/2011 L4, Comet C/2012 S1 (ISON) has them giddy. If expectations hold, this comet may be visible in daylight, close to the brightness of Venus at -4 magnitude. Some are even saying this comet could be as bright as the full moon. Again, this comet will be passing very close to the sun and almost anything can happen with a ball of dust filled with ice only 1.5 million miles above the surface of the sun. You will have to wait until the end of November and December to see if this one performs as promised.

March Night Sky

 
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