The Sky This Week
U.S. Naval Observatory's Weekly Blog

Celebrate International Dark Sky Week!

by Geoff Chester, USNO Public Affairs
06 April 2021

Ursa Major under dark skies, imaged 2019 March 17 from Mollusk, Virginia
with a Canon EOS Rebel SL2 DSLR, 24mm f/2.8 EF-S lens, and an Omegon Mini-Trak LX2 mechanical star tracker

The Moon plays hard-to-get this week, waning in the early morning sky as the week opens and re-appearing in evening twilight as it ends.  New Moon occurs on the 11th at 10:30 pm Eastern Daylight Time.  Look for Luna a few degrees below bright Jupiter in deep twilight before dawn on the 7th.  If you have a flat western horizon and a good, clear sky you might be able to catch a very slender crescent Moon half an hour after sunset on the evening of the 12th.  She will only be about 22 hours past New Moon, so her crescent will be little more than a hairline.  She should be much easier to spot on the following evening.

This week we celebrate International Dark Sky Week, a time when people are encouraged to turn out their outdoor lights to enjoy the beauty of the night sky overhead.  The event was founded in 2003 by a Virginia high-school student, Jennifer Barlow, who wanted to try to encourage her neighbors to turn down their nighttime lighting.  The event quickly became a national effort through the encouragement of the International Dark Sky Association, and in 2009 it became a world-wide event in celebration of the International Year of Astronomy.  It is now not only sanctioned by IDA, but it is actively encouraged by the American Astronomical Society and the International Astronomical Union, the two largest organizations of professional astronomers.  Its goals are simple.  As Jennifer put it, "The night sky is a gift of such tremendous beauty that should not be hidden under a blanket of wasted light. It should be visible so that future generations do not lose touch with the wonder of our universe.  It is my wish that people see the night sky in all of its glory, without excess light in the sky as our ancestors saw it hundreds of years ago."  The negative effects of light pollution began to receive attention from the astronomical community in the 1980s.  Since that time many studies have shown that it is more than just a nuisance to stargazers.  It affects many ecosystems in detrimental ways, with dire effects on bird and animal migrations and breeding cycles as well as human circadian rhythms and physical health.  In addition, it consumes vast amounts of energy from fossil fuels, most of which is wasted in lighting up the sky and adds to the already high concentrations of carbon in our atmosphere.  It is the goal of International Dark Sky Week to draw attention to this issue, which can easily be reversed for everyone’s benefit.  Please dim your lights and enjoy the nights!

Spring’s constellations greatly benefit from dark skies.  Unlike the bright stars of winter, which lie along the plane of the Milky Way, the stars of spring lie in the direction of the galactic pole.  Essentially our line-of-sight passes through the thin disc of our home galaxy, so we see relatively few bright stars.  With a handful of exceptions most of the spring constellations’ outlines are “fleshed out” by third- and fourth-magnitude stars.  A case in point is the constellation of Ursa Major, the Great Bear.  Most of us can see the seven stars of the Big Dipper asterism, but they make up less than half of the constellation’s shape.  From a dark site, though, the rest of the Bear takes shape, from her triangular head to her long legs terminating in claws.  One of my favorite spring constellations can only be seen under good dark sky conditions.  To find it first locate the sickle and triangle that make up the constellation of Leo, the Lion.  Just to the east of Denebola, the star that marks the Lion’s tail, you will find a large scattering of faint stars that represent Coma Berenices, or “Berenice’s Hair”.  This is the only constellation named for an historic figure and represents the golden tresses sacrificed by Berenice II, Queen of Egypt, for the safe return of her consort, Pharaoh Ptolemy III, from battle.

Planet Mars continues his lonely eastward trek through the setting stars of winter.  This week he continues his journey across the stars of Taurus, the Bull.  By the week’s end he passes between the stars El Nath and Zeta Tauri, which mark the tips of the Bull’s horns.

Saturn and Jupiter now rise before the onset of morning twilight, and may be found low in the southeast as the sky begins to brighten.  Saturn leads Old Jove by about half an hour and lingers even as Saturn winks out in the increasing twilight glow.   
 
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