Skinny Moon Debuts in Evening Sky

StarWatch 856 for the week of Jan. 13, 2013.

Have you noticed the sun setting just a tad bit later? December 7 was the earliest sunset, 4:35 p.m., for souls living at 40 degrees north latitude.  We still haven’t gained that much daylight since the winter solstice, the shortest day of the year, but we are at least making the most progress at the correct time of the day—evening.  

Sunsets are now about 25 minutes later.  Sunrises, on the other hand, are only three minutes earlier in comparison to the latest sunrise which occurred on January 4.  The bottom line is that we still have a long way to go before spring, even though I remain optimistic.   

At the beginning of the week, you’ll notice a scimitar-shaped waxing crescent moon hugging the SW horizon about 45 minutes after sundown.  The horseshoe shape of the moon always points to the location of the sun below the horizon, while the tilt of the horseshoe gives a good approximation of the tilt of the moon’s orbital plane to the horizon.  

Bring binoculars along to help accent one of the most beautiful aspects of a young moon, earthshine. When the moon is near its new phase, the Earth as viewed from the moon is nearly full.  

The Earth consumes 16 times the sky area of a full moon and reflects light about five times more efficiently than the moon, making Earth appear about 80 times brighter (5 x 16 = 80) in the moon’s sky than a full moon appears in our sky.  

Light reflected from a nearly full Earth is reflected back to us by the moon, producing sufficient illumination on the dark face of the moon to allow the entire disk to be seen.  This ashen light, earthshine, is also referred to as “the old moon in the new moon’s arms.” 

Although Sunday through Tuesday will be the best time to catch the ashen light with the unaided eye, binoculars will still reveal its presence right through First Quarter (Friday) if conditions are clear enough.  More fun lunar observations next week or read ahead at www.astronomy.org.