Driftless Dark Skies: Good Morning (Evening) Earthshine

December 30th, 2016 by John Heasley

I saw the new moon late yestreen

Wi’ the auld moon in her arm

Ballad of Sir Patrick Spens    

Welcome 2017 with Earthshine!  You can easily see how sunshine from our star lights up parts of our moon as it goes through its phases and the dark side wanes and waxes.  And you can also see how this sunshine is reflected by our moon to light up our darkness here on Earth with moonshine.  But there is also sunlight reflected off the Earth which lights up the dark side of the moon.  Humans have long wondered at the sight of the old moon in the arms of the new moon.  It was Leonardo da Vinci over 500 years ago who first figured out the cause of the glow.  This is Earthshine, and you can see it best in the days just before and after the new moons on December 29 and January 27. 

photo from NASA.

The show starts on the evening of December 30.  Look for a slender moon low in the southwest just after sunset at 4:34 and before moonset at 6:16.  On New Year’s Eve, the moon will be a little wider and a little higher in the sky until moonset at 7:15.  As the sky darkens at twilight notice how Earthshine lights up the dark side of the moon and lets you make out some of its features.  Binoculars make it easier to see the lighter highlands and the darker “seas”. 

On January 1, the moon is to the right of Venus bright and brilliant in the southwest.  You can enjoy both until moonset at 8:18.  On January 2, the crescent moon is between Venus and Mars.  Mars is dimmer and ruddier than Venus.  On January 3, the alignment changes and the moon is now above and to the left of Mars and Venus.  Sunshine now illuminates more of the moon making it more difficult to see the Earthshine.  But you can continue to enjoy the moon as it sets later and waxes fuller.  On the evenings of January 11 and 12, follow the Full Wolf Moon traveling high in the sky. 

Early risers can catch Earthshine just before sunrise later in the month.  The Last Quarter Moon is to the left of Jupiter on January 19.  Both are visible from midnight until sunrise at 7:28.  Keep watching as the moon wanes in size, rises later, and moves closer to the sun.  On the morning of January 24, the waning crescent moon is low in the southeast to the left of Saturn.  Both are visible from moonrise at 4:33 until sunrise at 7:24.  By now, you should start seeing Earthshine again.  Look for the even skinnier moon rising in the southeast on January 25 at 5:24.  As it rises higher, look for Mercury below it.  You’ll want a clear horizon and maybe binoculars for this one. 

Don’t miss the grand finale on January 31 as the Crescent Moon, Venus, and Mars form a tight triangle in the southwest from sunset at 5:11 until moonset at 9:29.  Hope you enjoy beginning 2017 with Earthshine! 

John Heasley is an astronomy educator and stargazer who enjoys connecting people with the cosmos. He volunteers with NASA/JPL as a Solar System Ambassador. For more information about stargazing in southwest WI, like Driftless Stargazing LLC on Facebook and find out whenever there’s something awesome happening in the skies. Driftless Dark Skies appears monthly in the Voice of the River Valley.