The Snow Moon

For most of us living in the Northern Hemisphere, February is the snowiest month of the winter.  Many wonder why moonlight on the snow is so much brighter at Full Moon than when it’s half full.   Well, because of the difference in their brightness in the sky.  Right?  Yes—but it’s not quite that simple.  You might expect twice as much light coming from the Full Moon than at its First or Last Quarters.  But it’s not.  It’s 12 times as bright! The reason is both subtle and little-known.  When half-full, light from the Sun hits the Moon at a glancing right-angle.  Every surface feature from mountains to crater walls and valleys cast long dark shadows across the lunar landscape, reducing the amount of sunlight reflected back at us.  But at Full Moon, the Sun shines directly overhead onto every bit of the Moon and there are no shadows.


Full Moon happens on February 16th this month but will light up the snowy landscape for several evenings both before and after this date.  Bright moonlight appears to have a very subtle bluish cast to many.  But this is only a pleasant illusion, caused by contrast with surrounding lighting—especially if it’s the soft amber glow of sodium vapor lamps.  It largely disappears in the presence of the high intensity lighting of mercury vapor ones.


And while on the subject of the Full Moon, most people consider it to indeed be a lovely sight to behold.  But not everyone.  Some complain that it floods the sky with light and wipes out the constellations.   And indeed it does make the fainter ones difficult to see.  But not magnificent Orion!  This is the brightest and most radiant of all the heavenly star patterns.  And on early February evenings, it takes center stage with its multi-hued stellar gems piercing the brightest moonlight.


— James Mullaney

Former assistant editor at Sky & Telescope magazine & author of 10 books on stargazing. His latest, Celebrating the Universe!, is available from