Western Slope Skies - Brightness of the Moon

The Moon is very bright during the first week of January, and it’s tempting to go for a snow shoe trek or ski tour by moonlight.  But two weeks ago, the moon was hardly visible at all.  There is one main factor that determines the visibility and brightness of the moon:  lunar phase. But, varying Earth-Moon distance also plays a role.

The Moon shines mostly by reflected sunlight as it orbits the Earth each lunar month.  The Moon has a sun-lit side and a night side, which migrate as the Moon moves along its orbit.   When the Moon is new, we are looking at the dark, night side, and we usually can’t see the Moon at all.   As the Moon moves eastward of the sun, we begin to see more and more of the day-lit side of the moon, and we perceive changing lunar phases, from crescent, to first quarter, to full, over a half-month period.  The sun-lit, lunar crescent is not very bright, but about 7 days after new Moon, the half-illuminated, first quarter Moon is bright enough to see in daylight.  On January 5th the Moon is full; we are now seeing its entire sun-lit side.  Because a full Moon has twice the sunlit area of a first quarter Moon, you might think that it would be twice as bright.  But, the full Moon is about 10 times as bright as a first quarter Moon!  This is mostly due to an effect called “shadow hiding.”  As we look at a full Moon, the sun is directly behind us, and we can’t see any dark shadows cast by lunar topography.  This makes the Moon appear extremely bright.     

Want to see a comet?  During January, Comet Lovejoy (aka 2014 Q2) will be moving northward in the evening sky.  The comet will appear as a fuzzy ball and may have a short tail.  Using binoculars and an on-line finder chart, you can find Comet Lovejoy on January 10 just west of Orion.  By, January 22, the comet will have moved northward into the constellation Aries.     

Western Slope Skies is produced by members of the Black Canyon Astronomical Society. This episode was written & recorded by Art Trevena.