The Third Quarter Moon

Half Moon

I put the lens of my point & shoot camera into the eyepiece of my telescope on Jan 14, 2008

Steve Margala

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Only in astronomy does “half” and “quarter” mean the same thing. At least, when we’re talking about the Moon. Read on …

Beginners, usually pointing telescopes at the full Moon, are often disappointed—unaware that the full Moon offers a bare cupboard because the sun then shines straight down like a flash camera to erase all shadows and highlights.

But all phases are not created equal. Sky watchers, take note: A fabulous Moon appears all this week. It’s now exploding with breathtaking detail for anyone with binoculars, spotting scope, or even the smallest telescope. The phase responsible for these sudden riches is the first quarter.

This is a Moon phase packed with misconception. Even its name is misleading: how many realize that the quarter Moon is the same thing as a half Moon? 

The Quarter Moon is the Same as a Half Moon

The quarter Moon is also called a “half Moon” because it looks like the Moon is only half lit—on one side of the Moon during the first quarter and on the other side during the third quarter because the Moon is on the other side of Earth. The third quarter usually happens around day 22 (twenty-two).

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A half Moon has something to offer:

  • Only a half Moon aims its terminator, the day-night line that is home to all the juicy detail, straight at us. This means highlighted craters then face you like actors hamming it up, instead of pointing, foreshortened, in other directions the way the rest of the lunar phases do.
  • You’d think a half Moon would be half as bright as a full Moon, right? Oddly enough, a half moon is only one tenth as bright as a full Moon. Yet why does it seem so bright? This is because the full Moon throws sunlight straight back at us like a movie screen, while the first quarter’s sideways illumination creates innumerable unseen shadows in the Moon’s powdery surface.

How to Best View the Half Moon

Point the cheapest telescope towards the half Moon. Stay below 60 power and the entire Moon will fill the field like a scene from 2001. This month, the optimum nights are Thursday to Sunday, November 19 to 22.

Ordinary binoculars reveal the lunar Apennines, that mountain range just above dead center, whose jagged Himalaya-sized peaks tower straight up at you like skyscrapers. Then there’s the badlands, the southern region, crazily pockmarked with a generous sampling of the 30,000 craters visible from Earth.

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The scene changes dramatically each night as the terminator slithers over the Moon’s surface at 10 miles per hour. (A lunar jogger with enough stamina could keep nightfall at bay!)

Our lunar revue ends next week with neither bang nor whimper, as the days around Thanksgiving usher in that romantic underachiever, the full Moon.

~ By  Bob Berman

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s blog on stargazing and astronomy. Wondering which bright objects you’re seeing in the night sky? Want to learn about a breathtaking sight coming up? Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, we’ll cover everything under the Sun (and Moon)!

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