What's Your Favorite Moon Phase?

Shedding Light on the Most Neglected Phase

May 10, 2021
Half Moon: Third-Quarter Moon Phase
NASA

If you’ve spent many years observing the Moon, you’ll know all its phases. Perhaps you even have a favorite Moon phase—whether it’s the slim crescent or a fully-illuminated disk. But let’s focus on an upcoming Moon phase that’s least familiar yet most full of wonders. Can you guess what it is? 

The last week of October offered astronomy headlines. We had a Full Moon, and a blue one at that, since it was the second full moon that month. And it happened on Halloween. There was also the switch from daylight time and the sudden onset of full darkness long before the clock strikes six. An eventful time.

Whenever an artist or cartoonist portrays a Moon, it’s reliably a Full Moon or a crescent. They’re the most recognizable lunar shapes, known by everyone. You’ll never see a Half Moon in a painting.

This paucity is reflected in conversations and literature. Ask friends to name any lunar phase that comes to mind and no one will say “half Moon.” Rarer still is that phase’s proper name, which is either “first quarter” when the Moon’s right side is illuminated, or “last quarter,” when the left side is lit. So things start off weird from the get-go, since half and quarter mean the same thing in Moonspeak. In no other area of science is this true.

And the oddities still don’t end, since not only can you correctly call the last quarter a half Moon, but you can also call it a third quarter Moon. Three names for the same phase.

And that’s what’s coming up this Sunday, November 8, 2020: A third quarter Moon. See your Moon Phase Calendar.

The Third Quarter Moon

I particular enjoy the last quarter Moon aka third quarter Moon. It’s not just the name that’s elusive. The last quarter doesn’t even rise until midnight, and doesn’t get high until dawn when few are awake. It’s for the few among us and a quiet time.

The third quarter Moon remains high for several hours after dawn—into the daytime. Ever noticed the Moon against the blue sky? No problem: The half Moon floats against the darkest-blue part of the morning sky, making it stand out vividly. Moreover, the blue sky behind it is maximally polarized, so wearing sunglasses makes it pop out even more dramatically.

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Image: The third quarter Moon.

Viewing the Third Quarter Moon

So happens, this particular last quarter Moon is almost as high and prominent as possible.

Go out soon after daybreak to see it overhead. Or gaze up any time Sunday morning—it’ll be prominent in the west until noon.

To take advantage of its specialness, use sunglasses to make it “pop,” or use them with binoculars to see craters at their best.

Or, if you have a small telescope, check it out up close. It will look odd because it’s illuminated by sunshine striking it the “wrong way.” That’s because we’re all used to seeing the other half Moon, the first quarter, the one that’s high and prominent each month at dinnertime.

The first quarter is so common and convenient, it’s deeply familiar. If you then see the last quarter Moon, its lighting looks weird by contrast. Almost spooky. Yet that lighting is ideal for showing off such unique features as the gorgeous crater Copernicus.

But its coolest facet is its position in the sky. This phase is the only time the Moon floats directly in front of us. It hovers in the exact direction our planet is moving through space. When you look at it this Sunday morning, you see the precise spot that we ourselves will occupy 3½ hours later.

So if you’re raking leaves at 9 AM and a neighbor or friend walks by and asks, “Where will you be for lunch?”—just point at the Moon.

Learn more about the quarter Moon and why it’s not called a half Moon.

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. 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, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe

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