Quarter Moon or a Half Moon? Clearing Up Confusion.

What Do We Mean by Quarter Moon?

Nov 19, 2018
Last Quarter Moon
Air-and-space.com

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Only in astronomy does a Half Moon and a Quarter Moon mean the same thing! And the Quarter Moon is actually more interesting to view than the Full Moon. Bob explains the Moon phases!

We’ve all looked up at the night sky and seen half of the Moon’s disk illuminated. If you had two half Moons and fit them together, you’d get a full Moon. But when you’re looking at a Half Moon, the official name is the “Quarter Moon.” It’s written that way in almanacs and newspapers. Sound confusing?

Think of the Moon going around the Earth as a runner going around baseball plates (first base, etc.). Earth is the pitcher. When the runner hits the ball, it goes to first base (one quarter of the way around) and then to second base (half way around), then to third base (third quarter around). At first base or third base, you get the quarter Moons.

With the Quarter Moon—what visually appears as the Half Moon—we can see exactly 50% of the Moon’s surface illuminated from Earth. It’s lit on right half of the Moon during the First Quarter and on the left side during the Third Quarter because the Moon is on the other side of Earth. The First Quarter happens around day 7 of a Moon’s cycle (one week in) and the Third Quarter usually happens around day 22 (three weeks in). See the Almanac’s Moon Phase Calendar.

Why the Quarter Moons Are Special

First of all, the Quarter Moon is much more interesting to me than the Full Moon.

With the Full Moon, most beginning astronomers can’t see much beyond the blinding orb. The Sun then shines straight down like a flash camera to erase all shadows and highlights.

First Quarter Moon

Everyone’s used to seeing the First Quarter Moon. This is the Moon that’s at its highest at sunset just around dinner time.

The shadowing is perfect. You see all the mountains and craters. It’s fascinating to look at. The First Quarter Moon explodes with breathtaking detail for anyone with binoculars, spotting scope, or even the smallest telescope.

Third Quarter Moon

With Third Quarter Moon, the left half is lit up. It doesn’t even rise until midnight and it’s not at its highest until dawn. Who’s up then? Nobody! Most of us don’t want to haul our telescopes out at 5 A.M. or 6 A.M. to look at the Half Moon when you could look at the “other Half Moon” (the First Quarter Moon) at six in the evening when it’s convenient. Everyone’s used to the First Quarter Moon.

If you do look at the Third Quarter Moon, it might look alien you’re used to the First Quarter Moon.  

phasesmoon.jpg

A Half Moon or Quarter 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. It’s lies directly ahead of us as Earth is zooming through the universe. 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. 

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.

moon_mountains_full_width.jpg

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!)

Yes, 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? 

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s blog on 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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Correction to First Quarter Moon

First Quarter Moon as Right Half Moon.

Just need some clarification

My Son is 5 yo in Kinder. He was taught about moon phases. He called Waning Crescent Moon as Right Crescent Moon and First Quarter Moon as Left Half Moon..how he should call Waxing Gibbous in terms of shape as a preschooler? I have doubt teaching my Son according to the standard astronomical moon phases because he might get confused and got wrong in the exam.

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