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Astro Bob: The Loneliness of the Last Quarter Moon

We've all seen a crescent, half, gibbous and full moon, but when was the last time you noticed the moon at last quarter?

Last quarter moon rise july 30 am.jpg
The moon, one night before last quarter phase, will clear the trees around 12:30 a.m. tomorrow morning, July 30th. Will you be awake to see it? (Stellarium)

Last quarter is a funny phase. It has the same shape as an evening half-moon but flipped. Instead of the right half lit by sunlight, the left half is. A first quarter moon has completed a quarter of its orbit around the Earth, while a last quarter moon has completed 3/4 of the trip. For that reason it's also known as the third quarter moon.

Moon phases July aug 2021 NASA JPL Caltech S.jpg
This diagram illustrates the primary and best-known lunar phases we'll see this month and through mid-August. (NASA / JPL-Caltech)

Most skywatchers are active during the evening hours, so we routinely catch sight of the crescent, half-moon, waxing gibbous (the egg-shaped moon between half and full) and full moon. If you're a night owl you'll also see the waning gibbous moon just after full. Schoolkids and early-rising workers are familiar with the morning crescent in the brightening dawn sky, especially from mid-fall to early spring when the sun rises late.

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July 31 last quarter moon.jpg
The last quarter moon shines high in the southern sky at sunrise. (Stellarium)

The last quarter moon inhabits the wee hours, rising around midnight standard time (1 a.m. daylight time). If my neighborhood is any gauge, nearly everyone's in bed by then. When I'm out with the telescope observing comets and other deep-sky wonders I'll often lose track of time. Not when the moon's at last quarter. At its rising my parental subconscious tells me it's late and time for bed. If I don't pack up soon, I'll pay for my pleasure.

That said, it's also true that the last quarter moon is visible at dawn just like the crescent, standing due south at sunrise. OK, so maybe I'm exaggerating at little at how difficult it is to see moon's last half. But I can vouch that nighttime sightings are uncommon unless you're a fox, an owl or an amateur astronomer.

Moon phases wiki Orion 8 CC BY SA 3.0 S V2.jpg
As the moon orbits the Earth, the changing angle it makes with the sun causes its phase to vary from new to crescent to half to full and back again. In the diagram the moon's location in its orbit (top half) is shown along with its corresponding phase at bottom. When the moon is new again at far right, the little green patch indicates that it has to orbit a little further to the left to re-align with the sun. That's because the Earth has also been moving along its orbit during this time. (Orion 8 / CC BY-SA 3.0 with additions by the author)

Before full phase the moon rises before sunset, so it's automatically visible as soon as the sky gets dark. At full, the moon lies directly opposite the sun and rises at sunset. After full, the moon rises after sunset. Depending on the angle the moon's path makes to the horizon the delay between successive moonrises can be as short as 20 minutes and as long as 90. Forty minutes to an hour is typical.

Two days after full, the moon rises about 1.5 hours after sunset. When it reaches third quarter on July 31st — seven days past full — it comes up around 12:30 a.m. local daylight time. The moon rises later because it's orbiting the Earth. Each night it moves about one outstretched fist to the left or east.

One night after full, the moon has traveled about a fist farther east. To see it rise again, the Earth has to rotate an additional 45 minutes. Two days after full, the Earth must rotate 90 minutes (2 x 45) to return the moon to view. By the time of last quarter, it has to spin for more than 4 1/2 hours for the moon to rise! No wonder that phase gets so little attention.

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Terminator last quarter first quarter panel final.jpg
At third quarter phase, the terminator marks the advancing line of lunar sunset. The moon's phase is thinning toward crescent. At first quarter, the sun is rising along the terminator, and its phase is increasing toward full. Notice how much more shadow detail you can see in craters near the terminator compared to beyond. (Bob King)

While first quarter and last quarter phases are both halves, one is waxing or increasing in phase towards full, while the other is waning toward crescent. The line of advancing sunrise on the waxing moon is called the terminator. Post-full moon, the terminator becomes the line of advancing sunset. It's essentially the boundary between day and night on the moon.

Craters and mountain peaks along the terminator cast dramatic shadows because the sun is either just rising (first quarter) or setting (last quarter) here. The contrast between shadow and low-angled sunlight highlights every tiny bump and irregularity, making the day-night boundary the best place to observe craters and other lunar features.

This is especially true at the quarter phases, when the terminator cuts a straight line across the moon's face, highlighting craters directly in our line of sight. There's no foreshortening. A round crater looks round, not squeezed into a narrow oval the way we see them at other phases.

16 day old moon Dec 1 2020 S foreshortening.jpg
Because the moon is a sphere craters appeared stretched or foreshortened especially those near its edge. (Bob King)

The moon is a sphere. When we face it squarely, everything is fully open to view. But when we look off towards its edge, features "crunch" together and narrow, hiding details. The gibbous or crescent moon terminator will still expose dramatically-lit craters (above), but they're all smooshed together. That's why first and last quarter phases are the best for viewing the moon in a telescope or pair of binoculars.

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Tonight, the moon — one day before last quarter — will rise right around midnight. I hope you'll get the opportunity to become better acquainted with this reclusive, nocturnal light.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob .

"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at nightsky55@gmail.com.
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