Moon Phases

What is a last quarter moon?

One half the moon's face in sunlight, lighted portion facing downward, left side marked N for north.
View at EarthSky Community Photos. | Dr Ski in Valencia, Philippines, caught the last quarter moon shortly after it rose around midnight on the morning of September 22, 2019. This moon phase is perfect for helping you envision the location of the sun … below your feet. Thanks, Dr Ski!

The last quarter moon falls one week after the full moon. From Earth, we see the moon half-lit. Actually, we’re seeing one quarter of the moon – hence the name – because the rest of the lit part is on the far side where we can’t see it. A last quarter moon looks like half a pie. It is also called third quarter moon.

See the moon phase for every day in 2021 on EarthSky’s famous lunar calendar. Order yours before they’re gone!

A last quarter moon appears half-lit by sunshine and half-immersed in its own shadow. It rises in the middle of the night, appears at its highest in the sky around dawn, and sets around midday.

Why last quarter isn’t a “half moon”

A last quarter moon provides a great opportunity to think of yourself on a three-dimensional world in space. Watch for this moon just after moonrise, shortly after midnight. Then the lighted portion points downward, to the sun below your feet. Think of the last quarter moon as a mirror to the world you’re standing on. Think of yourself standing in the midst of Earth’s nightside, on the midnight portion of Earth.

Between full moon and last quarter, you’ll see the waning gibbous moon. After last quarter and before new moon, the waning crescent moon appears.

On a last quarter moon, the lunar terminator – the shadow line dividing day and night – shows you where it’s sunset on the moon, whereas on a first quarter moon, the terminator shows sunrise. As viewed from above or below the moon’s orbital plane, the terminators of Earth and the moon align at both first and last quarter.

Craters and other features, including a short straight white line on a dark flat mare floor.
View at EarthSky Community Photos. | September 22, 2019, photo by Dr Ski. He wrote: “The moon’s southern limb at last quarter. The Straight Wall is either black or white depending on the angle of the sun’s rays. At lunar sunset (now), it’s white. Around full moon, Tycho is one of the easiest craters to find due to the impact rays emanating from it. It’s like the hub of a spoked wheel! At last quarter, Tycho becomes unremarkable. Clavius, on the other hand, becomes remarkable at high magnification.”
Labeled craters and mountain ranges at the edge between dark and light.
View at EarthSky Community Photos. | September 22, 2019, photo by Dr Ski. He wrote: “The Sea of Rains at last quarter. The lunar Alps and Apennines are bisected by the moon’s meridian. You can get an idea of the height of these mountains by how far they extend into the dark side of the terminator. At an elevation of over 5,000 meters [16,000 feet], the Apennines are twice as tall as the Alps.”

Also, a last quarter moon can be used as a guidepost to Earth’s direction of motion in orbit around the sun.

In other words, when you look toward a last quarter moon high in the predawn sky, for example, you’re gazing out approximately along the path of Earth’s orbit, in a forward direction. The moon is moving in orbit around the sun with the Earth and never holds still. But, if we could somehow anchor the moon in space … tie it down, keep it still … Earth’s orbital speed of 18 miles per second (29 km/sec) would carry us across the space between us and the moon in only a few hours.

Diagram with arrow showing Earth moving leftwards toward moon.
Earth is traveling in its orbit directly toward the last quarter moon. The arrow showing the path of the earth in its orbit curves, but only very slightly because the sun is 400 times as far away as the moon. The last quarter moon is high overhead at dawn. Diagram by Guy Ottewell.

Want to read more about the last quarter moon as a guidepost for Earth’s motion? Astronomer Guy Ottewell talks about it here.

A great thing about using the moon as a guidepost to Earth’s motion is that you can do it anywhere … as, for example, in the photo below, from large cities.

Daytime sky. High small moon, left half visible, above conical-top water tower and tall tan brick chimney.
Ben Orlove wrote from New York City: “I was sitting in the roof garden of my building, and there was the moon, right in front of me. You were right, this is a perfect time to visualize … the Earth’s motion.”

As the moon orbits Earth, it changes phase in an orderly way. Follow the links below to understand the phases of the moon.

New moon
Waxing crescent moon
First quarter moon
Waxing gibbous moon
Full moon
Waning gibbous moon
Last quarter moon
Waning crescent moon

Bottom line: A week after the full moon, the last quarter moon phase appears between the waning gibbous and waning crescent phases.

Read more: 4 keys to understanding moon phases

Posted 
January 1, 2021
 in 
Moon Phases

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Deborah Byrd

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