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.
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.
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.
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.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.
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