What is a last quarter moon?
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 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.
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.
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.
As the moon orbits Earth, it changes phase in an orderly way. Follow the links below to understand the phases of the moon.
Bottom line: A week after the full moon, the last quarter moon phase appears between the waning gibbous and waning crescent phases.