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. The moon reaches its exact half-illuminated phase, as viewed from Earth, on December 29, 2018, at 09:34 UTC; translate UTC to your time.
On a last quarter moon, the lunar terminator – the shadow line dividing day and night – shows you where it’s sunset on the moon.
A last quarter moon provides a great opportunity to think of yourself on a three-dimensional world in space. For example, it’s fun to see 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 middle of Earth’s nightside, on the midnight portion of Earth.
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 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 talked about it recently, too.
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: The next last quarter moon falls on December 29, 2018, at 09:34 UTC; translate UTC to your time.
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.