Enjoying EarthSky? Subscribe.

270,436 subscribers and counting ...

Last quarter moon is December 10

Watch for the last quarter moon Saturday night after midnight or Sunday morning. The moon will be showing us half of its lighted half, or day side.

The moon was almost exactly at last quarter when Jenney Disimon in Sabah, North Borneo captured this photo. The terminator line, or line between light and dark on the moon, appears straight.

Last quarter moon will come on December 10, 2017 at 07:51 UTC; translate to your time zone. A last quarter moon always 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. 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.

After the last quarter phase, the moon begins edging noticeably closer to the sun again on the sky’s dome. Next new moon will come on December 18, 2017 at 06:30 UTC; translate to your time zone.

A great thing about this observation is that it can be done from anywhere. 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 these links to understand the various phases of the moon.

Four keys to understanding moon phases

Where’s the moon? Waxing crescent
Where’s the moon? First quarter
Where’s the moon? Waxing gibbous
What’s special about a full moon?
Where’s the moon? Waning gibbous
Where’s the moon? Last quarter
Where’s the moon? Waning crescent
Where’s the moon? New phase

Moon in 2017: Phases, cycles, eclipses, supermoons and more

Deborah Byrd

MORE ARTICLES