The last first quarter of 2017 will come on December 26 at 09:20 UTC; translate UTC to your time zone. No matter where you are on Earth, watch for it on the evening of December 25 appearing in your evening sky half-illuminated (or nearly so), looking like half a pie. Or watch on the evening of December 26 or 27, when the terminator line (line between light and dark on the moon) will appear ever-so-slightly convex.
A first quarter moon shows half of its lighted hemisphere – half of its day side – to Earth. At quarter moon, the part of the moon we see is half-illuminated by sunlight and half-immersed in the moon’s own shadow.
We call this moon a quarter and not a half because it is one quarter of the way around in its orbit of Earth, as measured from one new moon to the next. Also, although a first quarter moon appears half-lit to us, the illuminated portion we see of a first quarter moon truly is just a quarter. We’re now seeing half the moon’s day side, that is. Another lighted quarter of the moon shines just as brightly in the direction opposite Earth!
And what about the term half moon? That’s a beloved term, but not an official one.
A first quarter moon rises at noon and is highest in the sky at sunset. It sets around midnight. First quarter moon comes a week after new moon. Now, as seen from above, the moon in its orbit around Earth is at right angles to a line between the Earth and sun.
As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.
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
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.