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Where’s the moon? 1st quarter

The moon reaches its 1st quarter phase on December 26 at 09:20 UTC.

Last month’s 1st quarter moon from Wisconsin – November 26, 2017 – from Suzanne Murphy. On a 1st quarter moon, the terminator line – or line between light and dark on the moon – appears straight.

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.

Tom Wildoner caught this photo of the moon on November 26, 2017 and wrote: “One of my favorite areas to photograph on the moon near the first quarter! I captured this view of the sun lighting up the mountain range called Montes Apenninus. The moon was near the first quarter phase on the evening of November 26, 2017 casting a nice shadow on the backside of the mountains. This mountain range is about 370 miles (600 km) long with some of the peaks rising as high as 3.1 miles (5 km).”

Here’s something else to look for on a 1st quarter moon. Aqilla Othman in Port Dickson, Negeri Sembilan, Malaysia caught this photo on May 3, 2017. Notice that he caught Lunar X and Lunar V. These are similar features on the moon that fleetingly take an X- or V-shape when the moon appears in a 1st quarter phase from Earth.

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!

Here’s a closer look at Lunar X and Lunar V. Photo taken May 3, 2017 by Izaty Liyana in Port Dickson, Negeri Sembilan, Malaysia. What is Lunar X?

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.

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

Deborah Byrd

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