news sciences: What is a 1st quarter moon?

A half illuminated moon. — View at EarthSky Community Photos. | Greg Diesel-Walck at Ormond-by-the-Sea, Florida captured this first quarter moon on January 9, 2022. Thank you, Greg!

Here are the characteristics of a first quarter moon:

– It’s the moon phase halfway between new moon and full moon.

– It’s a waxing moon.

– As viewed from anywhere on Earth, a first quarter moon appears at its highest in the sky at sunset. It sets around the middle of the night.

– It’s called a quarter moon, but, from Earth, it looks half-illuminated, like half a pie. Or you might say that – at first quarter moon – we’re seeing half the moon’s day side or a quarter of the whole moon.

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The moon, with just over half lighted, against a deep blue sky. — View at EarthSky Community Photos. | Steven Arthur Sweet of Toronto, Ontario, Canada, captured the moon just past 1st quarter on October 23, 2020. See more of Steven’s work at Lunar 101-Moon Book.

Half-lit Earth from north, and half-lit moon with divisions between dark and light sides lined up. — As seen from the north side of the moon’s orbital plane, the terminators (lines between sunny and shadowed sides) of the Earth and moon align at first quarter moon, and only the near half of the moon’s day side is visible from Earth. Not to scale. Click here to see animation.

There’s another reason we call this moon a quarter and not a half. It’s because it’s one quarter of the way around in its orbit of Earth, as measured from one new moon to the next.

And what about the term half moon? That’s a beloved term, but not an official one.

Do you have access to a small telescope? Here are some telescopic sights to see on a first quarter moon:

Half of the moon with tiny labeled X and V shapes along straight edge. — Lunar X and Lunar V appear when the moon is near its 1st quarter phase. They aren’t really Xs and Vs on the moon. They’re just high areas, catching sunlight, creating an example of pareidolia on the moon. Aqilla Othman in Port Dickson, Negeri Sembilan, Malaysia, caught them both in May 2017. Notice that he caught Lunar X and Lunar V.

Closeup of boundary between light and dark areas of the moon with Lunar V and X labeled. — Here’s a closer look at Lunar X and Lunar V. Photo by Izaty Liyana in Port Dickson, Negeri Sembilan, Malaysia. What is Lunar X?

Telescopic closeup of band of mountains on moon with a few large craters. — Tom Wildoner wrote: “One of my favorite areas to photograph on the moon near the 1st quarter! I captured this view of the sun lighting up the mountain range called Montes Apenninus. The moon was casting a nice shadow on the back side 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).”

As the moon orbits Earth, it changes phase in an orderly way. Follow the links below to understand the phases of the moon.

New moon
Waxing crescent moon
First quarter moon
Waxing gibbous moon
Full moon
Waning gibbous moon
Last quarter moon
Waning crescent moon

Bottom line: The first quarter moon occurs halfway between new moon and full moon. One quarter of the moon is visible from Earth, meaning that you see half the daylit side of the moon, looking like half a pie.

Check out EarthSky’s guide to the bright planets.

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