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Where’s the moon? Waxing crescent

The young moon is back in the evening sky, waxing toward a lunar eclipse on January 31.

Young moon of January 18, 2018 from Aimilianos Gkekas at Meteora near Kalambaka, Greece.

The moon has returned to the evening sky now. You’re likely to see it in the coming evenings. Astronomers call it a young moon – a waxing crescent – when the moon appears in the west shortly after sunset. The moon is now waxing toward the second full moon of this month, what in modern folklore has come to be called a Blue Moon. This full moon will also present the second-closest full moon of 2018, a supermoon. And it’ll undergo an eclipse visible from North America. A super Blue Moon eclipse!

The super Blue Moon eclipse happens before sunrise on January 31 for North America and Hawaii. It happens after sunset on January 31 for the Middle East, Asia, Indonesia, Australia and New Zealand.

Read more: Super Blue Moon eclipse coming up January 31

Trish Collins wrote on January 18: “A clear night allowed a wonderful view of the young moon setting over Fire Island, New York.”

Doug Waters caught the January 18, 2018 moon setting over the Beaufort (North Carolina) Airport runway and in the distance, the Port of Morehead City.

Where will you see the moon this week? It’s in the west shortly after sunset.

Some people think a moon visible in the west after sunset is a rising moon. But it’s not; it’s a setting moon. All objects in our sky rise in the east and set in the west, due to Earth’s spin under the sky. When you see a waxing crescent, you know the Earth, moon and sun are located nearly on a line in space. If they were more precisely on a line, as they are at new moon, we wouldn’t see the moon. The moon would travel across the sky during the day, lost in the sun’s glare.

Helio C. Vital – in Rio de Janeiro, Brazil – caught the January 18, 2018 young moon, too. We all see the same moon phase on the same day (more or less), but, in contrast to the Northern Hemisphere, the Southern Hemisphere’s lighted crescent moon has a different orientation to the western horizon after sunset.

This week, for all of us around the globe, the waxing crescent moon is just off the Earth-sun line and so appears in the west after sunset.

Note also that a crescent moon has nothing to do with Earth’s shadow on the moon. The only time Earth’s shadow can fall on the moon is at full moon, during a lunar eclipse. There is a shadow on a crescent moon, but it’s the moon’s own shadow. Night on the moon happens on the part of the moon submerged in the moon’s own shadow. Likewise, night on Earth happens on the part of Earth submerged in Earth’s own shadow.

Judy Lundquist in Lawrenceburg, Kentucky caught the January 18, 2018 young moon with earthshine. She wrote: “After days of overcast and snow, we finally got a clear sun and moonset!”

Because the waxing crescent moon is nearly on a line with the Earth and sun, its illuminated hemisphere – or day side – is facing mostly away from us. We see only a slender fraction of the day side: a crescent moon. Each evening, because the moon is moving eastward in orbit around Earth, the moon appears farther from the sunset glare. It is moving farther from the Earth-sun line in space. Each evening, as the moon’s orbital motion carries it away from the Earth-sun line, we see more of the moon’s day side. Thus the crescent in the west after sunset appears to wax, or grow fatter each evening.

Joe Kingore in Joplin, Missouri caught the January 18, 2018 young moon with earthshine, too.

Earthshine is the pale glow on the darkened portion (night side) of a crescent moon. It’s caused by light reflected from Earth’s day side onto the moon. After all, when you see a crescent moon in Earth’s sky, any moon people looking back at our world would see a nearly full Earth. Read more: What is earthshine?

Richard Hasbrouck captured the January 18, 2018 young moon from Truchas, New Mexico. Our thanks to all who submitted photos!

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

Check out EarthSky’s guide to the bright planets.

Deborah Byrd

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