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Young moon returns to evening sky

The moon is back in the evening sky, and will soon sweep past the planet Saturn.

Young moon on the evening of October 20, 2017 via Hector Barrios in Hermosillo Mexico.

In the days following new moon, a young moon – a waxing crescent – always appears in the west shortly after sunset. The moon has returned to the evening sky now, and you’re likely to see this little moon this weekend, or in the coming week.

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.

The moon will be passing Saturn in the days ahead, as shown on our chart below.

If you look shortly after sunset on October 21, you might see the moon low in the west, above the sunset point. In the days ahead, the moon will be waxing, appearing higher in the sky each evening after sunset, passing the bright red star Antares in the constellation Scorpius and the golden planet Saturn.

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.

2017 started out with a beautiful waxing crescent moon. This day-lapse composite image combines the earthshine moon from New Year’s Day with the crescent moon from the following day. A wide-field image with Venus at sunset and more information on how to make day-lapse images is available from Robert Pettengill of Austin, Texas.

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.

The pale glow on the darkened portion (night side) of a crescent moon is called earthshine. Is 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?

Chirag Upreti caught this image of the waxing crescent moon on June 25, 2017. He wrote: “The waxing crescent moon illuminated at ~6% descends behind the Eastern Sierra mountains as the radio antennae in the Owens Valley Radio Observatory actively scan the skies to reward human curiosity.”

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.

Photos of the August 21, 2017 eclipse here

How ISS astronauts saw the August 21 eclipse

Deborah Byrd

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