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Watch for the young moon

The young moon is back in the west after sunset. Very lovely, and passing near some bright stars in the days ahead.

Young moon over the West Philippine Sea by Jv Noriega.

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A waxing crescent moon – sometimes called a young moon – is always seen in the west after sunset.

In general, a waxing moon is seen one day to several days after new moon. It’s always seen in the evening, and it’s always seen in the west. On these days, the moon rises one hour to several hours behind the sun and follows the sun across the sky during the day. When the sun sets, and the sky darkens, the moon pops into view in the western sky.

The moon is now waxing toward first quarter. Next first quarter moon will be May 3, 2017 at 02:47 UTC. Next full moon is May 10 at 21:42 UTC; translate UTC to your time zone.

April 28, 2017 waxing crescent moon over Normandy, France from our friend Mohamed Laaïfat Photographies.

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.

But a waxing crescent moon is far enough away from that Earth-sun line to be visible near the sun’s glare – that is, in the west after sunset.

Gowrishankar Lakshminarayanan created this composite image on April 28, 2017. He wrote: “This is a 13-image composite of the waxing crescent moon setting behind the Empire State Building in NYC. The moon sequence is placed 2 minutes, 30 seconds apart.”

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.

Very young moon with a pale earthshine glow, April 27, 2017, by Edmund Buras.

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?

Watch for the moon near bright stars in the first week of May! On May 1, the moon will be sweeping near the bright stars Castor and Pollux in the constellation Gemini. Read more.

As the moon waxes, it’ll grow larger in phase. First quarter moon is May 3. That night – and on May 4 and 5, when the moon is in a waxing gibbous phase – it’ll be moving past the constellation Leo the Lion. Look for Leo’s brightest star Regulus. Read more.

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