A waxing crescent moon – sometimes called a young moon – is in the west after sunset in the week following new moon.
A waxing crescent moon is 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.
Also, this moon – waxing inexorably toward full moon and then to the waning moon phases and finally back to a new moon – will ultimately cover the sun in the much-anticipated total solar eclipse of August 21, 2017.
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.
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.
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?
As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.
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 created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.