What is the ghostly image at the top of this post? It’s a new moon. Its lighted half is facing entirely away from Earth.
The image is as if you flew in a spaceship to a place where you could see the night side of the moon. When the moon is new, its night face is facing us on Earth … and we can’t see the moon at this time.
On the day of new moon, the moon rises when the sun rises. It sets when the sun sets. It crosses the sky with the sun during the day. That’s why we can’t see the new moon in the sky. It’s too close to the sun’s glare to be visible. Plus its lighted hemisphere is facing away from us.
Each new lunar cycle is measured beginning at each new moon. Astronomers call one lunar cycle a lunation.
We can’t see the new moon from Earth, except during the stirring moments of a solar eclipse. Then the moon passes in front of the sun, and the night side of the moon can be seen in silhouette against the disk of the sun. Meanwhile, if you could travel in a spaceship to the opposite side of the moon, you’d see it shining brightly in daylight.
Once each month, the moon comes all the way around in its orbit so that it is more or less between us and the sun. If the moon always passed directly between the sun and Earth at new moon, a solar eclipse would take place every month.
But that doesn’t happen every month. Instead, in most months, the moon passes above or below the sun as seen from our earthly vantage point.
Then a day or two later, the moon reappears, in the west after sunset. Then it’s a slim waxing crescent visible only briefly after sunset – what some call a young moon.
As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.