I have a friend whose young son gets incredibly excited when he can see the moon during the day. After several excited shouts of “day moon!” the youngster asks his mom why he can sometimes see the moon when the sun is up, and not just at night. Good question.
Objects in the sky appear to pass overhead on a daily basis because the earth is rotating, or spinning on its axis. For example, at the equator, the sun appears above the horizon for approximately 12 hours a day (though this varies according to the season).
This is true for the moon as well – but the moon is also traveling in an orbit around the Earth.
“And because the moon travels around the Earth, its 12 hours above the horizon aren’t always the same as the sun’s 12 hours,” says Stephen Reynolds, an astrophysicist at NC State who was kind enough to talk to me about this subject. So, sometimes the moon is above the horizon only at night, sometimes only during the day, and sometimes a little of both.
However, any time the moon is above the horizon, you should be able to see it (though it’s a little less obvious in the bright light of day). With one exception.
While we talk about the moon “shining,” it is actually reflecting the light of the sun. And the side of the moon that is lit up is the side that is facing the sun.
Depending on the position of the sun and moon, those of us here on Earth can see different amounts of that “lit” side of the moon. That’s why we see different phases of the moon, such as a full moon or crescent moon.
If the moon is above the horizon, the only time you won’t be able to see it is when it is a “new” moon – meaning that the lit surface of the moon is facing away from the Earth.
Note: Many thanks to Stephen Reynolds, Alumni Distinguished Undergraduate Professor of Physics at NC State, for taking the time to talk with me about “day moons.” Any errors in the above post are mine alone.