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.
For the rest of the story: http://web.ncsu.edu/abstract/science/wms-day-moon/