Dear Eli,

I love to go for a walk with my dog roommate. Sometimes we see the moon in the sky—even though it’s daytime.

I asked my friend Michael Allen why that happens. He’s an astronomer at Washington State University.

He told me that we have to ask “what is a day” to answer your question.

“The sun lights up the earth,” Allen said. “Since the earth is a round ball, only half of the ball can ever be lit up at any one time.”

The side of the earth that the sun shines on experiences day. The side that faces away from … » More …