The Earth's moon shines so bright in the sky that it is something you can sometimes see during the day. But the Moon doesn't make its own light. The light you see coming from the Moon is reflected sunlight. The Moon is a sphere. When light shines on a sphere, the sphere is half lit and half dark. It doesn't matter where you position the sphere. It is always half lit and half dark.
The same is true for the Moon. It is always and half dark. The half that is lit is toward the Sun. The half that is dark is the side away from the Sun. It takes about four weeks for the Moon to orbit Earth. Look at the diagram below to think about where you are on Earth when you see the Moon during day or night.
The Moon doesn't always appear to be the same shape. That is because half of the Moon is always dark. The other half is lit by the Sun. As the Moon orbits the Earth, observers on Earth see different amounts of the lit half. The different shapes of the Moon are called phases. The phases change in a regular pattern as the Moon orbits Earth. The Moon completes its orbit and goes through its cycle phases in just over four weeks.