The amount of sunlight that illuminates the Moon fluctuates as the Moon travels through its orbit. The Moon experiences several phases each month, during which more or less of its surface area is visible from the Earth. As the Moon waxes or wanes, it appears to change shape from one night to the next.
The Moon completes a single orbit of the Earth once every 29.5 days. A new moon occurs when the side of the Moon visible from the Earth is receiving no direct sunlight. As the Moon waxes, more of its surface becomes visible each night until the full moon, when the side of the moon facing Earth is completely illuminated. This process is known as the lunar phase cycle and is commonly measured in quarterly increments of approximately seven days.
Because the Moon is a sphere, it experiences the same type of day and night cycle as the Earth. The moon is tidally locked with the earth, which means its orbital and rotational periods have the same duration such that the same half of the lunar surface is always facing the earth. Twice each year, the Moon travels through the Earth's shadow, resulting in a lunar eclipse.