It takes almost 27 days for the Moon to complete one trip around the Earth. This is known as the sidereal month. But it takes almost 29 days to change from a new moon to the next new moon. This is known as the synodic month. There is a difference of roughly two days. Wonder why?
We already know that the Moon’s phases change according to the sunlight that falls on it and new moon occurs when the Moon is placed between the Earth and the Sun.
While the Moon moves around it, the Earth moves around the Sun. The Earth moves farther in space during the time the Moon completes one orbit. So, for the Moon to come between the Earth and the Sun again, it has to move a little farther. It takes two days for the Moon to travel this extra distance.
Picture Credit : Google