Different shapes of the bright, visible part of the moon as seen from the earth during a whole month are called phases of moon. If we observe the moon continuously every night for a month, we’ll find that there is one day in the month when the moon cannot be seen in the night.

•The day on which the moon is not visible at all is called the new moon day.

•On next day, only a small curve like portion of the moon appears in the night sky. This is known as crescent moon.

•Thereafter, every night, the size of the bright, visible part of the moon appears to become bigger and bigger, giving us many shapes including the half moon and more than moon.

•After 15 days, we can see the whole bright disc of the moon in the night sky. So there is also one day in the month when the moon is visible as a perfectly round ball of light in the sky. The day on which the whole bright disc of moon is visible to us on earth is called the ‘full moon day’.

•Thereafter, every night the size of the moon goes on becoming smaller and smaller and after 15 days the moon is not visible again. This moon we again change in full moon after 15 days, and the process is repeated endlessly.

Causes of phases of moon

As moon revolve around the earth once every month and moves around the sun along with earth, different amounts of its sun-lit surface are turned towards the earth leading to a change in the appearance of the moon and formation of faces of the moon. Depending upon the relative positions of the sun, the moon, and the earth, we see different amounts of the sun-lit surface of the moon from the earth.