The moon does not appear similar on different days rather its shape changes every day. The different shapes of the moon are called the phases of moon.New Moon: On the new moon day, no moon is visible in the sky.
Waxing Crescent: Within a few days of the new moon day; the moon appears like a crescent. This is called the waxing crescent.1st Quarter: Within about a week from the new moon day; the moon appears as a semicircle. This is called the 1st quarter moon.
Waxing Gibbous: Within about 10 days from the new moon day; the moon appears as a circle with some portion cut off. This is called the waxing gibbous moon.
Full Moon: Within about 15 days from the new moon day; the moon appears as a complete circle. This is called the full moon.
Waning Phase: The waxing phase of the moon is followed by the waning phase. During this phase; the full moon reduces in size and goes through the waning gibbous, 2nd quarter and waning crescent phases. Within about 29 days from the new moon day; the next lunar month begins with the next new moon.
Reason for Phases of Moon: The periods of rotation and revolution are same in case of the moon. This means that the moon takes the same time in completing one rotation and one revolution. Due to this, only one surface of the moon is visible from the earth.