Moon is the only satellite of the earth. It revolves around the earth and also around itself. It takes 29.5 days to complete revolution around the earth.
The moon we see at night is due to illumination by the sun. Since, the moon revolves around the earth, it looks in different shapes on different days. This different shape is due to different amounts of the illuminated part of the moon appearing to us. At any point of time, we can see one side of the moon.
Phases of the moon
In total there are 8 phases of the moon like the
1. New moon
2. Waxing Crescent
3. First Quarter
4. Waxing Gibbous
5. Full Moon
6. Waning Gibbous
7. Third Quarter
8. Waning Crescent
Certain phases repeat themselves like two crescents, gibbous and half phases each cycle with different sides being illuminated.
New moon: Here the moon appears dark or invisible to us. But in fact the other side of the moon is illuminate and not seen by us. This phase occurs because, the moon lies in between the earth and sun. The one side of moon facing sun has light while its back side is seen as dark to those on the earth. So dark side of the moon faces the earth and light side faces the sun. Some call it as no moon day as moon is invisible in the sky.
Waxing Crescent: This is the next stage and only a small part of the Moon appears. This is lighted by the sun and gradually grows larger on successive days from right to left.
The size of the moon appear to be smallest at this phase.
First Quarter: This is exact half of the moon from the right side being lighted, while the remaining half on the left is dark. With each passing day, the lighted portion grows.
Waxing Gibbous: From right to left, the extent of illumination is increased.
More than half portion of the Moon is illuminated and as days pass, more and more becomes lighted.
Full Moon: This phase occurs when the earth is between sun and moon. Here the illuminated side of the Moon faces the Earth.
The entire moon in complete circular form can be seen on this day. This day is called full moon day.
Waning Gibbous: Form right to left the moon looks bright. As the days go on the darkness grows further. More than half of the Moon appears lighted. This become with less and less on following days.
Third Quarter: This is opposite of first quarter and only the left half of the Moon appears lighted. This lighted portion grows smaller and smaller on successive days.
Waning Crescent: From right to left of the moon, only a small part is illuminated. The remaining area looks dark. The lighted Moon is visible but grows smaller and smaller on each successive day.