Why does moon change shape?
Half of the moon is always lit by sunlight because it completes one revolution and rotation around its axis in 29.5 days and the other half is in darkness. We are able to see only that part of moon which is receiving sunlight and during its revolution the vision of sunlit area varies. Hence we observe changes in the shape of moon.
These changes are known as phases of moon. When moon comes between the earth and the sun the part of moon receiving sunlight cannot be seen by us. Therefore we see no moon on that night, and the rising and setting of moon is at same time as the sun. This is called as new moon.
When moon completes its half orbit around the sun it appears as half circle because now we are able to see only half sunlit area of moon. This is called first quarter and moon rises in the noon and sets in the midnight.
When moon reaches where the earth is between moon and sun we are to see the whole area of moon which is receiving light from the sun. This is called full moon. It rises when sun sets and sets with the rising of sun.
After one week of full moon it again appears half circle we see half of its sunlit area. Moon has only to cover its last quarter of the orbit around the earth. This is called last quarter in this moon rises in midnight and sets during the noon.
Lunar eclipse not always takes place when there is a full moon because it is higher and lower than the earthâ€™s orbit. So the orbit is not in the line and earth shadow doesnâ€™t fall on the moon. This revolution of the moon around the earth appears to us as the moon is changing its shape.