Why Does The Size Of The Moon Appear To Change?

We know that the moon looks like a bright disc on full moon day, but it goes on fading and disappears on the new moon night. Again it starts increasing in size and becomes a full bright disc on full moon day. These changes are called ‘the phases of the moon’.

Do you know why it happens?

In reality, the size of the moon neither increases nor decreases, it simply appears to be changing, as it only changes its position with respect to the sun. We know that moon revolves round the earth and it is the only satellite of the earth. Moon completes one revolution round the earth in 29 1/2 days. The moon is illuminated by the sunlight.

We can see only one side of the moon from the earth. When the moon comes in between the sun and the earth then the bright side of moon is not visible to us from the earth and only the dark side of the moon faces the earth. In this case we cannot see the moon. It is consider as the new moon day. As the moon’s position gets changes because of its motion, some lighted part of its surface becomes visible to us. About half of the illuminated lunar disc becomes visible to us after one week of the new moon. Slowly the shining part of the moon’s surface goes on increasing therefore on the full moon day the fully illuminated whole disc can be seen from the earth.

On full moon day the earth is in between the moon and the sun. The bright part of the lunar surface goes on decreasing during the next fifteen days and it completely disappears on the fifteenth day. In this way the cycle continues and we see the different phases of moon.

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