Why do we se only one side of moon?

moon_side Moon is the natural satellite of the earth which orbits around the earth in about 29 days.

We see only one side of moon this is because of gravitational tides. Earth and moon exert tidal forces on one another. Earth and moon are very close to each other so the earth tidal effects had used the rotational energy of moon in the beginning causing the rotation of moon to get slower and reaches to the lowest energy where moon spinning exactly once in its orbit and this facing only one side towards earth. In simple words we can say that the earth’s gravity has affected the moon’s rotation till it was gravitationally locked in synchronous revolution pattern. Millions of years ago moon’s rotation was faster than the present times.

Moon takes29.5 days for one rotation around its axis and complete revolution around the earth because of this we are able to see only one face of the moon from every part of the earth. This can be easily understood by a simple experiment. Take a ball and mark its half with a pencil. Now assume your hand to be earth and ball as moon. Facing the same side of Ball towards hand form a circle around the hand. You will notice that it has completed its circle around earth as well as rotation around its own axis and in the whole procedure only one side faced the hand.

Same happens in the case of moon and earth. So we can say that we are able to see only one face of moon and not the other, but the other side is not dark because it also receives sunlight during rotation around earth. If the moon doesn’t rotate at all then we will be able to see the entire surface of moon at different times.

We see phases of moon because sometimes we are able to see the part of moon which is receiving sunlight and call it as full moon but sometimes only a small part of moon is lit by sun out of the whole side facing towards the earth. These are known as half moons or gibbous. But it does not matter that whether the moon is new moon or gibbous or half moon only one side of the moon is always seen from the earth.

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    Earth has gravity and tries to pull the moon. Moon in its turn has gravity and tries to pull the Earth. Due to the gravity of both Earth & Moon the present distance between Earth & Moon is maintained. Earth rotates on its Axis and due to this rotation the lines of attraction between Earth & Moon is broken. This rotation of Earth causes Moon to revolve around the Earth. (The principle is similar to moving 1 magnet by moving another magnet without physical contact and this is the principle which is working between Earth & Moon). The present distance between Earth & Moon causes Moon’s perod of revolution to be about 29 days. If the distance had been less, the period for 1 revolution would have been less and vice versa. Moon only revolves around the Earth and it is wrong to say that the time for 1 rotation and 1 revolution is exactly the same which makes only 1 face visible from the Earth. If Moon had taken about 29 days for 1 rotation, than the other face of the Moon should have been visible after 14.5 days. The revolution of Moon can be explained as follows. Suppose a child is playing with her Mother and keeps circling her holding her for support. Than the face of the child is always facing the Mother and the back of the child is not visible at any point of time. This is what is happening with respect of Earth & Moon. Therefore we can conclude that Moon only revolves around the Earth and that there is no rotation on its Axis and the force for rotation is the action of Earth’s rotation on its Axis.

    M.Vishwanath – thank you for your excellent explanation, easy for me to understand, and easy to explain to my 13 year old son, who is so inquisitive and curious about anything in space since a small child. Thank you for your knowledge and insight. I wish I had a teacher like you when I was at school. Do you teach, or share other on-line teachings? Regards. Cardillo, 17 Oct 2010.

    Dear Mr. Cardillo, Thank you very much for the compliments. I am really glad that my answers regarding the moon was so helpful to you and your son. I wish to state that I am a banker by profession and am employed in Union Bank of India, Hennur Road Branch, Bangalore. I am interested in Astronomy and so many other subjects not related to my profession and I get answers to so many questions when I sit in deep contemplation. Thank you once again and wish you all the best.

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