Why does the Moon always faces its one side to earth?

  

      The moon is an outer celestial object which revolves around the earth. It revolves around the earth, on an average of once in every 27 days due to the Earth’s heavier gravitational pull. The moon generally has two sides in it, namely the near side and the Far side. The side which we see now on the moon is called the near side and the side we don’t ever see is called the far side of the moon.

Credit (Near Side): By Gregory H. Revera - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11901243

Credit (Far Side): By Jay Tanner - Unknown source, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=34227359

    Even though the moon revolves as much as many times, it always faces its near side to the Earth. So we always happen to see the one side of the moon. This is because the rotation of the moon matches with the revolution of the moon. Here rotation means, an object is rotating itself at a constant speed whereas, revolution means an object is orbiting another object at a specific speed. So, if the revolution of the moon, takes 27 days to revolve around the earth, the rotation also takes 27 days. So the near side of the moon always faces the earth.

Rotation of Moon around the earth with its one side facing to earth.

       This strange thing happens mainly due to the Earth’s gravitational pull. The word Gravity means a force exerted by an object on another object to pull it towards. Here the gravity of an object depends upon two factors, which are mass and distance. If an object has more mass, it has more gravitational strength to pull an object to it, and if it has less mass it has less gravitational strength to pull an object to it. Also attracting another object depends upon the distance. If the distance between two objects increases by 2 times; the gravity decreases by 4 times. So the gravity decreases as the increase in distance. So, if anything in the universe has masses in them and if they are at an equivalent distance, those objects attract each other.

        The same above factors happen to the moon and earth. In space, both earth and the moon have masses in them and they are present at an equivalent attractable distance, so they both exert forces and attract each other. But here the mass of the Earth is greater than the mass of the moon. So the Earth exerts more gravitational pull on the moon than the moon exerts on the earth. As we saw earlier, that gravity also depends upon the distance. So the gravitational pull exerted by the earth on the moon varies with the distance of the surface of the moon because the moon itself is very big. So at each part of the moon, the gravitational pull of the earth varies. So on the near side of the moon, the gravitational pull is heavier and gets weaker on moving to the far side of the moon. Due to this phenomenon, the centre of gravity of the moon lies below the centre of mass.  Here the centre of gravity is defined as the point on the moon where the total gravity of the earth is concentrated, whereas the centre of mass is defined as the point of mass where the total mass of the moon is present, or it can be expressed as the point on the moon where the moon can be balanced by distributing its mass on either side. So as the earth’s gravitational pull always concentrates the centre of gravity and it is on the near side of the moon, it does not allow the near side of the moon to move away from it. If the moon tries to move a little bit fast or slow,  the gravity of the earth pulls back the moon to the original position (as same as the apple falls on the ground by earth's gravity when we throw it upwards). So the moon always faces its one side to the earth due to the centre of gravity.

Gravitational Interaction between the Earth and the Moon

     Due to the constant heavier gravitational pull by the earth, the moon gets deformed from its original shape into an elongated shape. So the moon slightly looks in an oval shape. In the same way, the moon also exerts some gravitational pull on the earth, but as its gravitational pull is so weaker it cannot affect the whole earth, but it can affect the ocean tides on the earth. That is why during night time sea waves get larger, due to the Moon’s gravitational pull. Generally, the tides rising up the Earth's surface travels a bit forward to the moon due to the rotation of the Earth.

 

Tides caused on the earth due to the gravitational pull of the moon