Why is moon drifting away from earth into higher orbit ?
It may amaze to you that moon’s orbit is increasing at rate of 3.8 centimetres per year or +38.247±0.004 mm/year . It gives an idea that moon’s orbit might closer to Earth billion years ago . That increments in length of moon’s orbit would continue from billion years ago .
But what does make it to increase its orbit ? Answer lies in “tidal force” . Moon is sufficiently large and nearer to Earth to cause tides on Earth and raising the Earth’s ocean in form of high tides which is noticeable to us . But there is also unnoticeable effect of moon’s gravity on Earth . Its gravity affects the solid part also ( since moon’s mass is 1/80th of Earth’s mass so we should consider as two body system here ) . Farther point of Earth experiences lesser gravitational field of moon than nearer point because gravitational force is inversely proportional to squared of distance . This differences in gravitational impact on different point stretch the geometry of Earth and cause a bulge out at nearer point . Since it happened due to tidal force of moon , that’s why it is called “tidal bulge” .
This tidal bulge start to exerts force on moon and pulling ahead the moon into its orbit by causing the torque on it because earth’s rotation ( 30km/s) is higher than moon’s rotation ( 1km/sec ) . Due to Earth’s faster rotation , some parts its angular momentum transfer into moon’s orbit ( whole process takes place in “Earth – Moon system” ) which increases tidal acceleration of moon and that let it to recede in higher orbit . But due to law of conservation of angular momentum in Earth – moon body system , Earth’s rotation decreases and lengthens the day by 23 microsecond per year which is unnoticeable .
Vice versa is also true . Gravitational field of Earth also creates “tidal bulge” on moon and decreases the rotation and makes it to synchronize with moon’s orbital velocity . The whole process is called “tidally locked ” That is why moon’s rotation and its revolution period are same .
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