same side always
Why Same Side?
In the evening on a full-Moon Day, we often admire the beauty of the rising full Moon in the eastern sky. But have you ever noticed that the Moon always shows the same side (known as the near side) to Earth-bound observers? Even during the waxing or waning phase, we see the near side only.
Have you ever wondered why? Doesn’t the Moon rotate on its own axis? If it rotates on its own axis, we should be able to see the other side (far side) too; Right? So why do Earth-bound observers only ever see the near side of the Moon?
What is going on? Continue reading.
Tidal Locking
First, let us investigate why the Moon shows us only one side all the time.
This occurs because Earth’s greater gravitational pull locked the Moon gravitationally. Scientists call this phenomenon tidal or gravitational locking. As a result of the gravitational locking, the Moon’s rotational period, on its own axis, became equal to its orbital period – the time to complete one orbit around the Earth.
As we know, the lunar month is 29.5 days which is the time period between a new moon and the next new moon. This is synodic month.
However, the sidereal month is only 27.322 days which is the actual time period to orbit the Earth. In other words, sidereal month is the time the Moon takes to traverse 360 degrees around Earth. (We will discuss synodic versus sidereal month in another post.)
As mentioned above, because of tidal locking the Moon takes exactly 27.322 days to orbit the Earth as well as to make one rotation (spin) on its own axis. Astronomers call this synchronous rotation or spin-orbit coupling. Because of the synchronous rotation, we see the same face of the Moon all the time.
The animation below illustrates the synchronous rotation of the Moon.
Synchronous rotation
We can notice the following points watching the animation:
- As the Moon completes one orbit around the Earth, the Moon also completes one rotation on its own axis.
- In such a gravitationally locked state, the near side (marked in yellow) always faces the Earth
If you are not convinced on point 1, watch closely how the imaginary red-arrow, on the Moon, turns 360 degrees as the Moon completes one orbit around the Earth keeping the near side (marked in yellow) towards the Earth all the time.
More on Tidal Locking
So, in conclusion: Earth’s gravitational pull is responsible for tidal locking which in turn puts the Moon in synchronous rotation; and this synchronous rotation is the reason the Moon shows us only one side.
Okay we get it now. But why did the Moon get tidally locked to the Earth in the first place?
To answer that question, we need to go back billions of years. When our Earth and its moon system originally formed about 4.5 billion years ago, the Moon was much closer to the Earth. In addition, the Moon was rotating much faster. Earth’s greater gravitational pull on the near side of the Moon, compared with the pull on the far side, slowed down the Moon’s rotation until the Moon was tidally locked to the Earth. Scientists believe that this process could have taken hundreds of millions of years. You can read more on the origin of the Moon here.
Once the Moon got into such a locked state, the Earth has been holding it in place as if an invisible string is attached to it. From then on, they continued to perform this cosmic dance for billions of years!