Precession and sound in space

Precession | the third motion of Earth

Generally, we are only familiar with two motions of Earth - one is a rotational motion on its axis, and another is a motion to orbit the sun, but there is a third motion that is too slow such we cannot observe his full one cycle, even if it takes our whole life and several rebirths. We call this third motion as Precession.

Precession is a slow but regular and gravity-induced orientation of the earth's axis, which completes his one full cycle in about 26,000 years. Precession is similar to a wobbling top. Hipparchus( 190BC-120BC ), one of the greatest astronomer in history, had discovered this motion. He observed that the celestial north pole of the celestial sphere is somewhat wobbling, then it clearly means the earth itself wobbling. Today, the celestial north pole is touching Polaris star (it will very close in around 2100 years), but in 14,000 years, it will pick up the star Vega.

Left - A Wobbling top, Right - Precession, the celestial north pole or north pole is touching Polaris star. And in 14,000 years, it will touch up the star Vega.

At our home planet - Earth, these all three motions are not crystal clear. And That's why ancient astronomers thought that it is the sky that moves around the stationary Earth, not the Earth that goes around under the sky.

The cause of Precession is pretty logical that the gravity of the moon and the Sun(Of course, there are also little the effect of others planet and stars gravity). We know that Earth is not a total sphere, and tilt towards the Sun, which causes an extra gravitational fore on the tilt part. And so Sun's gravity has a great contributor to the Earth's wobbling. It also a fact that if the Earth is the complete sphere, there no Precession.

The word "precession" is no longer suitable to describe the third motion of Earth, but still, we use it. We are also using this word to describe for any shift of a spin axis around a cone.

Can we hear sound in space?

Let me first define - what is sound?
In simple words, Sound is a vibration that propagates as a wave in medium(liquid, gas, or solid).
To understand this, consider the example of water. The ripples on the surface of the water is nothing but a vibration which distribute as a wave, and water is the medium that carries this wave.

The sound which we hear is a vibration as wave in air(medium). Sound travels in air as a series of compression and refraction.

In order to hear in space we should fill it with some medium (gas, liquid or solid).
We know space is a vacuum. Yes, no we can't hear anything in space because sound needed a medium to travel. 
But in actuality, space is not a complete void or perfect vacuum. Space is filled with dust and a very small amount of particles per cubic meter, and that's why - we can still hear the sound if force is so strong to propagate the wave.

Black holes, quasars, galaxy mergers, supernovas, molecular hydrogen clouds, and stellar collisions make sounds in space. And if it is near to the earth or has a strong vibration we can hear it. In recent years scientists developed ways to hear this sound which a human ear can't sense.
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