Chuseok, the full moon is already running towards the end of the year. The moon was originally part of the Earth. The moon was created a long time ago when a celestial body the size of Mars called ‘Theia’ collided with the Earth and part of it fell off. Surprisingly, the moon created in this way is now protecting the Earth. In fact, it is almost a miracle that higher life forms like humanity can exist on Earth. The moon plays an important role in this miracle.
The Earth and the Moon have a strong gravitational effect on each other
Role in stabilizing the Earth’s rotation axis
Thanks to this, extreme summer and winter damage
The universe is full of secrets we don’t know yet
Life on Earth requires numerous conditions to be met In order for life to exist on Earth, numerous conditions, not just the Moon, must meet exquisitely. First of all, a planet cannot be too close to a star, or too far away. As far as we know, liquid water is one of the essential conditions for life. If the planet is too close to the star, all the water will evaporate into water vapor, and if the planet is too far away, all the water will freeze into ice. Depending on the brightness of the star, the area where liquid water can exist on the planet is called the ‘Goldilocks zone’. For reference, Goldilocks is named after the British fairy tale ‘Goldilocks and the Three Bears’. In our solar system, the only planets that fall into the Goldilocks zone are Earth and Mars. Mars has weaker gravity than Earth, so it is presumed that the original water gradually escaped into space.
Also, the planets in our solar system are very well distributed. Inner planets such as Mercury, Venus, Earth, and Mars are all made of rock and are relatively small. On the other hand, exoplanets such as Jupiter, Saturn, Uranus, and Neptune are all made of gas and are very large. In this situation, exoplanets act as a kind of ‘gravitational vacuum cleaner’ that sucks in celestial bodies penetrating into the solar system. If there were no exoplanets, the inner planets would be subject to asteroid collisions much more frequently.
To put things in perspective, our solar system is located at a very reasonable distance from the center of the galaxy. If our solar system were too close to the center of the galaxy, it would have been strongly exposed to X-rays and gamma rays from the black hole, and if it was too far away, the density of heavy elements such as iron that form planetary cores would have been very low. However, as mentioned earlier, the closest friend that protects the Earth and living things is the moon.
Interestingly, the secret to the moon’s ability to protect the Earth lies in the fact that we can only see one side of the moon. The moon’s orbital period and rotation period are the same. It may seem obvious, but the Earth’s orbital period is 365 days and its rotation period is 1 day. On the other hand, the moon’s orbital period and rotation period are exactly the same, 27.3 days. This means that as the moon orbits the Earth, it always rotates with the same side facing the Earth. That way we can only see one side of the moon. But why on earth is that so?
An unusually large Earth satellite, the Moon
Compared to the satellites of other planets in the solar system, the Moon is the largest compared to its parent planet, the Earth. In other words, the Earth and the Moon exert a strong gravitational force on each other. To exaggerate slightly, the Earth and the Moon could crush each other. This is a tidal phenomenon. Tides often refer to the periodic rise and fall of Earth’s sea level due to the moon’s gravity, but conversely, the Earth also distorts the Moon to some extent. Roughly speaking, the Earth and the Moon are both slightly distorted, like eggs, and face each other in an elongated direction. The fact that the moon’s orbital period and rotation period are the same means that the moon’s orbit and rotation are precisely fixed so that the Earth and the moon always face each other in this elongated direction. Technically, this phenomenon is called ‘tidal locking’. So how does tidal locking protect the Earth?
As mentioned earlier, the reason tidal locking occurs is because the moon’s gravity is strong enough. The Earth’s axis is tilted by 23.5 degrees. This tilt of the rotation axis, which is neither too big nor too small, creates appropriate changes in seasons and gives birth to life. The presence of the moon, which exerts strong gravity, plays a role in stabilizing the Earth’s rotation axis. Recent research suggests that if there were no moon, the Earth’s rotation axis could be tilted by nearly 90 degrees. The fact that the rotation axis is tilted close to 90 degrees means that the seasons alternate between extreme summer and winter. In other words, there is continuous daylight in summer and continuous nighttime in winter. For reference, Uranus’ rotation axis is actually tilted 97.7 degrees. We should be thankful for the moon.
The universe is fine-tuned
In fact, it may be the universe itself that helps us exist. For example, physical laws consist of fundamental constants such as the speed of light, the charge of an electron, Planck’s constant, the gravitational constant, and Boltzmann’s constant. If the values of these fundamental constants had been slightly different, there is a high probability that the universe as we know it today would not have existed. Among these, the value of the energy density of empty space in space, the so-called cosmological constant, is very precisely fine-tuned. If the cosmological constant had been larger than it is now, the universe would have remained an empty space with almost no matter. Conversely, if it had not existed at all, the universe would have collapsed early due to its own gravity. For reference, another name for the energy of empty space in space is dark energy. How on earth did this fine tuning happen? I do not know. The universe is full of secrets we still don’t know.
Park Kwon, Professor, Department of Physics, Institute for Advanced Study
