GS12: Why is our moon drifting away?
Updated: Jun 18
In our last article on the moon, we talked about a few questions. How big is the moon relative to the Earth? The relative size ratio of planets. Why do we see only one face of the moon? What is the difference between the near-side and far-side of the moon? What is hidden behind the far side of the Moon? If you haven't read it yet, the link is below.
For a long time, people thought that the moon was a large asteroid that was stuck in the orbit due to the gravitational pull of the Earth. After careful calculations, however, it was found that no asteroid can have exactly the same orbit as does the moon. This answer was given by the Angular momentum of the Earth-Moon system.
Angular momentum tells us about two things, how fast a rotating object is rotating, and how hard it would be to change its rotation.
For any closed system, the conservation of angular momentum is always followed. That means when there are no perturbations caused by external objects in the system the angular momentum remains constant. The same happens for the Earth-Moon system. The spinning, orbiting system of the Earth and the Moon has angular momentum as well and it is conserved. Well, not perfectly, a small amount of energy is lost against tidal friction. Apart from it, the total angular momentum of the system has remained the same for the last 4.5 billion years.
Why is the distance between the Earth and Moon increasing? Why is Earth slowing down?
Remember our enthusiastic skater, let's call him again. The momentum of his system remains conserved while skating. The angular momentum of a system depends on the way mass is distributed and how fast the system is rotating. No matter whether he folds his arms or opens them up (which is actually the shifting of mass), the momentum remains conserved. This is reflected simply by the speed of rotation of the skater. The similar happens with the Earth-Moon system as well but with a little twist.
Tidal locking is responsible for the synchronous rotation of the Moon.
This is why you can only see one face of the moon.
As you know, due to the gravitational attraction of the moon, oceanic tides are formed. The tidal bulge of water pulled from the surface creates a tiniest inclination of the Earth-Moon axis, by a small angle alpha, as the bulge is dragged ahead of the position due to Earth's rotation. Consequently, there exists a mass in the bulge which exerts torque between Earth and the Moon. The bulge nearer to the Moon pulls it stronger than the bulge farther away, this torque boosts Moon in its orbit and slows down the rotation of Earth.
Also, to preserve the angular momentum of the system, this causes the moon to move away from the Earth. This unique relationship between the Earth and the moon is known as Tidal Locking. This phenomenon is also observed because of the large Moon to Earth-size ratio; therefore, this may not be vividly observed for other planet and its moon except the Charon-Pluto system. As a result of this, the duration of the eclipse is getting shorter when compared to ancient eclipse observation.
Edmond Halley was the first to suggest, in 1695, that the mean motion of the Moon was getting faster, by comparison with ancient eclipse observations, but he gave no data.
How do geologists perceive this phenomenon?
With every rise in sea level, as a result of the Earth-moon interaction, there are very thin layers of sandstone or siltstone which are deposited known as tidal rhythmites, one layer created each time the tide goes in and out. If these layers are preserved over time, then geologists may count the layers and determine how many lunar months per year there were when that rock was being deposited. If the rock is radiometrically dated, then we can even know about the months there were in a year at a given time in the past.
With the help of this analysis, 2.45 billion years ago, geologist determined that a day on Earth were 19 hours long. Amazing! This also speaks about how the rotation of the Earth has slowed down in the 4.6 billion years and days have gotten longer. As the Moon pulls the tides around the Earth, the Earth's rotation is slowed by tidal friction. As the rotation slows, to conserve angular momentum, the Moon moves slightly away from the Earth.
There is an art which is very famous among scientists these days. It is an art of extrapolation. With the set of recorded observations and data, scientists extrapolated the points back to the time when Earth was still forming to know the duration of a complete day. It has been concluded that the day length in the early solar system was only about 5 hours long! However, now with more data, it has been found that Earth was never spinning that fast as it required the Moon to be very close to the Earth, an instance described mostly by the Poets. Instead, it was found that the Earth and Moon separated rapidly, and lengthening was fast. Based on the tidal rhythmites now we know that Earth and moon were separated by 1/3rd of the present, and by 620 million years ago it was the only 2/3rd of what it is today. And today, with the help of laser ranging, it is known that the Moon is receding from the Earth at a rate of 3.82 ± 0.07 cm per year.
This is not just a case with our moon. Every planet in our solar system which has a moon suffers the same loss. In fact, the loss is more pronounced if they have several moons, poor giant planets. In fact, for the past few days, researchers claimed that Titan, a moon of Saturn, is drifting 11 cms away each year. This is the biggest drift noted for any planet's moon which is believed to be 100 times faster than what was thought. This has also provided another insight that the early expansion in the Solar System occurred at a much greater speed than what was anticipated. And now, since, you have read the whole article, I believe, it is easier for you to understand why does a moon drift away from its planet?
Angular momentum plays a key role, which is already discussed by us, in the validation of the theory of the formation of Moon. The Giant Impactor theory, the one which plausibly explains the formation of the moon, sounds much logical than an asteroid being captured in the orbit around the Earth, according to the conservation of angular momentum.