This article on the lunar retreat is a guest article by Anja Sjöström, an IB diploma student from Switzerland.
Why is the moon moving away from Earth?
The moon, Earth’s only natural satellite located at about 384,000 kilometers from our planet, is slowly drifting away from Earth. The moon currently sustains the function of stabilizing Earth’s wobble or polar motion, the movements, and drifting of Earth around its axis due to its uneven distribution of mass and slightly oblate spheroid shape. This assures a relatively stable and regulated climate. In short, if Earth did not have the moon, it would, even though the rate would be almost unnoticeable, start slowing down and becoming extremely unstable.
We often take the Earth-Moon distance as a fixed constant. However, in the greater scheme of things, the Moon has been moving away from Earth ever since its creation. The moon’s creation is most often explained using a theory called the ‘large impact theory’. Due to the extremely similar composition between the moon and planet Earth, scientists believe that about 4.5 billion years ago a Mars-sized object collided with Earth.
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A collision from which the left-over debris would have coalesced into what we today know as the moon, settling into orbit around Earth at an initial distance of about 22,500 kilometers. As such, the initial distance, and the one at which the moon finds itself today, greatly differ due to an ongoing process known as a lunar retreat.
To our day, the moon can be observed to be drifting away from Earth at a speed of about 3.78 cm a year. However, this speed has not remained constant over the years, and our system has experienced several periods of the peak in lunar retreat during which the moon was suspected to have moved away from Earth at a rate as high as 7 cm per year due to either increased geological activity on Earth, such as continents breaking apart and periods with increased fluctuations between ice ages and greenhouse periods, or heavy impacts of meteorites upon the moon. These events would exacerbate tidal activity between the moon and the Earth.
The previously mentioned effect of tides is at the core of the explanation for the lunar retreat. As the moon and Earth both exert a gravitational attraction on each other, maintaining the moon on its orbit, our oceans are consistently being pulled in the direction of the moon. This movement of the oceans, this tidal bulge, exerts a frictional force upon Earth, slowing down its rotation. As such, the moon’s angular momentum increases in order to compensate and is gradually pushed into a slightly higher orbit.
How was this phenomenon observed?
The moon’s retreat was speculated over 300 years ago by Edmond Halley, a famous English astronomer. He had emitted this hypothesis after studying various photographs and/or records of ancient eclipses. Comparing them to those occurring more recently, he realized that the shadow of the moon differed in size. This was later confirmed by the United States and the USSR in the 1970s when their mirrors bouncing off laser signals on the moon started indicating a receding speed of 3.78 centimeters per year.
What are the repercussions?
Fossil records can be studied to show that at the creation of the moon, when it was much closer to Earth, our planet was spinning much faster, making the days about five hours long. As such, on a large time scale, our days will start getting longer and longer which will in turn start affecting the stability of our planet’s ecosystems and climate.
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In addition, the previously mentioned ‘wobble’ that the moon stabilizes will start to become much more noticeable as the moon has a mitigated gravitational effect on Earth due to a larger distance from it. This will yield a measurable change in seasons with much greater temperature swings and longer exposure to extreme conditions such as warm weather during the summer when facing the sun, or freezing conditions in winter when turning away from the sun.
Thanks to technology and our relatively high resilience, humans need not worry for compensation is easily possible. However, many species will be finding themselves unable to alter their lifestyle fast enough to survive in changing conditions.
Consequences on the future of solar eclipses
As the moon grows further away from Earth, we are getting closer to the day total solar eclipses will no longer be possible. Scientists from NASA say that the last observable total solar eclipse will be seen in about 600 million years. Such eclipses rely on the apparent diameter of the moon and the Sun from Earth which coincides in size, the Sun is about 400 times wider but being situated about 400 times further away from Earth than the moon. If the moon retreats too much, humans will see the moon become smaller and smaller, and when aligned with the sun, will no longer be able to cover the entirety of the Sun and block all of its light.
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