The story returns to popping on social networks and newspapers with a certain regularity: in China there would be such a gigantic infrastructure that it has changed our planet forever. It is the Three Gorges dam, a huge artificial basin that feeds the largest hydroelectric system in the world. Colossal to the point of having influenced the position of the north geographical pole, the rotation of our planet and, as a consequence of this, the duration of the days. How true is there in this story?
The Damn of Discord
The three gorges dam, in fact, is an engineering project of immense size, which has served almost 10 years to be made, and about 20 to start producing energy. The construction takes its name from the three gorges crossed by the blue river in the area: the quang throat, the wuxia throat and the xyling throat. He has a very long story: imagined already in the early 1920s by Sun Yat-Sen, one of the two founders of Kuomintang, as a strategic asset to modernize post-imperial China, its construction was approved in 1992 by the then Premier Li-Meng, which began in 1994 and ended only in 2003. And from the beginning it was accompanied by strong controversies.
To create the artificial basin, in fact, it was necessary to submerge, in whole or in part, more than 1,300 archaeological sites, 13 cities, almost 1,400 among countries and villages, forcing over 1,400 people to leave their homes and destroying natural habitats inhabited by many species at risk of extinction (such as literature, a dolphin of fresh water that lived in the waters of the blue river, and is considered functionally extinct from the 2006). Next to these direct consequences of the construction of the three gorges dam, at least since 2005 he has heard another periodically, of planetary flow rate: the slowdown of the terrestrial rotation.
NASA’s article
Retracing the story of this news backwards, which continues to shine in newspapers and social media with a certain regularity, the initial source is an article published in 2005 on the NASA Jet Propulsion Laboratory website, in which the effect that had the terrible earthquake of the Indian Ocean of 26 December 2004 on the rotation of our planet was calculated. In the article, two NASA researchers explain how every event that produces a move of a large mass, as can be a large earthquake or the creation of an artificial water basin, influences the rotation of the earth, the duration of the days, and the position of the north geographical pole in an imperceptible way.
The reason is simple: it is the effect of a physical principle called conservation of the angular moment, which explains different peculiarities of the bodies in rotation, such as the tendency of gyroscopes to maintain the orientation of their rotation axis; Or again, and it is the case that interests us, the fact that the more the mass of a body in rotation is distributed away from the rotation axis, the more its rotation slows down, as happens to the skaters on the ice when they accelerate and decele them in their piroette, lengthen and shortening the distance of the arms and legs from the body.
The effects on terrestrial rotation
In the article, the researchers use the three -gorge dam as an example, calculating that the movement of 42 billion tons of water at 175 meters above sea level (as happens when the dam basin is completely filled) this removal of a part of the terrestrial mass from the rotation center (like a skater that expands the arms) produces an elongation of the days of 0.06 microseconds, and A movement of the position of the north pole of about two centimeters.
It is therefore true that, theoretically, the inauguration of the dam may have influenced the duration of the days. But it is important to consider that the effects are so contained as to be impossible to measure. And that the pelvis does not contain a quantity of fixed water all day and all year, and therefore the effects would still be changing. And above all, that it is not the only mass shift that influences the rotation of our planet. According to the same authors of the NASA article, the 2004 Indonesian earthquake has in fact lengthened the days of 2.68 microseconds. And instead focusing on the results of human activities, it was calculated that the extraction of water resources globally reached such an entity, as to have moved 80 centimeters to the east of the geographical northern pole between 1993 and 2010. Putting things in perspective, therefore, the effects of the Chinese dam are negligible in any practical sense of the term.