We often think that time is an absolute and immutable measurement that tells us the passing of days without any variation. But we tend to forget that in fact time is given by what Einstein masterfully described in his Theory of General Relativity. They are the movements derived from the gravitational fields of the stars, which determine the passing of the seconds and unfortunately, the movements of the Earth are not at all simple, so much so that in school we are usually explained only superficially what really happens.
The Earth not only has two movements, in reality it has five and there are those who dare to say that it has many more but we have not been able to count them yet. Therefore, time is subject to a multiplicity of factors and changes gradually, subject to the movements of our planet which, it must be said, depends on two other entities; the sun and the moon.
Over the past 50 years, a total of 27 leap seconds have been added to our clocks to reset measurements and keep civil calendars in sync with . It is not that we now have 27 extra seconds in the day, but that leap seconds, additional or leap seconds, are seconds that are added to the calendar of a certain year to readjust human measurements.
A more correct way to say it is that on 27 occasions a second has had to be added to the years, because it slowed down and took longer to complete the days of said year. At these times, the clocks tick an unusual tick which can be for example day lengths such as 23:59:59, 23:59:60, 00:00:00.
What causes changes in the Earth’s rotation?
The Earth is not alone, we must get used to the idea that everything that happens on our planet, specifically speaking of movements through its orbit, is mainly given by the Earth-Moon system. The orbital dynamics between the two orbs is a vicious circle that generates tidal forces and also the slowdown in the rotational spin of our planet.
There are three main interactions that intervene in the tidal force, the attraction of the Sun, the Moon and the rotation of the Earth. Also comes into play is the fact that our planet does not have a completely spherical shape, but rather is geoid with a flattening at the poles. In that sense, the attraction that the Moon exerts on our planet is not always the same, but depends on its position around the planet.
When the Earth’s bulges are closer to the Moon, the latter’s gravity has a greater pulling effect. But let’s not forget that the Earth is subject to the Sun and rotates on its own axis during the rotation movement, this causes the protuberance to not be exactly aligned with the movement of the Moon, but rather lags behind its position. Consequently, the net attraction that the satellite exerts on the Earth is backwards, generating a slowdown in the rotation of our planet. This is why sometimes the Earth takes longer than normal to complete the rotations on itself, lengthening the length of the days.
Changes over time
Since the 1960s, the day has lengthened by at least 3 milliseconds, going from 86,400 to 86,400,003 seconds. And although it seems like an almost ephemeral disturbance, let’s remember that it rotates incessantly, therefore if each day of rotation lasted 1 millisecond longer, the measurement of the human clock would lose 1 second every 1000 days and would require adding 1 leap second to recover it.
Simply put, time is not an exact measurement, the day is not the robotic passage of 86,400 seconds in perpetuity, but due to changes in the Earth’s rotation, this measurement is constantly changing. And in fact, it will continue to change because the Moon moves further and further away from us, although that is another story.