We often think that time is an absolute and immutable measure that counts the passing of days without any variation. But we tend to forget that, in fact, time is given by what Einstein described masterfully in his Theory of General Relativity. It is the movements derived from the gravitational fields of the stars that 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 only given a superficial explanation of what really happens.
The Earth does not have only two movements, it actually has five, and some 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 readjust measurements to keep civil calendars in sync with the Earth’s rotation. It is not that we now have 27 extra seconds in the day, but the leap seconds are seconds that are added to a certain year’s calendar to readjust human measurements.
A more correct way of saying it is that on 27 occasions a second has had to be added to the years because the Earth’s rotation slowed down and took longer to complete the days of that year. At these times, clocks tick unusually, which can be for example day lengths such as 23:59:59, 23:59:60, and 00:00:00:00.
What causes changes in the Earth’s rotation?
The Earth is not alone, we have to get used to the idea that everything that happens on our planet, specifically talking about 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 the tidal forces and also the slowdown in the rotational spin of our planet.
There are three main interactions involved in the tidal force, the attraction of the Sun, the Moon, and the rotation of the Earth. In addition, the fact that our planet is not completely spherical in shape, but is a geoid with a flattening at the poles comes into play. In this 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 flaring bulges are closer to the Moon, the Moon’s gravity has a stronger pull effect. But let’s not forget that the Earth is attached to the Sun and rotates on its own axis during the rotational motion, so the bulge is not just aligned with the motion of the Moon, but behind its position. Consequently, the net attraction that the satellite exerts on the Earth is backward, generating a slowdown in the rotation of our planet. This is why sometimes the Earth takes longer than normal to complete the revolutions around itself, lengthening the length of the days.
Changes in time
Since the 1960s, the day has lengthened by at least 3 milliseconds, from 86,400 to 86,400.003 seconds. And although it seems an almost ephemeral perturbation, let us remember that the Earth rotates and rotates without ceasing, therefore if each day of rotation lasted 1 millisecond longer, the measurement of the human clock would lose 1 second every 1000 days, and it would be required to add 1 intercalary second to recover it.
In short, time is not an exact measurement, the day is not the robotic course 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 as the Moon moves farther and farther away from us, although that is another story.
Story originally published in Spanish in Ecoosfera
