Earth movements
The earth is subjected to movements of various kinds. The main movements of the Earth are defined with reference to the Sun and are: rotation, translation, precession, nutation, Chandler wobble and perihelion precession.
Rotation movement
It is a movement carried out by the Earth rotating on the terrestrial axis, which cuts the surface at two points called poles. This rotation is from west to east, that is, for an observer located in space over the terrestrial north pole, this movement is left-handed (counterclockwise). A complete turn, taking the stars as a reference, lasts 23 hours and 56 minutes 4.1 seconds and is called a sidereal day. If we take the Sun as a reference, the same meridian passes in front of our star every 24 hours, called the solar day. The approximately 3 minutes and 56 seconds difference is due to the fact that in that period of time the Earth has advanced in its orbit and must rotate more than one sidereal day to complete one solar day.
The first reference taken by man was the Sun, whose apparent movement, caused by the rotation of the Earth, determines day and night, giving the impression that the sky revolves around the planet. In the colloquial use of the language, the word day is used to designate the period that in astronomy is called solar day, and corresponds to solar time.
Translational movement
It is the movement by which the planet Earth rotates in an elliptical orbit around the Sun in 365 days and a little less than 6 hours. For an observer located in space above the Earth's North Pole, this movement is also left-handed (counterclockwise), and logically, seen from the Earth's South Pole, this movement is right-handed (like clockwise).. As the calendar registers 365 whole days, the beginning of each year is brought forward, which is approximately compensated (since it does not take into account the equinoctial precession. See below) making one out of every slightly more than four years, to which It is called a leap year and has 366 days. The cause of the translation movement is the action of gravity and originates a series of changes that, like the day, allow the measurement of time. Taking the Sun as a reference, it results in what is called a tropical year, the period necessary for the seasons of the year to repeat themselves. It lasts 365 days, 5 hours, 48 minutes and 45 seconds. The movement it describes is an elliptical trajectory of 930 million kilometers, at an average distance from the Sun of practically 150 million kilometers, 1 au (astronomical unit: 149 597 871 km) or 8,317 light minutes. From this it follows that the Earth moves through its orbit at an average speed of 106,200 km/h (29.5 km/s).
The Earth's trajectory or orbit is elliptical. The Sun occupies one of the foci of the ellipse and, due to the eccentricity of the orbit, the distance between the Sun and the Earth varies throughout the year. In the first days of January the maximum proximity to the Sun is reached, producing perihelion, when the distance is 147.5 million kilometers, while in the first days of July the maximum distance is reached, called aphelion, when the distance is 152.6 million kilometers.
As can be seen in the graph above, the Earth's axis forms an angle of about 23.5º with respect to the normal of the ecliptic, a phenomenon called obliquity of the ecliptic. This inclination, combined with the translation, produces long periods of several months of continuous light and darkness at the geographic poles, as well as being the cause of the seasons of the year, derived from changes in the angle of incidence of solar radiation and in the duration of the hours of light that this obliquity produces.
Precession of the equinoxes
The precession of the equinoxes (the slow and gradual change in the orientation of the Earth's axis of rotation) is due to the precession of the Earth caused by the moment of force exerted by the Earth-Sun system as a function of the inclination of the earth's axis of rotation with respect to the plane of the earth's orbit (around 23°43' currently). This movement is carried out completely every 25,776 years (so that approximately every 130 centuries the seasons would be reversed, but the difference between the sidereal year and the tropical year is incorporated and corrected by the Gregorian calendar) and an observer in space, located above the north pole, you would see it as a right turn (in the same sense as the clockwise one).
The inclination of the Earth's axis varies from 23° to 27°, since it depends (among other causes) on telluric movements. In February 2010, a variation of the Earth's axis of approximately 8 centimeters was recorded, due to the 8.8° Richter earthquake that affected Chile. While the tsunami and consequent tsunami that hit Southeast Asia in 2004, displaced 17.8 centimeters to the Earth's axis.
Nutation movement
Precession is even more complex if we consider a fourth movement: nutation. This happens with any symmetrical or spheroidal body rotating on its axis; A spinning top (spinning top) is a good example, because when it falls the precession begins. As a consequence of the falling movement, the top's spike rests on the ground with more force, so that the vertical reaction force increases, which eventually becomes greater than the weight. When this happens, the center of mass of the top begins to accelerate upward. The process is repeated, and the movement is made up of a precession accompanied by an oscillation of the axis of rotation up and down, which is called nutation.
In the case of the Earth, nutation is the periodic oscillation of the Earth's pole around its average position on the celestial sphere, due to the external forces of gravitational attraction between the Moon and the Sun with the Earth. This oscillation is similar to the movement of a spinning top (spinning top) when it loses power and is about to fall.
The Earth moves about nine seconds of arc every 18.6 years, which means that in one full turn of precession, the Earth will have made 1385 loops. To get an idea of this movement, let's imagine that, while the axis of rotation describes the conical movement of precession, it in turn covers a small ellipse or loop in a period of 18.6 years.
In one full turn of precession (25,776 years) the Earth goes through more than 1,300 loops of nutation. The nutation movement of the Earth was discovered by the British astronomer James Bradley.
Chandler's Wobble
This is a small wobble of the Earth's axis of rotation that adds 0.7 arc seconds over a period of 433 days to the precession of the equinoxes. It was discovered by the American astronomer Seth Carlo Chandler in 1891, and the causes that produce it are currently unknown, although several theories have been proposed (climate fluctuations causing changes in the distribution of atmospheric mass, possible geophysical movements under the earth's crust, variations of salt concentration in the sea, etc.). The sum of the Chandler Wobble and other minor effects is called polar motion.
Motion of perihelion precession
In the movement of translation, the earth describes an ellipse around the Sun, which occupies one of the foci of said ellipse, but the other focus is not static, it also slowly rotates a small angle of 3.84 arcseconds per century, around the Sun, in the same sense of the orbit and this rotation of the free focus of the ellipse is known as apsidial precession or perihelion precession or advance, which is the moment of least distance from the Earth to the Sun. Logically the aphelion, or the moment of greatest distance from the Earth to the Sun, also undergoes this advance, which, even though it is angularly the same, is even greater tangentially. This movement has a period of about 34,285,714 years.
Orbital variations
The orbital variations or cycles of Milanković describe the joint effects that changes in Earth's movements cause in the climate over thousands of years. The term was coined after studies by Serbian astronomer and geophysicist Milutin Milanković. In the 1920s, he theorized that the resulting variations caused cyclic changes in solar radiation that reached the Earth's surface and that this had a significant impact on the patterns of climate change on Earth.
Some similar astronomical theories had been anticipated during the nineteenth century by Joseph Adhemar, James Croll and others, but their verification was complex because of the absence of relevant fossil data and because it was not clear what periods were important in the past to check it out.
Currently, geological materials on the surface of the Earth that have not changed for thousands of years are being studied by specialists to find out the changes in terrestrial weather. Although many of them are consistent with the hypothesis of Milankovitch's theories, there is a set of the same as the predictable hypotheses are not able to explain.