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The notion of axis It has more than a dozen meanings. It may be the dipstick what provides support to a body while turning , going through it. Land , for its part, is that linked to the land (as opposed to water or sky) or planet Earth .

In the field of geography and of the astronomy , the idea of Earth's axis . It is a imaginary bar around which our planet revolves . If this axis of rotation extends to the celestial sphere, it allows to determine the points known as poles .

The land rotates on its axis in a rotatory motion . By convention it is said that a complete return of the planet delay 24 hours (that is, one day), although this period of time is not accurate since, while rotating on the axis, the land It is also advancing in its orbit.

The earth's axis, which has an extension of 12,713 kilometers , presents an inclination of 23°5' regarding the perpendicularity to flat of the ecliptic. If extended, set the celestial poles . It is important to mention, however, that the orientation of the earth's axis changes cyclically.

The nutation it is a slight oscillation movement that registers the axis in periods of 18.6 years, while with a period of 25 767 years a movement of precession . These displacements change their orientation. Because of these phenomena, the earthquakes and to other causes, the inclination of the axis of the land can go from 23° to the 27° .

The origin of the word nutation It is found in the Latin verb nutare, which can be translated as "oscillate" or "nod." East movement It is irregular and occurs in symmetrical objects that perform a rotation movement on its own axis, as is the case of the Earth, although not only the planets suffer, but also objects and devices of daily life such as the tops and gyros In more technical words, it is a movement that does not alter Euler's first angle.

The Euler's angles they are a group of three coordinates that are used to determine the orientation of a reference system with respect to another, both being normally mobile and orthogonal axes. The nutation phenomenon was discovered by James Bradley , an astronomer from England, in the year 1728, although it was two decades before he made it publicly known. According to his observations, he concluded that the cause of nutation was in the attraction that both the Moon and the Sun exert.

The earth's axis periodically oscillates around its average position, because of these gravitational attraction forces, making a movement that closely resembles that of a spinning top. One of the consequences of this phenomenon is that every 18.6 years, the axis oscillates about 9 "of arc and 17", on each side, with respect to the average oblique values ​​of the ecliptic and displacement of the Aries point, respectively.

Continuing with the concept of precession , which is also called precession nutation movement, we can say that it is associated with the disturbance that suffers the earth's axis in its direction. We can distinguish two types of precession:

* without external moments : This type of precession occurs when the body is in rotation around an axis that is not the largest or the lowest moment of inertia (a measure of rotational inertia that can be represented when the movement occurs around a main axis of inertia);

* due to external moments : in this case we can talk about free moving symmetrical top, when two of the moments of inertia coincide; or of gyroscope, when the angular momentum is a vector whose module is the product of angular velocity at the moment of inertia.

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