The motion of revolution (generally with an elliptical trajectory) is the movement that a planet or other celestial body makes around a center of mass.
The term can therefore refer to the motion of the Earth around the Sun, but also to the motion of a satellite around a planet or that of a star around the galactic center.
In ancient times, in which a geocentric model of the universe prevailed, celestial bodies were attributed a motion of revolution that had the Earth as its center. The various planets were placed, as if they were set, on nine immaterial concentric spheres, similar to orbits, which by rotating made them move with them. The outermost sphere of all was the one that contained the fixed stars, whose rotation was due directly to a spiritual cause, namely God, or to following the angelic hosts, each of which was concerned with moving one of the nine lower spheres. Speaking for example of the sky of Venus, Dante addresses the Principalities as follows:
In the prose commentary he specifies that in his song "they induce one to hear what I mean by certain Intelligences, or true in a more used way, we mean Angels, who are in revolution of the sky of Venus, yes as movers of that". With the adoption of the heliocentric model, the phenomenology of the motions of revolution was described by Kepler through the theory of the elliptical orbits of the planets, of which the Sun now occupied one of the two foci; subsequently Isaac Newton provided theoretical support to these empirical observations, attributing the planetary motions to the presence of a gravitational field generated by the Sun. This theory, however, did not explain some motions of revolution, such as that of mercury, as Einstein's general relativity would have done. .
The terrestrial revolution
The Earth, like the other planets in the solar system, travels an elliptical orbit having an eccentricity of just 0.017 around the Sun (which occupies one of the two foci) in a counter-clockwise direction (when viewed from the north celestial pole). The Earth reaches perihelion (the point of maximum proximity) at the beginning of January, and aphelion (the point of maximum distance) at the beginning of July (note that the alternation of the seasons is not due to the variation of the distance from the Sun , since in the northern hemisphere of the planet the hot season coincides with the period of maximum distance from the Sun).
The year can be identified in two different situations:
the solar year, that is the time that must pass for the sun to return to the zenith of the same tropic, this lasts 365 days, 5 hours, 48 minutes and 46 seconds;
the sidereal year, that is the time that must pass for the sun to return to the same position with respect to the stars, this lasts 365 days, 6 hours, 9 minutes and 10 seconds.The orbital circumference, as a whole, measures about 940 million kilometers . Its average speed is approximately 30 km / s.
Indeed, both the Earth and the Sun describe elliptical orbits, of which one of the foci is the center of mass of the system formed by the two bodies. This is located on the junction of the centers of the two stars, inside the Sun, only 450 km (approximately) from its center since the mass of the star is equal to about 334 000 times that of our planet. It is therefore not a big mistake when we say that the Earth travels an elliptical orbit of which the Sun occupies one of the foci.
Not traveling in a circular orbit, even if the eccentricity is small, means that the speed is not uniform: a planet goes faster to perihelion than to aphelion. This variation follows Kepler's second law: the area swept by the Sun-Earth vector ray is constant for equal time intervals, ie the "areolar" velocity is constant. This derives from the conservation of the angular momentum that occurs in the motions due to a central force.
Movements of the Earth
Period of re