rereading an old book of navigazoione air, I came across a curiosity which perhaps says a lot about that hint of change in seasons that our grandfathers, our fathers and ourselves complain. In fact, the Earth, in addition to the known rotation around its axis, the effect is the alternation of day and night and the movement of revolution, a period of one year and counterclockwise, giving rise to the seasons, makes many other changes, but one in particular is interesting. And 'the movement that the Earth rotates over 26,000 years, called "precession of the equinoxes" (slow movement of the axis of rotation). This movement consists of a rotation of the Earth that, while continuing to maintain the same inclination to the ecliptic (apparent path described by the Sun on the celestial sphere, in his year course), deviates by hand by hand from North Star, at which it is today and what will end up between 26,000 years, while having described a cone, called precession. The effect is the sliding of the knots equinoctial (curved line, which combines the imaginary spring and autumn equinoxes along the trajectory of the Sun) of about 50''grade (1.5 km) each year along the 'ecliptic, in consequence of which there will be a shift of the seasons compared to the months of the year: that is, between 13,000 years the equinox (one of two opposite sides of the sky, where the length of day and night is equal to) Spring will fall on September 23, because its nodal point will have made around half of the ecliptic, while the autumnal equinox falls on March 21.
Now, even if they consider me immortal, I think I have some difficulty in seeing this change, however, it may already be underway. Too bad, I would have liked ...!
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