Astronomy Answers: From the Astronomical Dictionary

# Astronomy AnswersFrom the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the precession

praecessio = [Latin] going first

The rotation axis of the Earth is slowly spinning around the poles of the ecliptic because of the gravitational attraction between the equatorial bulges of the Earth and the Moon and Sun. It takes about 26,000 years before the axis has spun around once. This spinning motion is called precession.

One result of the precession is that the vernal equinox slowly moves between the stars in the sky, at a speed of one degree each 71.6 years. This motion is called the precession of the equinoxes. The places between the stars in the sky where the Sun is at the beginning of the seasons are tied to the vernal equinox, and so are also influenced by the precession. Since about the year −68, the vernal equinox moves through the modern constellation of Pisces (the Fishes), and it will move into the constellation of Aquarius (the Waterman) around the year 2597.

Another result of the precession is that the poles of the sky also move between the stars. At the moment, the celestial north pole is quite close to the bright star alpha Ursae Minoris (alpha UMi, also called Polaris or the North Star).