Precession of the Earth's Axis
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Every calendar year, the winter solstice occurs at a slightly different point on the Earth's orbit around the Sun. The same is true for the summer solstice, or either of the equinoxes. This is due to the "precession" of the Earth's axis, which has a period of 25,772 years. After half that period, or 12,886 years, the winter solstice occurs when the Earth is at a point on its orbit opposite that of 12,886 years earlier. The night sky for a given day of the calendar year is changed significantly throughout the 25,772-year period of precession. The Earth's orbit around the sun is elliptical, with a very small eccentricity.[more]
The perihelion (closest to the Sun) and aphelion (farthest from the Sun) also precess such that they drift from the solstice. This precession has a period of about 100,000 years. Some important dates are 1246 CE, in which year the winter solstice and the perihelion coincided (shown as ); 21,982 CE, the year in which they will again coincide; and 27,018, the year in which the Earth will be back at at the winter solstice. In this Demonstration, "year" is the CE year within the Gregorian calendar and "day" is the day within a given year. By setting the day to 1, you can see the precession of the Earth's axis throughout the 25,772-year cycle, starting with the year 1246 CE.[less]
Contributed by: David von Seggern (July 2015)
(University of Nevada)
Open content licensed under CC BY-NC-SA
 Wikipedia. "Axial Precession." (Jul 13, 2015.) en.wikipedia.org/wiki/Axial_precession.
 The United States Naval Observatory. "The Seasons and the Earth's Orbit." (Jul 13, 2015.) aa.usno.navy.mil/faq/docs/seasons_orbit.php.
 NOAA Paleoclimatology. "Orbital Dynamics." (Jul 13, 2015.) www.ncdc.noaa.gov/paleo/ctl/clisci100ka.html.