Editing Axial precession (section)

==Effects==
[[File:Earth axial precession.svg|thumb|300px|The coincidence of the annual cycles of the apses (closest and further approach to the Sun) and calendar dates (with seasons noted) at four equally spaced stages of a fictitious precessionary cycle of 20,000 years (rather than the Earth's true precessionary cycle of 26,000 years). The season dates are those in the north. The tilt of fictitious Earth's axis and the eccentricity of its orbit are exaggerated. Approximate estimates. Effects of weak planetary precession on the stages shown are ignored.]]
The precession of the Earth's axis has a number of observable effects. First, the positions of the south and north [[celestial pole]]s appear to move in circles against the space-fixed backdrop of stars, completing one circuit in approximately 26,000 years. Thus, while today the star [[Polaris]] lies approximately at the north celestial pole, this will change over time, and other stars will become the "[[pole star|north star]]".<ref name="Astro 101"/> In approximately 3,200 years, the star [[Gamma Cephei]] in the Cepheus constellation will succeed Polaris for this position. The south celestial pole currently lacks a bright star to mark its position, but over time precession also will cause bright stars to become [[South Star]]s. As the celestial poles shift, there is a corresponding gradual shift in the apparent orientation of the whole star field, as viewed from a particular position on Earth.

Secondly, the position of the Earth in its orbit around the Sun at the [[solstice]]s, [[equinox]]es, or other time defined relative to the seasons, slowly changes.<ref name="Astro 101"/> For example, suppose that the Earth's orbital position is marked at the summer solstice, when the Earth's [[axial tilt]] is pointing directly toward the Sun. One full orbit later, when the Sun has returned to the same apparent position relative to the background stars, the Earth's axial tilt is not now directly toward the Sun: because of the effects of precession, it is a little way "beyond" this. In other words, the solstice occurred a little ''earlier'' in the orbit. Thus, the [[tropical year]], measuring the cycle of seasons (for example, the time from solstice to solstice, or equinox to equinox), is about 20 minutes shorter than the [[sidereal year]], which is measured by the Sun's apparent position relative to the stars. After about 26&nbsp;000 years the difference amounts to a full year, so the positions of the seasons relative to the orbit are "back where they started". (Other effects also slowly change the shape and orientation of the Earth's orbit, and these, in combination with precession, create various cycles of differing periods; see also [[Milankovitch cycles]]. The magnitude of the Earth's tilt, as opposed to merely its orientation, also changes slowly over time, but this effect is not attributed directly to precession.)

For identical reasons, the apparent position of the Sun relative to the backdrop of the stars at some seasonally fixed time slowly regresses a full 360° through all twelve traditional constellations of the [[zodiac]], at the rate of about 50.3 [[Arcsecond|seconds of arc]] per year, or 1 degree every 71.6 years.

At present, the rate of precession corresponds to a period of 25,772 years, so a tropical year is shorter than a sidereal year by 1,224.5 seconds {{nowrap|(20 min 24.5 sec &asymp; (365.24219 &times; 86400) / 25772).}}

The rate itself varies somewhat with time (see [[#Values|Values]] below), so one cannot say that in exactly 25,772 years the Earth's axis will be back to where it is now.

For further details, see [[#Changing pole stars|Changing pole stars]] and [[#Polar shift and equinoxes shift|Polar shift and equinoxes shift]], below.