Editing Axial precession (section)

==Polar shift and equinoxes shift==
[[Image:Outside view of precession.jpg|thumb|upright=1.3|Precessional movement as seen from 'outside' the celestial sphere]]
[[File:Precession animation small new.gif|right|thumb|upright=1.3|The 25,700&nbsp;year cycle of precession as seen from near the Earth. The current north [[pole star]] is [[Polaris]] (top). In about 8,000&nbsp;years it will be the bright star [[Deneb]] (left), and in about 12,000&nbsp;years, [[Vega]] (left center). The Earth's rotation is not depicted to scale – in this span of time, it would actually rotate over 4&nbsp;million times.]]

The images at right attempt to explain the relation between the precession of the Earth's axis and the shift in the equinoxes. These images show the position of the Earth's axis on the ''[[celestial sphere]]'', a fictitious sphere which places the stars according to their position as seen from Earth, regardless of their actual distance. The first image shows the celestial sphere from the outside, with the constellations in mirror image. The second image shows the perspective of a near-Earth position as seen through a very wide angle lens (from which the apparent distortion arises).

The rotation axis of the Earth describes, over a period of 25,700&nbsp;years, a small {{blue|blue circle}} among the stars near the top of the diagram, centered on the [[ecliptic coordinates|ecliptic]] north pole (the {{blue|blue letter '''E'''}}) and with an angular radius of about 23.4°, an angle known as the ''[[obliquity of the ecliptic]]''. The direction of precession is opposite to the daily rotation of the Earth on its axis. The {{brown|brown axis}} was the Earth's rotation axis 5,000&nbsp;years ago, when it pointed to the star [[Thuban]]. The yellow axis, pointing to Polaris, marks the axis now.

The equinoxes occur where the celestial equator intersects the ecliptic (red line), that is, where the Earth's axis is perpendicular to the line connecting the centers of the Sun and Earth.The term "equinox" here refers to a point on the celestial sphere so defined, rather than the moment in time when the Sun is overhead at the Equator (though the two meanings are related). When the axis ''[[precession|precesses]]'' from one orientation to another, the equatorial plane of the Earth (indicated by the circular grid around the equator) moves. The celestial equator is just the Earth's equator projected onto the celestial sphere, so it moves as the Earth's equatorial plane moves, and the intersection with the ecliptic moves with it. The positions of the poles and equator ''on Earth'' do not change, only the orientation of the Earth against the fixed stars.

<span id="equinox_shift_rate_anchor" class="anchor"></span>
[[File:Equinox path.png|left|upright=2|thumb|Diagram showing the westward shift of the [[March equinox]] among the stars over the past 6,000&nbsp;years.]]
As seen from the {{brown|brown grid}}, 5,000&nbsp;years ago, the [[March equinox]] was close to the star [[Aldebaran]] in [[Taurus (constellation)|Taurus]]. Now, as seen from the yellow grid, it has shifted (indicated by the {{red|red arrow}}) to somewhere in the constellation of [[Pisces (constellation)|Pisces]].

Still pictures like these are only first approximations, as they do not take into account the variable speed of the precession, the variable [[Axial tilt|obliquity]] of the ecliptic, the planetary precession (which is a slow rotation of the [[ecliptic plane]] itself, presently around an axis located on the plane, with longitude 174.8764°) and the proper motions of the stars.

The precessional eras of each constellation, often known as "''Great Months''",<!--essentially the same thing as Astrological Ages--> are given, approximately, in the table below:<ref>{{cite book |last=Kaler |first=James B. |author-link=James B. Kaler |year=2002 |title=The Ever-Changing Sky: A guide to the celestial sphere |type=Reprint |page=152 |publisher=Cambridge University Press |isbn=978-0521499187 |url=https://books.google.com/books?id=KYLSMsduNqcC&pg=PA152}}</ref>

{| class=wikitable
! rowspan=2 | Constellation
! colspan=2 | Approximate year
|-
! Entering
! Exiting
|-
| Taurus
| 4500 BC
| 2000 BC
|-
| Aries
| 2000 BC
| 100 BC
|-
| Pisces
| 100 BC
| 2700
|-
| [[Age of Aquarius|Aquarius]]
| 2700
| 5300
|}