For #402, try describing each axis of ICRS
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@ -236,33 +236,28 @@ a permanent frame of reference
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that is a high-precision replacement
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for the old J2000.0 system
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that was popular at the end of the 20th century.
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The orientation of the axes is roughly:
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The ICRS is one of three related concepts
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that you will often see mentioned together
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in technical publications:
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* *x-axis* — points at the Vernal Equinox:
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the position of the Sun in our sky
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at the moment the Sun’s annual journey along the ecliptic
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carries it from south to north across the celestial equator
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and Spring begins in the Northern Hemisphere.
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To express the same thing more technically,
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this axis points towards
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the “ascending node of the ecliptic on the celestial equator”
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as of the beginning of the year 2000.
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In ancient times this point in the sky was called “the first point of Ares.”
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* | *Barycentric Celestial Reference System* (BCRS) —
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a coordinate origin whose relativistic frame of reference
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is the one that was carefully defined in IAU 2000 Resolution B1.3
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which puts the coordinate origin
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at the gravitational center of the Solar System.
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The direction in which the coordinate axes might point
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is left unspecified.
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* *y-axis* — is aimed at the point 90° east of the Vernal Equinox
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on the celestial equator.
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It can be hard to remember which way is east in the sky, so:
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if you were standing at the center of the celestial sphere
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with the north pole above your head
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and the x-axis in front of you,
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this axis would be 90° to your *left.*
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* *International Celestial Reference Frame* (ICRF) —
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a precision reference frame
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that radio astronomers have helped us define,
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that will become forever more exact
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as we measure better and better positions
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for a list of very distant radio sources.
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Wherever the origin of your coordinate system might lie,
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you can use the ICRF to define
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where your *x*-axis, *y*-axis, and *z*-axis should point.
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* *International Celestial Reference System* (ICRS) —
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A coordinate system whose origin is defined by the BCRS
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and whose axis directions are defined by the ICRF.
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In essence, the ICRS = ICRF + BCRS.
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* *z-axis* — points towards the celestial North Pole.
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Instead of using an acronym,
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Skyfield uses the class name :class:`Barycentric`
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