From 72ff2c2348c6f814bab70bba0a0fc08ecd9f05a6 Mon Sep 17 00:00:00 2001 From: Brandon Rhodes Date: Thu, 9 Jul 2020 21:09:29 -0400 Subject: [PATCH] For #402, try describing each axis of ICRS --- skyfield/documentation/positions.rst | 43 ++++++++++++---------------- 1 file changed, 19 insertions(+), 24 deletions(-) diff --git a/skyfield/documentation/positions.rst b/skyfield/documentation/positions.rst index 501773c..07b2b74 100644 --- a/skyfield/documentation/positions.rst +++ b/skyfield/documentation/positions.rst @@ -236,33 +236,28 @@ a permanent frame of reference that is a high-precision replacement for the old J2000.0 system that was popular at the end of the 20th century. +The orientation of the axes is roughly: -The ICRS is one of three related concepts -that you will often see mentioned together -in technical publications: +* *x-axis* — points at the Vernal Equinox: + the position of the Sun in our sky + at the moment the Sun’s annual journey along the ecliptic + carries it from south to north across the celestial equator + and Spring begins in the Northern Hemisphere. + To express the same thing more technically, + this axis points towards + the “ascending node of the ecliptic on the celestial equator” + as of the beginning of the year 2000. + In ancient times this point in the sky was called “the first point of Ares.” -* | *Barycentric Celestial Reference System* (BCRS) — - a coordinate origin whose relativistic frame of reference - is the one that was carefully defined in IAU 2000 Resolution B1.3 - which puts the coordinate origin - at the gravitational center of the Solar System. - The direction in which the coordinate axes might point - is left unspecified. +* *y-axis* — is aimed at the point 90° east of the Vernal Equinox + on the celestial equator. + It can be hard to remember which way is east in the sky, so: + if you were standing at the center of the celestial sphere + with the north pole above your head + and the x-axis in front of you, + this axis would be 90° to your *left.* -* *International Celestial Reference Frame* (ICRF) — - a precision reference frame - that radio astronomers have helped us define, - that will become forever more exact - as we measure better and better positions - for a list of very distant radio sources. - Wherever the origin of your coordinate system might lie, - you can use the ICRF to define - where your *x*-axis, *y*-axis, and *z*-axis should point. - -* *International Celestial Reference System* (ICRS) — - A coordinate system whose origin is defined by the BCRS - and whose axis directions are defined by the ICRF. - In essence, the ICRS = ICRF + BCRS. +* *z-axis* — points towards the celestial North Pole. Instead of using an acronym, Skyfield uses the class name :class:`Barycentric`