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Celestial Ephemeris Pole

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Title Celestial Ephemeris Pole
Author(s) J. Sanz Subirana, J.M. Juan Zornoza and M. Hernández-Pajares, Technical University of Catalonia, Spain.
Level Basic
Year of Publication 2011

This is also a quasi-inertial system, like the Conventional Celestial Reference System (CRS). The difference between the two systems is that the pole of this one is the true one (i.e., instantaneous earth's rotation axis). Its origin is the Earth's centre of mass, its fundamental plane is the one defined by the true pole, therefore is the true equator, and its principal direction is toward the true vernal equinox (i.e., the intersection between the mean ecliptic plane and the true equator plane).

If all external forces were null, Celestial Ephemeris Pole (CEP) would be a fixed pole in Space (i.e., it would not precess or nutate). Nevertheless the position of CEP relative to the earth's crust would change due to the irregular mass distribution of the earth and its variation. But, if the earth were spherical and homogeneous, it would be a fixed pole in Space, whatever the external forces be null of not, and also on the earth's surface. This pole is a suitable reference for the external observer.

The International Astronomical Union (IAU) resolution B1.7 recommended that after 1 January 2003, the CEP be replaced by the Celestial Intermediate Pole(CIP). The CIP definition is an extension of that of the CEP in high frequency domain and coincides with that of the CEP in the low frequency domain [Denis et al., 2004] [1]. It is consistent with the IAU2000A model for precession and nutation.


  1. ^ [Denis et al., 2004] Denis, D., McCarthy and Petit, G., 2004. IERS Conventions (2003). IERS Technical Note 32. IERS Convention Center, Frankfurt am Main.