Until 1983, the angular value of the obliquity for any date was calculated based on the work of Newcomb, who analyzed positions of the planets until about 1895: Astronomers produce new fundamental ephemerides as the accuracy of observation improves and as the understanding of the dynamics increases, and from these ephemerides various astronomical values, including the obliquity, are derived. The angular value of the obliquity is found by observation of the motions of the Earth and planets over many years. Currently about 23.4°, it varies slightly due to changes in the plane of the Earth's orbit with planetary precession. Obliquity of the ecliptic is a name used by astronomers for the inclination of Earth's equator to the ecliptic, or of Earth's rotation axis to a perpendicular to the ecliptic.
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