The right ascension and declination are defined relative to the plane of the Earth’s equator and to the vernal equinox. Unfortunately, neither of these is fixed in the sky because of precession. The Earth is not quite spherical but is slightly oblate. Because of this, the gravitational force between the Earth and the Sun produces a couple which causes the axis of rotation of the Earth to PRECESS. The Earth’s rotation axis keeps a constant angle to the plane of the ecliptic (the plane of the Earth’s orbit round the Sun), but the direction of the rotation axis slowly changes so that it completes one revolution in about 26,000 years. This makes it imperative that a measured position is recorded together with the date or EPOCH of the measurement. The epochs of 1900.0 and 1950.0 have been used extensively: 1950.0 refers to a position on 1 January 1950. The amounts by which right ascension (RA or a) and declination (Dec or 6) change are given approximately by
RA change = 3.074 + 1.336 sin ? tan ? seconds of time per year
Dec change = 20.041 cos ? arc seconds per year.
Earth’s axis about its mean position. These are largely due to the influence of the Moon and planets on the orbit of the Earth. The principal component of this nodding motion, called NUTATION, causes objects to be displaced by a maximum of 9 arc seconds with a period of 18.6 years.
The motion of the Earth on its orbit causes small changes in the apparent position of an astronomical object. This change is known as STELLAR ABERRATION and causes a maximum shift of about 20 arc seconds when the direction of motion of the Earth is perpendicular to the direction to the object.