That apparent change in direction of a star, planet, or other celestial body which is caused principally by the Earth's revolution about the Sun. That part of stellar aberration which has an annual period. (The Sun's barycentric motion combines with the Earth's revolution to cause a very small aberration which is also covered by the term. ) During the year, a star in the plane of the Earth's orbit (the ecliptic) appears to move from one end of a straight line to the other and back again. The length of this line is twice the constant of aberration. A star 90° from the ecliptic appears to move in a circle which has a diameter twice the constant of aberration. Stars at intermediate angles above or below the ecliptic appear to move in ellipses with major diameters twice the constant of aber-ration and with minor axes getting longer with increasing angular distance of the star from the ecliptic. Because the Earth moves in an elliptical rather than a circular orbit, the apparent paths of stars not on the ecliptic are not quite elliptical.