detectable using Bradley's instruments.

For stars near the pole of the ecliptic - for which the velocity of the Earth is always perpendicular to the Earth-star line - the effect of this so-called stellar aberration is a deviation from the star's mean position of constant magnitude, with the direction changing as the Earth goes round its orbit. Thus, such stars trace out small circles of radius 20.5". For stars in the ecliptic plane, the displacement vanishes when the Earth is moving directly toward or away from the star, and is a maximum when the Sun, Earth and star are collinear. Thus, such stars oscillate backward and forward about their mean position, the maximum displacement being 20.5". Stars between these extremes move round ellipses, the eccentricity of which depends on the ecliptic latitude of the star, but with semi-major axes always of 20.5", a quantity known as the 'constant of aberration'.

In 1728, Bradley determined that the phenomenon was due to a combination of the motion of the Earth relative to the 'fixed' stars, and the fact that light is

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