The exact instants of equinoxes or solstices are the times when the apparent ecliptic longitude (including the effects of aberration and nutation) of the Sun is 0°, 90°, 180°, or 270°.
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This discovery of what became known as the aberration of light was, for all realistic purposes, conclusive evidence for the movement of the Earth, and hence for the correctness of Aristarchus' and Kepler's theories; it was announced to the Royal Society in January 1729 (Phil. Trans. xxxv. 637).