ABSTRACT
The phenomenon of the parallax is caused by the finite distance of a planet or star from the observer. Of all the phenomena described so far altering the apparent direction of the source, the parallax is the first to give direct information on the distance and nature of the heavenly body, not only on the motions of the observer nor on fundamental properties of light and of space-time. The determination of the parallaxes is a fundamental astronomical measurement. The direct determination of the solar parallax πʘ poses great difficulties, due to its small amount and the lack of precise reference points on the surface. The methods based on Kepler’s third law provide the mass of the Moon in addition to the solar parallax, because it is the Earth–Moon barycenter that follows the Keplerian orbit around the Sun. Annual parallaxes are the fundamental method for the direct determination of the stellar distances.
