<P> Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects . Created by the different orbital positions of Earth, the extremely small observed shift is largest at time intervals of about six months, when Earth arrives at exactly opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations . The parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of Earth and the Sun, a baseline of one astronomical unit (AU). </P> <P> Stellar parallax is so difficult to detect that its existence was the subject of much debate in astronomy for thousands of years . It was first observed by Giuseppe Calandrelli who reported parallax in α - Lyrae in his work "Osservazione e riflessione sulla parallasse annua dall'alfa della Lira". Then in 1838 Friedrich Bessel made the first successful parallax measurement ever, for the star 61 Cygni, using a Fraunhofer heliometer at Königsberg Observatory . </P>

Measuring parallax from earth gives us a maximum baseline distance of approximately