Hubble precision stellar distance measurement has been extended 10 times further into the Milky Way. By 1910 it had computed 16 parallax distances to other stars, out of only 108 total known to science at that time. Ī large heliometer was installed at Kuffner Observatory (In Vienna) in 1896, and was used for measuring the distance to other stars by trigonometric parallax. Those three results, two of which were measured with the best instruments at the time (Fraunhofer great refractor used by Struve and Fraunhofer heliometer by Bessel) were the first ones in history to establish the reliable distance scale to the stars. Henderson published his results in 1839, after returning from South Africa. Friedrich Bessel, a friend of Struve, carried out an intense observational campaign in 1837-1838 at Koenigsberg Observatory for the star 61 Cygni using a heliometer, and published his results in 1838. Few years later ― 1835-1836 ― followed Friedrich Georg Wilhelm von Struve at Dorpat (nowadays Tartu) university observatory, who measured the distance of Vega and published his results in 1837. The first successful stellar parallax measurements were done by Thomas Henderson in Cape Town South Africa in 1832-1833, where he measured parallax of one of the closest stars ― alpha Centauri. In the second quarter of the 19 th century, technological progress reached to the level which provided sufficient accuracy and precision for stellar parallax measurements. Measurement of annual parallax was the first reliable way to determine the distances to the closest stars. This angle is approximately that subtended by an object 2 centimeters in diameter located 5.3 kilometers away. The nearest star to the Sun (and also the star with the largest parallax), Proxima Centauri, has a parallax of 0.7685 ± 0.0002 arcsec. The angles involved in these calculations are very small and thus difficult to measure. Annual parallax is normally measured by observing the position of a star at different times of the year as Earth moves through its orbit. The parsec (3.26 light-years) is defined as the distance for which the annual parallax is 1 arcsecond. the angle subtended at a star by the mean radius of Earth's orbit around the Sun. Stellar parallax is most often measured using annual parallax, defined as the difference in position of a star as seen from Earth and Sun, i.e. The split lens of the Bamberg Heliometer (late 19th century) Thomas Henderson, Friedrich Georg Wilhelm von Struve, and Friedrich Bessel made first successful parallax measurements in 1832-1838, for the stars alpha Centauri, Vega, and 61 Cygni. Stellar parallax is so difficult to detect that its existence was the subject of much debate in astronomy for hundreds of years. 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). 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 opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations. Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects. (1 AU and 1 parsec are not to scale, 1 parsec = ~206265 AU) Stellar parallax is the basis for the parsec, which is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond.

Parallax angle