page_title: Stellar parallax
section_title: 19th and 20th centuries
hierarchical_section_title: Stellar parallax / 19th and 20th centuries
caption_attribution_description: English: EB1911 - Heliometer - Fig. 10
context_page_description: Stellar parallax is the apparent shift of position of any nearby star 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 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.
  Stellar parallax is so difficult to detect that its existence was the subject of much debate in astronomy for hundreds of years. Friedrich Bessel made the first successful parallax measurement in 1838, for the star 61 Cygni, using a Fraunhofer heliometer at Königsberg Observatory.
context_section_description: 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 angle subtended at a star by the mean radius of Earth's orbit around the Sun. The parsec (3.26 light-years) is defined as the distance for which the annual parallax is 1 arcsecond. Annual parallax is normally measured by observing the position of a star at different times of the year as Earth moves through its orbit. Measurement of annual parallax was the first reliable way to determine the distances to the closest stars. The first successful measurements of stellar parallax were made by Friedrich Bessel in 1838 for the star 61 Cygni using a heliometer.
  Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, mostly by use of the filar micrometer.  Astrographs using astronomical photographic plates sped the process in the early 20th century.  Automated plate-measuring machines and more sophisticated computer technology of the 1960s allowed more efficient compilation of star catalogues. In the 1980s, charge-coupled devices (CCDs) replaced photographic plates and reduced optical uncertainties to one milliarcsecond.
  Stellar parallax remains the standard for calibrating other measurement methods (see Cosmic distance ladder). Accurate calculations of distance based on stellar parallax require a measurement of the distance from Earth to the Sun, now known to exquisite accuracy based on radar reflection off the surfaces of planets.
  The angles involved in these calculations are very small and thus difficult to measure. 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. This angle is approximately that subtended by an object 2 centimeters in diameter located 5.3 kilometers away.
  A large heliometer was installed at Kuffner Observatory (In Vienna) in 1896, and was used for measuring the distance to other stars by trigonometric parallax. By 1910 it had computed 16 parallax distances to other stars, out of only 108 total known to science at that time.
caption: Diagram of a heliometer from the 1911 Encyclopædia Britannica, which would be a view looking towards the split lens of a heliometer
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