How was the distance to a star first measured?

Astronomers first measured the distance to a star by parallax, the small apparent shift in a star's position as Earth moves around the Sun. The first reliable published measurement was by the German astronomer Friedrich Bessel in 1838, for the star 61 Cygni. He found a shift of 0.314 arc seconds, which placed the star about 10 light-years away. [1] [2]

Parallax is the apparent change in direction of a distant object when the observer changes position. The smaller the parallax angle, the more distant the object. Surveyors on Earth use a baseline between two stations, but baselines of that kind are useless for the stars, which are far too distant. [3]

The baseline for stars comes from Earth's orbit. Astronomers define a star's parallax as half of its total shift between opposite sides of the orbit, so that the baseline is 1 astronomical unit, the average Earth–Sun distance of 149,597,870,700 metres. Nearby stars shift against more distant background stars, and distant stars shift less. [3] [4]

The angles are tiny. Even for the nearest stars the parallax is a fraction of an arc second, and one arc second is 1/3600 of a degree. Robert Hooke described the problems of looking for annual star motion in 1674, but telescopes were not sensitive enough to detect the shifts until the 19th century. One by-product of the search was James Bradley's discovery of the aberration of light in 1725. [3] [2] [1]

Bessel used a heliometer made by Joseph Fraunhofer. This is a telescope whose lens is cut in half, and the halves slide apart to produce two adjustable images, which let him measure very small differences in the relative positions of stars. [1]

Bessel is usually credited, because he was the first to publish a reliable measurement. Two others have claims. Wilhelm Struve published data on Vega in 1837 that were not considered highly reliable, then a new value in 1840 of 0.261 arc seconds, about twice the modern value of 0.130. Thomas Henderson measured Alpha Centauri in the early 1830s at the Cape of Good Hope but published only in 1839, reporting about one arc second. The modern values are roughly 0.755 and 0.797 arc seconds for its two stars. [1]

Accounts differ on how to describe this. One textbook says that Bessel, Henderson and Struve independently measured the parallaxes of 61 Cygni, Alpha Centauri and Vega in 1838. The ESA account gives different publication dates for Struve and Henderson and treats Bessel as the first to publish reliably. [3] [1]

Measuring parallax is not simple. Astronomers must correct for atmospheric refraction and for the effects of seeing, and must account for each star's own proper motion across the sky. [5]