Although astrometry is over 2,200 years old, it wasn’t until 1807 when we truly realised just how far away stars really were.
In December 2013, the European Space Agency (ESA) launched the Gaia space telescope to survey a billion stars in the Milky Way galaxy. Gaia continues a long-standing global culture to catalog the positions, distances and motions of stars in detail.
Astrometry is one of the oldest natural sciences and dates back to the father of trigonometry, Hipparchus. Hipparchus published the first extensive star catalog in 190 BC, containing the positions of 850 stars. He only had an astrolabe and an armillary sphere at his disposal.
Hipparchus’s star catalog was used for many centuries – only to be superseded in 1627 by Tycho Brahe. Brahe’s catalog consisted of the positions of a thousand stars, with a precision of half-arc-minute. (There are 60 arc-minutes in one degree. The Moon occupies about half a degree, i.e. 30 arc-minutes, in the night sky.) This was a hundred-fold improvement over Hipparchus’s measurements. It was possible because Brahe used more sophisticated instruments: quadrants and sextants.
Star catalogs were not just of scientific value. They were also extremely useful for precision sea navigation, in a time of frenzied colonisation. This prompted multiple European groups to publish expanded, and increasingly precise, star catalogs.
Then came the telescope.
This device could allow skygazers to measure the apparent changes in a star’s position against a fixed background, as Earth moved around the Sun.