Nicolas-Louis La Caille (born 15 March 1713), also known as Lacaille, came to the Observatoire de Paris in 1736/7, when the observatory was under the leadership of Jacques Cassini (1677-1756), son of Jean-Dominique (1625-1712).

Longitudes can be accurately determined on Earth by using the timings of the eclipses of Galilean satellites of Jupiter, as previously shown on the west coast of France and discussed in works on clocks by Huygens (1629-1695) in the Low Countries (1655) and tables compiled by Jean-Dominique Cassini (1668).

Following the foundation of the Royal Observatory in Greenwich in 1675, ‘in order to the finding out of longitude of places for perfecting navigation and astronomy’, John Flamsteed (1646-1719), the first Astronomer Royal, began to work under the King’s authority in the Octagon Room of the Observatory, employing Tompion’s clocks, in view of the determination of the longitudes at sea in employing the method of the lunar distances.

In the mid-1730s, two expeditions under the command of the Académie Royale des Sciences left France to reach as close as possible to the equator (in Peru) and the north pole. The 1735 Peru expedition was equipped with a petit cadran anglais (a new Hadley’s quadrant or octant). The Lapland expedition (1736-1737) was followed by Lacaille’s new measurements of the Méridienne de France (1739/1740) after an expedition (1738) along the west coast of France to accurately determine both longitude and latitude.

Following Edmond Halley, (1658-1749), who twice travelled in the southern hemisphere, Lacaille headed for South Africa; while aboard, he tested the lunar distance method. During his voyage, Lacaille determined the positions of about 10,000 stars, leading him being called ‘Father of the Southern Skies’, and also determined the longitudes and latitudes of every place he visited, including islands, for the benefit of navigators: Bourbon, de France, Ascension, nowadays La Réunion, Mauritius. He also contributed to the determination of the Earth-Moon distance, in cooperation with Lalande (1732-1807), observing from Berlin.

Lacaille’s Cœlum Australe Stelliferum was published the year after his death (21 March 1762). Having reduced only a small part of his observations of stars before his death, aged 49, the British Association for the Advancement of Science decided to reduce and publish the catalogue of his 9766 stars in 1847. With his large sextant, having a power larger than Halley’s instrument, Lacaille could view a much greater number of stars, leading to an anglo-french cooperation with his catalogue in the service of navigation in the southern hemisphere.