How it began
In February 1677 Edmond Halley, a 20-year-old Oxford student, arrived on the island of St Helena in the South Atlantic to survey the southern stars. He had sailed from London in late 1676 with a large sextant with telescopic sights, a smaller quadrant and several refracting telescopes, all paid for by his father. His observatory is thought to have stood on a ridge now called Halley's Mount near the center of the island. He observed for about a year and was back in London in May 1678.
Before it existed
At the end of the previous century Tycho Brahe had measured the positions of almost a thousand stars visible from northern Europe with naked-eye sighting instruments. For the southern sky the only observations were rough and incomplete listings made about 75 years earlier by the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman. John Flamsteed had just begun a telescopic survey of the northern stars at Greenwich, and Halley set out to extend it to the southern celestial pole.
How it works
Halley measured star positions with a sextant fitted with telescopic sights, which made his observations considerably more accurate than those of Tycho. To cover many stars in one year he took some of the best-determined stars of Tycho as reference points, so errors in those positions limited the overall reliability. The finished catalogue listed 341 stars in 24 constellations, including Robur Carolinum, a new constellation of his own devising.
Who pushed back
The weather on St Helena was far cloudier than Halley had hoped and allowed no more than about an hour of observing per night. He complained that his instruments and notebook quickly became soaked with dew. The catalogue therefore held far fewer stars than he had hoped for. Few other astronomers adopted his constellation Robur Carolinum.
What changed first
A star chart was printed by July 1678. In November 1678 Robert Hooke presented the chart and catalogue to the Royal Society, which elected Halley a member that month. The results were published early in 1679 as Catalogus Stellarum Australium. Flamsteed called him the southern Tycho. Halley was also the first to record the variable star now known as Eta Carinae.
What lasted
Abraham Sharp, assistant to Flamsteed, later recalculated the positions of the stars and in 1725 published 265 of them as a southern supplement to the Catalogus Britannicus, which made that work the first all-sky catalogue. Halley was not credited there. A plaque placed in 1977 marks the low stone walls that remain at the observatory site on St Helena.
