Flamsteed, Atlas coelestis

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Flamsteed was England’s first Astronomer Royal, charged with improving star positions accurately enough to determine longitude at sea. In 1676, he completed the building of the Greenwich Observatory. 

Flamsteed’s star atlas, posthumously published, became the most celebrated and influential star atlas of the 18th century. At the time, it was the largest star atlas ever printed. Its 28 copperplate engravings include 25 double-page star charts and 2 double-page planisphere maps. 

Constellation figures are viewed from the front, matching the traditional names of stars. Sir James Thornhill was among the artists who designed the constellation figures in a Rococo style that was soon copied in Paris and Berlin. 

Flamsteed determined star positions using observing instruments equipped with telescopic sights (a first among major atlases). 

Isaac Newton relied on Flamsteed’s star coordinates, made available to him at an earlier date, for his theory of universal gravitation and explanation of the motion of the Moon. 

More than 3,000 stars are presented, double the number in Hevelius.

A Note on Celestial Coordinates

Earlier atlases were based on the great circle of the Sun’s annual path around the sky, called the “ecliptic.” Celestial “longitude” measures in degrees along the ecliptic, and celestial “latitude” measures in degrees perpendicular to the ecliptic. The ecliptic is angled at 23.5 degrees from the Earth’s equator. Because the ecliptic and the Earth’s equator do not coincide, celestial latitude and celestial longitude do not coincide with terrestrial latitude and longitude.

Flamsteed’s atlas was the first major atlas to use a grid based not on the ecliptic, but on the Earth’s geographical coordinates. Flamsteed thus introduced the convention now used in modern star atlases where the Earth’s equator, projected into the sky as the celestial equator, is (now) marked off in “hours” and “minutes” of “Right Ascension,” corresponding to terrestrial longitude. The Earth’s latitude circles are projected into the sky as circles of “Declination,” measured in degrees north or south of the celestial equator.