John Senex’s north celestial hemisphere of 1721/22

John Senex’s celestial charts

Astronomers in the early 18th century lacked good charts of the heavens, for celestial mapping was still in its infancy. The best generally available source of star positions was Tycho Brahe’s catalogue of a thousand stars as edited by Johannes Kepler and published in the Rudolphine Tables of 1627, which formed the basis of most charts and globes of the time.

It was hoped that better data would soon be forthcoming when John Flamsteed (1646–1719) was appointed Astronomer Royal in 1675 to produce the first telescopic star catalogue from the new observatory at Greenwich, but he was still at work on it 30 years later. In the meantime, Johannes Hevelius in Poland had produced a naked-eye catalogue of some 1,500 stars which, along with an atlas, was published posthumously in 1690, but this was rare, bulky, and expensive. As Smithsonian historian Deborah Jean Warner eloquently summarized it in her book The Sky Explored (1979): ‘The charts of Seller[note] were unsatisfactory, those of Hevelius were unobtainable, and Flamsteed was reluctant to issue an imperfect copy of his great work.’

While Flamsteed delayed, the need was filled by a surreptitious joint effort between Edmond Halley and Isaac Newton. Newton obtained a preliminary copy of the catalogue being prepared at Greenwich by Flamsteed. Halley, a close colleague of Newton’s, edited this catalogue without Flamsteed’s approval and published it in 1712 under the title Historiae Coelestis (subtitled Stellarum Fixarum Catalogus Britannicus). So enraged was Flamsteed by this blatant piracy that he bought up and destroyed about three-quarters of the 400 copies printed.

Zodiacus Stellatus

Flamsteed might have hoped that would be the end of it, but the data from the bootleg catalogue lived on in a series of popular charts produced by the London cartographer John Senex (1678–1740). The first of these, published in 1718, was a chart of the zodiac drawn in three strips across two sheets, of particular use for following the passage of the Moon and planets in front of the stars. As well as astronomy these had practical application in navigation, since at that time the method of lunar distances, which involved measuring the position of the Moon against the star background, was the favoured method of finding longitude at sea. They were only the second zodiac charts to be produced, and far superior to Seller’s first efforts of 1679.

Senex was careful to conceal the source of his star data, saying in a note at the foot that he had ‘procured a Copy of the Britannick Catalogue of Fixt Stars’ from ‘a Person of Quality’. There was, though, no doubt who the ‘Person of Quality’ was. As Flamsteed’s long-time assistant Joseph Crosthwait wrote disapprovingly from Greenwich in 1720, the year after Flamsteed’s death: ‘Senex is so much a tool of Dr. Halley's, and affronted Mr. Flamsteed so much in his life-time, by engraving the Zodiacus Stellatus, and putting his own name to it, in order to screen Dr. Halley from the law.’ Senex and Halley were already well acquainted, for Senex had published Halley’s famous book Synopsis of the Astronomy of Comets in 1705.

Celestial hemispheres

Two celestial hemispheres followed three or four years later (they were being engraved in 1721, but which year they were published is uncertain). The northern chart claimed to show ‘all the Stars contain’d in the Britannick Catalogue (as Publish’d by Dr. Halley)’. On the southern chart, the Flamsteed stars were supplemented by those catalogued by Halley himself from St Helena in 1677/78.

According to the heading, the stars on this southern chart were ‘carefully layd down … by Joseph Harris’, in reference to the young Welsh astronomer Joseph Harris (1703/4–64). However, he is not recorded as working with Senex until early 1725, having moved to London at the end of 1724, so it may be that this southern chart was produced some years after the northern one. Halley was by then Astronomer Royal, having succeeded Flamsteed in 1720, and hence no longer available to help. These hemispheres were centred on the celestial poles, but Senex also produced a pair of hemispheres centred on the ecliptic poles (see here and here); on these, the constellation figures were drawn with fainter outlines.

Senex’s charts proved popular among astronomers and mariners, being both affordable and readily obtainable, and were reprinted and reissued for the rest of the century. William Herschel was using them when he discovered the planet Uranus in 1781 (he later bought a copy of Flamsteed’s official Atlas Coelestis). Herschel, incidentally, referred to them as ‘Harris’s Sheet Hemispheres’ and ‘Harris’s maps’ but he misunderstood their true authorship: Harris’s involvement was restricted to the southern half of the pair, and might only have involved adding Halley’s observations of the southern stars. His name was not mentioned at all on the ecliptic charts.

A large planisphere

Senex’s last chart appeared posthumously in 1746. This was a planisphere some two feet (60 cm) across centred on the north celestial pole and extending down to –38°, thereby showing all the sky visible from London during the year. (Note: A label at bottom left wrongly captions it ‘The southern hemisphere projected on the plane of the ecliptic’, whereas it was in fact the northern hemisphere and the projection was equatorial, with the celestial pole at the centre.) A companion sheet contained a circular mask that could be cut out and placed on the planisphere to determine the rising and setting times of the stars, as in the modern planispheres used by amateur astronomers.

Whereas the star positions in the earlier hemispheres had an epoch of 1690, on this planisphere they were updated to 1740, the year Senex died; presumably he had completed, or nearly completed the work by the time he died. His engraved plates were obtained from Senex’s widow by the London cartographers John Bowles and his family and Robert Sayer who continued to reissue his work. This time there was no question of piracy: the planisphere proudly proclaims that it was published ‘according to Act of Parliament September 10, 1746’. 

Occasionally the various Senex sheets were bound together to form a quasi-atlas, but mostly they were sold separately. As a consequence, few have survived to remind us of their role, and that of Senex, in the development of astronomy in the 18th century.

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John Senex’s north and south celestial hemispheres of 1721/22, based on Edmond Halley’s pirated catalogue of Flamsteed’s observations published in 1712.
On the northern chart Senex introduced the figure Ramus Cerberus, held in the grasp of Hercules. (Images courtesy Daniel Crouch Rare Books)

John Senex’s south celestial hemisphere of 172?