John Flamsteed
1646-1719
English Astronomer
John Flamsteed is remembered for his accurate and extensive work in positional astronomy. As the first British Astronomer Royal, he pioneered the systematic use of telescopic sights and produced the first great star catalog based on telescopic observations.
Flamsteed was born on August 19, 1646, at Denby, England. He attended the Derby Free School but was forced to leave when stricken by a severe rheumatic condition. So serious was the illness that his health was compromised for the rest of his life. During his extended period of convalescence Flamsteed developed an interest in astronomy, which he perused through self-study (1662-69).
In 1670 he entered his name at Jesus College, Cambridge, to pursue an M.A. degree. That same year he submitted a small ephemeris of lunar occultations to the Royal Society that was published in the Philosophical Transactions. This gained him some recognition and led to his acquaintance with Jonas Moore, who presented him with a micrometer and high quality telescopic lens. These instruments enabled Flamsteed to begin serious observational work that firmly established his reputation before his graduation from Cambridge (1674).
One of the major problems of this time was the accurate determination of longitude at sea. This was of particular interest to England, whose merchant fleet was quickly becoming the world's largest. Calculating longitude required comparing local time with a standard reference time. It had been suggested that the Moon's motion against the stellar background could be used to determine this standard. Flamsteed was asked by Moore to comment on a proposed method for exploiting this suggestion. Flamsteed's judgment was that no method would work until stellar and lunar positions were more accurately known. Moore used this assessment to successfully petition King Charles II to establish a national observatory at Greenwich and appoint Flamsteed the first Astronomer Royal on March 4, 1675.
John Flamsteed. (Library of Congress. Reproduced with permission.)Greenwich Observatory was completed in 1676, but Flamsteed was given only a meager salary and no provisions for assistants or instruments. Moore eased the situation somewhat by donating 2 clocks and a 7-foot (2.1-meter) sextant. Nevertheless, Flamsteed worked alone for the first 13 years, recording over 20,000 observations. His observations were six times more accurate than Tycho Brahe's (1546-1601), and he was the first astronomer to make systematic use of clocks for taking measurements. An inheritance from his father's estate solved his financial problems and allowed him to commission a 140-degree mural arc with which he precisely determined Greenwich's latitude, the ecliptic's obliquity, and the position of the equinox. He then devised a method for observing absolute right ascensions—the celestial equivalent of terrestrial longitude—that removed all stellar positional errors from parallax, refraction, and latitude. Flamsteed also deduced a 25.25-day solar rotational period from sunspot observations as well as producing tables of atmospheric refraction, tides, and the Moon's elliptic inequality according to Kepler's second law.
The unsurpassed accuracy of Flamsteed's data placed them in high demand. Isaac Newton (1642-1727) and Edmond Halley (1656-1742) pressed for publication so they could test their theories, but Flamsteed resisted because his reductions were incomplete. He was eventually obliged to deposit his observations with the Royal Society, after which Halley unlawfully published a muddled and incomplete version (1712). Flamsteed publicly burned 300 of the 400 copies printed, then commenced work on his own edition. Before his death on December 31, 1719, he had completed enough work on the three-volume Historia Coelestis Britannica that his assistants were able to see it to completion in 1725. It contained the positions of nearly 3,000 stars and established Greenwich as one of the world's leading observatories. A set of star maps—Atlas Coelestis (1729)—based on the catalog also appeared posthumously.
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