Flamsteed Astronomy Society

page 1 of 3

The post of Astronomer Royal is 330 years old

On March 4, 1675, by Royal Warrant, John Flamsteed was appointed first Astronomical Observator.   Flamsteed was charged that he should “forthwith apply himself with the most exact care and diligence to the rectifying of the tables of motion of the heavens, and the places of the fixed stars, so as to find the so-much-desired longitude of places for the perfecting of the art of navigation.”

In the late 17th Century seafarers, once they were out of sight of land, were still mostly lost.   They could only estimate their position by ‘ded’ (deduced) reckoning — keeping a record of their estimated courses, times,  and speeds to calculate a position at the end of each leg of sailing.   This was a hit and miss technique at best and many deep-ocean voyages were ending in shipwreck leading to much loss of ...  wealth.   The challenge to find an accurate position at sea was a question for big business.   The theory was known.   Distance north or south (measured by latitude) could be found by taking sights of the Sun or pole star.  This was a technique long understood.  For many years navigators had sailed down a parallel — travelled north or south until they arrived at the latitude of their destination and then turned and sailed east or west until arrival — or disaster.   The hard question was whether to turn left or right — to go west or east, and for how far?   Distance east or west is measured by longitude, and to determine longitude it was necessary to find the local time at the present position, and the time at Greenwich at the same moment (time by the Sun mind — no such thing as International Time Zones then).  The difference between local time and Greenwich time would give longitude — 15 degrees of longitude for each hour difference.  But in 1675 when you’re in the mid-Atlantic, how to find the time at Greenwich?   No radio, no radar, no cell phones. 

Two schemes were favoured.  If you could take on your voyage a clock that would work accurately at sea then you could set the clock to Greenwich time before departure and it would always show you Greenwich time.  All that remained was to find local time through Sun or star sights and take the difference to calculate longitude.  But nobody knew how to build an accurate clock that would run reliably at sea.  The only clocks then accurate enough were pendulum clocks and a pendulum won’t run well for long on a bouncing sailing ship.  Many had tried, including Huygens and Hooke, but it would take until 1764 and Harrison’s fourth marine timekeeper H4 before a clock would provide the solution.   In the meantime the best alternative was to use some celestial indicator — a clear event in the heavens that could be observed and its instant of occurrence compared with an accurate prediction at Greenwich time.  Eclipses would work but they’re too rare to be practical.   The Paris Observatory had developed the use of the relatively frequent eclipses of Jupiter’s moons to determine longitude on land, but observing Jupiter’s moons wasn’t practical from the heaving deck of a ship.  The best shot would be to use the position of the Moon against the background of bright stars.  The Moon shifts position by its own diameter each hour against the stars, enough change for a sighting to be used to find Greenwich time by comparison with an almanac, a technique called ‘lunar distances’.

In 1675 Le Sieur de St. Pierre, a friend of Charles II’s special pal, Louise de Keroualle then Duchess of Portsmouth (L), claimed to have perfected the technique and opportunistically petitioned Charles for reward and recognition.  To get him off his back Charles appointed a Commission to investigate.  Sir Jonas Moore, Surveyor-General of the Ordnance, was on the Commission and he had met Flamsteed in 1670 and been very impressed with the young astronomer’s talents.  Moore had become Flamsteed’s patron and asked him to study St. Pierre’s claims.  Flamsteed reported that St. Pierre was quite frankly, full of it, but that the technique was sound in theory.  What was needed was a careful programme of observation to measure the positions of the stars with sufficient precision, and to develop the ‘theory of the Moon’ to be able to predict lunar position accurately.

The face that saved a thousand ships?  Louise de Keroualle, Duchesse of Portsmouth


By Pierre Mignard

Flamsteed aged about 34

(Courtesy Royal Society)


He was 28 when appointed Astronomical Observator

Sir Jonas and Flamsteed were able to persuade Charles to sponsor the foundation of an observatory where Flamsteed could carry-out the programme of observations.   Charles may well have been influenced by his own investments in trans-Atlantic trade and by the earlier founding of the Paris Observatory.  Charles lived his life in a ‘keeping up with the Bourbons’ competition with Sun King Louis XIV, a race he could never win.  Nevertheless, Charles was minded to warrant a less-than-princely £500 raised by the sale of duff gunpowder.  Christopher Wren, who was doing a bang-up job with the new St. Paul’s Cathedral, was asked to find a site and construct a suitable building.