1686 - Trade Winds

Edmund Halley, an astronomer and polymath aged 30, presents at the Royal Society a map of the Trade Winds also known as Easterlies. As for the Atlantic ocean, they have been known by the Portuguese since the beginning of the exploration, that is in the first half of the 15th century. As for the Pacific ocean, this is Andrés de Urdaneta (†1568) who brought them to light in 1565 and plotted a map for the Manila galleon route between the Philippines and Acapulco.

Halley is already famed for his map of Southern stars that he carried out at the age of 21, which made him a fellow of the Royal Society at the amazing age of 22.

For that purpose, he set an observatory in the island of St Helena which lies by 16° South and 6° West from London meridian, in the sea that Halley, as most of accurate scholars, names the Æthiopic ocean. He had proposed Astronomer Royal John Flamsteed, 30 then, to expand his catalogue of Northern stars by Southern ones, which was the argument for his journey and science exile. His endeavour was supported by King Charles II and by Flamsteed, both impressed by his earliness and his high spirits. Aged 17, he had reported Flamsteed erroneous data he found in Tycho Brahe’s tables and, to crown the year, he had published a method to work out aphelia and eccentricities of planet trajectories.

¤ Then Flamsteed's title was King’s Astronomical Observator as he was appointed to lead the observatory of Greenwich as soon as completed, which happened in 1676.

From his stellar observations of 1676-77, Halley issued a first batch in 1679 and a second this year.

¤ His return in England generated a delightful incident. Oxford University refused to reintegrate him after his return from St Helena because he left without permission. The king intervened in the dispute to deliver his incensed Royal opinion and to stress out derision of the dean’s ludicrous decision, which permitted Halley to be granted a degree of Master of Arts just after having been elected a Fellow of the Royal Society in the same year 1678. Will we see that again some day ?

Halley got new patrons with Christopher Wren, 54, famous as an architect but also, primally, as an astronomer, and Robert Hooke, 51, the widest polymath ever if this word suffices to depict such a fertile mind. A dinner in 1683 between the three masterminds led to a passionate discussion concluded by a £2 wager to the one who would find the reason which leads the planets to adopt elliptical trajectories. It kept to Halley’s mind a full year. He thought that the reason should lie in the inverse square law but could not solve the question. Hooke, on his own, told he knew the answer without delivering it – old tactics of his to stall. It would have been useless and tactless to insist...  

¤ As everybody knows, Johannes Kepler (†1630) established by observing that trajectories were elliptical but he had no explanation.

¤ A £2 wager is a sum for Halley who earns about £50 a year. Money seems not to matter too much for him, which is fortunate since the Royal Society has just advised him that it cannot afford the promised extra salary of £50 a year for his new job of Secretary, which would have doubled his income as it doubled its amount of work. Wren, who is caring, certainly thought Halley would win the bet to venture it.

The following year, Halley dared to call on Isaac Newton, 43 years old, considered by a genius by the happy ones who can grasp what he says, and no less by the others, on the challenging question. It inspired the famous reply De Motu Corporum in Gyrum in return (1684). It initiated a further flow of thoughts in Newton’s wits which have come to fruition in 1685 and led to their immersion in the monumental Philosophiae Naturalis Principia Mathematica which have just come to light. It is noteworthy that the Principia have been published at Halley’s expense as the Royal Society, denying its promise of funding, cannot afford publication of Newton's magnum opus that our perfect gentleman rightly considers as an awesome work of art.

¤ Halley’s funding of Newton’s work is a great gesture to remember. Even greater for a low-paid scholar.

As for the trade winds and the monsoons, Halley’s memoir stems from an inquiry to ocean-going seafarers and from his own observations recorded during his travels between England and St Helena and his stay there. This is, as usual with him, an accurate depiction of phenomena based on a collection of cross-checked observations. He observed by himself that, in both hemispheres, the wind blows most of time, if not constantly, to West below 30° latitude. His inquiry confirms this fact, establishes the general features of the wind system and describes the irregularities like the monsoons.

The map that he has drawn is outstanding of clarity – a model of self-explanatory graphic – and of practicality. It should be delivered to every sea-going ship. Somebody should say that to Samuel Pepys, the Admiralty’s mainstay.

Halley also delivers a hypothesis for the trade winds – also called Easterlies as everybody knows – by inertia but he honestly underlines points which his theory cannot explain, e.g. the doldrums.

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IN RETROSPECT FROM TODAY

NOTE - On Halley’s hypothesis about the easterlies.

He was right in his depiction, he was right in his intuition of a global circulation of air masses, he was wrong in his explanation – what he himself suspected because of the doldrums. The deviation of wind is caused by the Coriolis effect, but this is not a result from Mathematician Gaspard Gustave de Coriolis (†1843) himself but from later works of William Ferrel (†1891) which included Coriolis deflection in atmospheric circulation.

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SUGGESTED BIBLIOGRAPHY

Edmund Halley – An Historical Account of the Trade Winds, and the Monsoons, observable in the Seas, between and near the Tropicks, with an Attempt to assign the Phisical cause of the Said Winds in Royal Society Transactions, pages 153-168 plus a map  – London, 1686

M. B. Deacon – The Contribution of Edmund Halley to Meteorology and Oceanography in the Bulletin of the Royal Meteorological Society – London, 1986

Bill Bryson – A Short History of Nearly Everything – London, 2003

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CREDIT

Isaac Whoods – Portrait of Sir Edmund Halley – oil on canvas, circa 1680 – © National Portrait Gallery