Edmond Halley

Edmond Halley

astronomer; second Astronomer Royal

Halley in his sixties, after the portrait by Godfrey Kneller. At the sitting he had a comet, a pair of transits and a hollow planet outstanding, and would not live to see any of the three settled.

Born	8 November 1656, London
Died	25 January 1742, Greenwich, aged 85[1]
Offices	Savilian Professor, 1704; Astronomer Royal, 1720
Also	Master and Commander, Royal Navy
Things he settled
Southern star catalogue	1678, at twenty-two
Publication of the Principia	Edited and paid for, out of his own pocket[2]
First isoline map	1701
Things he did not live to see
The comet's return	1758, on time, sixteen years after him
His name attached to it	1759, by a Frenchman
The transit campaigns he designed	1761 and 1769
His hollow Earth disproved	1774

Edmond Halley (8 November 1656 – 25 January 1742) was an English astronomer, geophysicist and naval captain, the second Astronomer Royal, and the man who got the Principia into print by badgering Newton into writing it and then paying the printer himself.^[2]

He is remembered for a comet he did not discover, under a name he never used, given to it by a French astronomer seventeen years after his death. That is not an injustice so much as the pattern of his career. Halley's habit was to work out what would happen and to say when, and the whens he chose were almost all beyond him.

Before the Principia

He went out to St Helena at twenty, because it was the southernmost place under the Crown, and came back with the first telescopic catalogue of the southern sky: 341 stars, published in 1678, which got him a degree by royal mandate and a fellowship of the Royal Society at twenty-two.^[3]

The great service came in 1684. Halley wanted to know what curve a planet would follow under an inverse-square attraction, could not do it, and went to Cambridge to ask. Newton said an ellipse, said he had calculated it, and could not find the paper. What Halley did next is the reason the rest exists: he did not let it drop. He got the calculation rewritten, then extended, then extended again into three books; he edited them, corrected the proofs, answered Hooke's claim of priority, and when the Royal Society found it had spent its publication budget on a history of fishes, he paid for the printing out of his own salary.^[2]

The Principia thus reached the world at Halley's expense, and in it was a figure for the density of the Moon that was wrong by a factor of three. Halley believed it, as everybody did, and built on it.

The comet

Halley worked through the recorded orbits of twenty-four comets and noticed that three of them were the same shape: the comets of 1531, 1607 and 1682 had travelled the same path, at intervals of about seventy-six years. He concluded that they were one object on a long ellipse, published the Synopsis of the Astronomy of Comets in 1705, and said when it would be back.

Wherefore, if, according to what we have already said, it should return again about the year 1758, candid posterity will not refuse to acknowledge that this was first discovered by an Englishman.
– Edmond Halley, 1705^[4]

It was recovered on Christmas night 1758 by a Saxon farmer with a telescope, sixteen years after Halley's death, and the following year Lacaille attached his name to it. The claim in the sentence above is the one part of it that is not true: the comet had been seen and recorded at every return for two millennia, by people from Chinese court astronomers to the embroiderers of the Bayeux Tapestry. What Halley discovered was not the comet. It was the date, which is a harder thing and is not what the name commemorates.^[5]

The compass, and what came out of it

The 1701 chart of the Atlantic. The curving lines join places where the compass errs by the same amount, and the double line through Bermuda and the Cape Verde Islands is where it does not err at all. It is the first published map on which a quantity is drawn as a line, and every contour, isobar and isotherm since is descended from it.

Halley commanded a Royal Navy vessel, the Paramore, on three voyages between 1698 and 1701 – the first sea voyages made for a purely scientific purpose – in order to measure the variation of the compass across the Atlantic. What he did with the readings was the innovation. Rather than tabulate them, he drew curve-lines joining the places where the variation was equal, and published the result in 1701 as a chart.

Nobody had done that before. The technique is now called an isoline and it is the basis of the contour, the isobar, the isotherm and every line on a map that joins equal values, and it was invented to display an error in an instrument.

Fourteen years later he did something adjacent, and on a deadline. In 1715 he computed the track of a total eclipse across England, had it engraved and sold beforehand so that readers could see whether they stood in it, the object being to have the same event timed at once from a great many known places. The path was about twenty miles out, the lunar tables being at fault rather than the method, and he printed a corrected sheet afterwards showing where the shadow had actually gone.

The compass readings also produced Halley's one conspicuous failure. The variation was not merely large but drifting, slowly, over decades, and something inside the Earth had to be moving to account for it. In 1692 he proposed that the planet is a shell around further shells, turning at slightly different rates, and that the space between them is lit and possibly inhabited: the hollow Earth, advanced in the Philosophical Transactions by the Clerk of the Royal Society, for a reason.^[6] He was encouraged in it by the Principia's lunar density, which appeared to leave the Earth too light for its size. It was disposed of in 1774, by weighing a Scottish mountain, thirty-two years after he died.

Sent forward

The comet is the famous instance of Halley's addressing people who did not yet exist. It is not the largest.

In 1716 he published a method for obtaining the distance to the Sun by timing the transit of Venus across it from widely separated stations on the Earth, and worked out what precision would be needed and how far apart the observers must be.^[7] The next transits were due in 1761 and 1769. He was nearly sixty, and said plainly in the paper that the work would fall to others; the campaigns went out as he had specified them, at the cost of two international campaigns, and returned a figure within a per cent of the modern one.

The earliest of these is the one that waited longest to be of use. In 1695, in a paper otherwise concerned with the ruins of Palmyra, he set the records of ancient eclipses against the places those eclipses ought to have been seen from, found that they did not agree, and concluded that the Earth's rotation is slowing. Nothing could be done with this until there were clocks steadier than the planet, which took until the 1930s; it is now the reason time has to be corrected by hand.

In 1718 he compared the positions of Sirius, Arcturus and Aldebaran against those Ptolemy had recorded, found that all three had shifted, and concluded that the fixed stars are not fixed.^[8] The displacements are a matter of arcminutes accumulated over sixteen centuries, and nobody had thought to look because the sky is the standing example of a thing that does not change.

The glass of wine

The office came to him with a debt attached, and it is the one part of his conduct that will not be defended here. In 1712 he and Newton had printed four hundred copies of Flamsteed's star catalogue without its author's consent, from observations Flamsteed held to be unfinished, and Halley wrote the preface and did not put his name to it. In 1716 Flamsteed got hold of three hundred of them and burned them, calling it a sacrifice to Heavenly Truth.^[9]

Flamsteed died in 1719, Halley took the chair the following year, and the widow removed the Greenwich instruments. They had been bought with her husband's own money and were his to leave, and she did not leave them here. He began again at sixty-three in an empty observatory, held the office for twenty-two years, and set about a full cycle of lunar observations, which takes eighteen years and which he completed, in his eighties.

He died at Greenwich on 25 January 1742. The account in the Dictionary of National Biography is the one worth having: tired of the physicians' cordials, he asked for a glass of wine, drank it, and expired.^[1]

The comet came back sixteen years later, on time.

See also

• The longitude problem – the errand his Atlantic voyages were part of
• John Flamsteed – his predecessor, whose catalogue he printed without asking and whose widow took the instruments
• Hollow Earth – his one bad idea, and its respectable reason
• The Schiehallion experiment – the weighing that ended it, thirty-two years too late for him to mind
• Isaac Newton – whose book he extracted, edited and paid for
• Moon – whose density, wrong in that book, encouraged the cavity
• Stigler's law of eponymy – under which the comet is named for the man who dated it rather than the man who found it
• Eclipse – which he mapped in advance, then remapped where it had been
• Sun – whose distance was measured by the method he left instructions for
• James Bradley – who succeeded him at Greenwich and found the Earth moving
• Time – where his slowing Earth ended up, as a second inserted by hand every year or two

References

1. ^ Dates in New Style. In the Old Style then used in England he was born 29 October 1656 and died 14 January 1741, the year not turning until March. The wine is from the Dictionary of National Biography, which has him tired of the doctors' cordials.
2. ^ The Royal Society had undertaken to publish the Principia and then discovered that its funds had gone on Francis Willughby's De Historia Piscium, a history of fishes which did not sell. Halley, its Clerk, was at one point paid his salary in unsold copies of it.
3. ^ Catalogus Stellarum Australium, 1678. The island was chosen for being the southernmost English possession and turned out to be under cloud for most of the stay, which is the standing hazard of St Helena and was not discovered until he got there.
4. ^ From the eighteenth-century English rendering of the Synopsis Astronomiae Cometicae (1705), which was written in Latin. Wordings differ slightly between translations; "candid posterity" is the usual one.
5. ^ The comet appears in the Chinese records from 240 BC, in the Bayeux Tapestry for the apparition of 1066, and in a good many places between. Halley's contribution was the identification of the returns with one another, which is the whole of the achievement and none of the name.
6. ^ See hollow Earth. The luminous interior is usually quoted to make him ridiculous; it follows from the hypothesis rather than decorating it, since a shell with a space inside must account for what the space contains.
7. ^ E. Halley, "Methodus singularis quâ solis parallaxis sive distantia à terra…", Philosophical Transactions, 454–464, 1716.
8. ^ E. Halley, "Considerations on the change of the latitudes of some of the principal fixt stars", Philosophical Transactions, 736–738, 1718.
9. ^ The volumes were printed as the Historia Coelestis in 1712, on the Royal Society's authority and at the Crown's expense. Flamsteed's bonfire in April 1716 took some thirteen thousand sheets, and he burned by preference the catalogue of fixed stars, the observations from his mural arc, and Halley's preface.

External links

• E. Halley, "An Account of the Cause of the Change of the Variation of the Magnetical Needle…" (1692) – the hollow Earth, as published
• E. Halley, "Methodus singularis quâ solis parallaxis…" (1716) – the transit-of-Venus method, written for astronomers not yet born
• E. Halley, "Considerations on the change of the latitudes of some of the principal fixt stars" (1718) – the fixed stars, found to be moving
• Kneller's portrait of Halley – Royal Museums Greenwich, via Wikimedia Commons; the likeness above is taken from it
• The 1701 chart reproduced above – Wikimedia Commons