George Biddell Airy | |
|---|---|
 George Biddell Airy in 1891 | |
| Born | (1801-07-27)27 July 1801 Alnwick, Northumberland, England |
| Died | 2 January 1892(1892-01-02) (aged 90) Greenwich, London, England |
| Citizenship | British |
| Education | Colchester Royal Grammar School |
| Alma mater | Trinity College, Cambridge |
| Known for | SeventhAstronomer Royal See full list |
| Awards | Smith's Prize(1823) Copley Medal(1831) RAS Gold Medal(1833, 1846) Lalande Prize(1834) Royal Medal(1845) Albert Medal(1876) |
| Scientific career | |
| Fields | Astronomy,mathematics |
| Institutions | Trinity College, Cambridge Royal Society |
| Academic advisors | George Peacock |
| 7th Astronomer Royal | |
| In office 1835–1881 | |
| Preceded by | John Pond |
| Succeeded by | William Christie |
Sir George Biddell Airy (/ˈɛəri/; 27 July 1801 – 2 January 1892) was an Englishmathematician andastronomer, as well as theLucasian Professor of Mathematics from 1826 to 1828 and the seventhAstronomer Royal from 1835 to 1881. His many achievements include work onplanetaryorbits, measuring the mean density of the Earth, a method of solution of two-dimensional problems insolid mechanics and, in his role as Astronomer Royal, establishingGreenwich as the location of theprime meridian.
Airy was born atAlnwick inNorthumberland, one of a long line of Airys who traced their descent back to a family of the same name residing atKentmere, inWestmorland, in the 14th century. The branch to which he belonged, having suffered in theEnglish Civil War, moved toLincolnshire and became farmers. Airy was educated first at elementary schools inHereford, and afterwards atColchester Royal Grammar School.[1] An introverted child, Airy gained popularity with his schoolmates through his great skill in the construction of peashooters.[2]
From the age of 13, Airy stayed frequently with his uncle, Arthur Biddell atPlayford,Suffolk. Biddell introduced Airy to his friendThomas Clarkson, the slave trade abolitionist who lived atPlayford Hall. Clarkson had an MA in mathematics from Cambridge, and examined Airy in classics and then subsequently arranged for him to be examined by a Fellow fromTrinity College, Cambridge on his knowledge of mathematics.[3][4] As a result, he entered Trinity in 1819, as asizar, meaning that he paid a reduced fee but essentially worked as a servant to make good the fee reduction.[5] Here he had a brilliant career, and seems to have been almost immediately recognised as the leading man of his year. In 1822 he was elected scholar of Trinity, and in the following year he graduated assenior wrangler and obtained firstSmith's Prize. On 1 October 1824 he was elected fellow of Trinity, and in December 1826 was appointedLucasian professor of mathematics in succession toThomas Turton. This chair he held for little more than a year, being elected in February 1828Plumian professor ofastronomy and director of the newCambridge Observatory.[1] In 1836 he was elected aFellow of the Royal Society and in 1840, a foreign member of theRoyal Swedish Academy of Sciences. In 1859 he became foreign member of theRoyal Netherlands Academy of Arts and Sciences.[6]
Some idea of his activity as a writer on mathematical and physical subjects during these early years may be gathered from the fact that previous to this appointment he had contributed three important memoirs to thePhilosophical Transactions of theRoyal Society, and eight to theCambridge Philosophical Society. At the Cambridge Observatory Airy soon showed his power of organisation. The onlytelescope in the establishment when he took charge was thetransit instrument, and to this he vigorously devoted himself. By the adoption of a regular system of work, and a careful plan of reduction, he was able to keep his observations up to date, and published them annually with a punctuality which astonished his contemporaries. Before long amural circle was installed, and regular observations were instituted with it in 1833. In the same year theDuke of Northumberland presented the Cambridge observatory with a fineobject-glass of 12-inch aperture, which was mounted according to Airy's designs and under his superintendence, although construction was not completed until after he moved toGreenwich in 1835.[1]
Airy's writings during this time are divided between mathematical physics and astronomy. The former are for the most part concerned with questions relating to the theory of light arising out of his professorial lectures, among which may be specially mentioned his paperOn the Diffraction of an Object-Glass with Circular Aperture, and his enunciation of the complete theory of therainbow.[citation needed] In 1831 theCopley Medal of the Royal Society was awarded to him for these researches. Of his astronomical writings during this period the most important are his investigation of the mass ofJupiter, his report to theBritish Association on the progress of astronomy during the 19th century, and his workOn an Inequality of Long Period in the Motions of theEarth andVenus.[7]
One of the sections of his able and instructive report was devoted to "A Comparison of the Progress of Astronomy in England with that in other Countries", very much to the disadvantage of England. This reproach was subsequently to a great extent removed by his own labours.[8]
One of the most remarkable of Airy's researches was his determination of themean density of the Earth. In 1826, the idea occurred to him of attacking this problem by means ofpendulum experiments at the top and bottom of a deepmine. His first attempt, made in the same year, at theDolcoath mine in Cornwall, failed in consequence of an accident to one of thependulums. A second attempt in 1828 was defeated by a flooding of the mine, and many years elapsed before another opportunity presented itself. The experiments eventually took place at theHarton pit nearSouth Shields in northern England in 1854. Their immediate result was to show that gravity at the bottom of the mine exceeded that at the top by 1/19286 of its amount, the depth being 383 m (1,257 ft). From this he was led to the final value of Earth'sspecific density of 6.566.[9] This value, although considerably in excess of that previously found by different methods, was held by Airy, from the care and completeness with which the observations were carried out and discussed, to be "entitled to compete with the others on, at least, equal terms."[8] The currently accepted value for Earth's density is 5.5153 g/cm3.[citation needed]
In 1830, Airy calculated the lengths of the polar radius and equatorial radius of the earth using measurements taken in the UK. Although his measurements were superseded by more accurate radius figures (such as those used forGRS 80 andWGS84) hisAiry geoid (strictly a reference ellipsoid, OSGB36) is still used by Great Britain'sOrdnance Survey for mapping of England, Scotland and Wales because it better fits the local sea level (about 80 cm below world average).[10][11]
Airy's discovery of a new inequality in the motions of Venus and the Earth is in some respects his most remarkable achievement. In correcting the elements ofDelambre's solar tables he had been led to suspect an inequality overlooked by their constructor. The cause of this he did not long seek in vain; thirteen times the mean motion of Venus is so nearly equal to eight times that of Earth that the difference amounts to only a small fraction of Earth's mean motion, and from the fact that the term depending on this difference, although very small in itself, receives in the integration of thedifferential equations a multiplier of about 2,200,000, Airy was led to infer the existence of a sensible inequality extending over 240 years (Phil. Trans. cxxii. 67). The investigation was probably the most laborious that had been made up to Airy's time inplanetary theory, and represented the first specific improvement in the solar tables effected in England since the establishment of the theory ofgravity. In recognition of this work theGold Medal of the Royal Astronomical Society was awarded to him in 1833[8] (he would win it again in 1846).
Theresolution of optical devices is limited bydiffraction. So even the most perfect lens cannot quite generate a point image at itsfocus, but instead there is a bright central pattern now called theAiry disk, surrounded by concentric rings comprising an Airy pattern. The size of the Airy disk depends on the light wavelength and the size of the aperture.John Herschel had previously described the phenomenon,[12] but Airy was the first to explain it theoretically.[13]
This was a key argument in refuting one of the last remaining arguments for absolutegeocentrism: thegiant star argument.Tycho Brahe andGiovanni Battista Riccioli pointed out that the lack ofstellar parallax detectable at the time entailed that stars were a huge distance away. But the naked eye and the early telescopes with small apertures seemed to show that stars were disks of a certain size. This would imply that the stars were many times larger than the Sun (they were not aware ofsupergiant orhypergiant stars, but some were calculated to be even larger than the size of the whole universe estimated at the time). However, the disk appearances of the stars were spurious: they were not actually seeing stellar images, but Airy disks. With modern telescopes, even with those having the largest magnification, the images of almost all stars correctly appear as mere points of light.
In June 1835 Airy was appointedAstronomer Royal in succession toJohn Pond, and began his long career at the national observatory which constitutes his chief title to fame. The condition of the observatory at the time of his appointment was such thatLord Auckland, the firstLord of the Admiralty, considered that "it ought to be cleared out," while Airy admitted that "it was in a queer state." With his usual energy he set to work at once to reorganise the whole management. He remodelled the volumes of observations, put the library on a proper footing, mounted the new (Sheepshanks)equatorial and organised a new magnetic observatory. In 1847 analtazimuth was erected, designed by Airy to enable observations of themoon to be made not only on themeridian, but whenever it might be visible.[14] In 1848 Airy invented the reflex zenith tube to replace thezenith sector previously employed. At the end of 1850 the great transit circle of 203 mm (8.0 in) aperture and 3.5 metres (11 feet 6 inches)focal length was erected, and is still the principal instrument of its class at the observatory. The mounting in 1859 of an equatorial of 330 mm (13 in) aperture evoked the comment in his journal for that year, "There is not now a single person employed or instrument used in the observatory which was there in Mr Pond's time"; and the transformation was completed by the inauguration ofspectroscopic work in 1868 and of the photographic registration ofsunspots in 1873.[8]
The formidable undertaking of reducing the accumulated planetary observations made at Greenwich from 1750 to 1830 was already in progress under Airy's supervision when he became Astronomer Royal. Shortly afterwards he undertook the further laborious task of reducing the enormous mass of observations of the moon made at Greenwich during the same period under the direction, successively, ofJames Bradley,Nathaniel Bliss,Nevil Maskelyne and John Pond, to defray the expense of which a large sum of money was allotted by the Treasury. As a result, no fewer than 8,000 lunar observations were rescued from oblivion, and were, in 1846, placed at the disposal of astronomers in such a form that they could be used directly for comparison with the theory and for the improvement of the tables of the moon's motion.[8]
For this work Airy received in 1848 a testimonial from theRoyal Astronomical Society, and it at once led to the discovery byPeter Andreas Hansen of two new inequalities in the moon's motion. After completing these reductions, Airy made inquiries, before engaging in any theoretical investigation in connection with them, whether any other mathematician was pursuing the subject, and learning that Hansen had taken it in hand under the patronage of theking of Denmark, but that, owing to the death of the king and the consequent lack of funds, there was danger of his being compelled to abandon it, he applied to the admiralty on Hansen's behalf for the necessary sum. His request was immediately granted, and thus it came about that Hansen's famousTables de la Lune were dedicated toLa Haute Amirauté de sa Majesté la Reine de la Grande Bretagne et d'Irlande.[8]
In 1851 Airy established a newPrime Meridian at Greenwich. This line, the fourth "Greenwich Meridian", became the definitive internationally recognised line in 1884.[15][16] It was superseded by theIERS Reference Meridian in 1984, which runs approximately 102 metres to the east.
In June 1846, Airy started corresponding with French astronomerUrbain Le Verrier over the latter's prediction that irregularities in the motion ofUranus were due to a so-far unobserved body. Aware that Cambridge AstronomerJohn Couch Adams had suggested that he had made similar predictions, on 9 July Airy urgedJames Challis to undertake a systematic search in the hope of securing the triumph of discovery for Britain. Ultimately, a rival search in Berlin byJohann Gottfried Galle, instigated by Le Verrier, won the race for priority.[17] Though Airy was "abused most savagely both by English and French"[18] for his failure to act on Adams's suggestions more promptly, there have also been claims that Adams's communications had been vague and dilatory[17] and further that the search for a new planet was not the responsibility of the Astronomer Royal.[19]
Using a water-filledtelescope, in 1871 Airy looked for a change instellar aberration through therefracting water due to theaether drag hypothesis.[20] Like all other attempts to detect aether drift or drag, Airy obtained a negative result.
In 1872 Airy conceived the idea of treating thelunar theory in a new way, and at the age of seventy-one he embarked on the prodigious toil which this scheme entailed. A general description of his method will be found in theMonthly Notices of the Royal Astronomical Society, vol. xxxiv, No. 3. It consisted essentially in the adoption ofCharles-Eugène Delaunay's final numerical expressions forlongitude,latitude, andparallax, with a symbolic term attached to each number, the value of which was to be determined by substitution in the equations of motion.[8]
In this mode of treating the question the order of the terms is numerical, and though the amount of labour is such as might well have deterred a younger man, yet the details were easy, and a great part of it might be entrusted to "a merecomputer".[8][a]
The work was published in 1886, when its author was eighty-five years of age. For some little time previously he had been harassed by a suspicion that certain errors had crept into the computations, and accordingly he addressed himself to the task of revision. But his powers were no longer what they had been, and he was never able to examine sufficiently into the matter. In 1890 he tells us how a grievous error had been committed in one of the first steps, and pathetically adds, "My spirit in the work was broken, and I have never heartily proceeded with it since."[8]
In 1862, Airy presented a new technique to determine thestrain andstress field within abeam.[21] This technique, sometimes called the Airy stress function method, can be used to find solutions to many two-dimensional problems insolid mechanics.[citation needed] For example, it was used by H. M. Westergaard[22] to determine the stress and strain field around a crack tip and thereby this method contributed to the development offracture mechanics.
Airy was consulted about wind speeds and pressures likely to be encountered on the proposed Forth suspension bridge being designed byThomas Bouch for theNorth British Railway in the late 1870s. He thought that pressures no greater than about 10 pounds per square foot (500 pascals) could be expected, a comment Bouch took to mean also applied to the first Tay Railway Bridge then being built. Much greater pressures, however, can be expected in severe storms. Airy was called to give evidence before the official inquiry into theTay Bridge disaster, and was criticised for his advice. However, little was known about the problems of wind resistance of large structures, and a Royal Commission on Wind Pressure was asked to conduct research into the problem.[23]
Airy was described in his obituary published by theRoyal Society as being "a tough adversary" and stories of various disagreements and conflicts with other scientists survive.Francis Ronalds discovered Airy to be his foe while he was inaugural Honorary Director of theKew Observatory, which Airy considered to be a competitor to Greenwich.[24][25] Other well documented conflicts were withCharles Babbage and SirJames South.[26][27]
In July 1824, Airy met Richarda Smith (1804–1875), "a great beauty", on a walking tour ofDerbyshire. He later wrote, "Our eyes met ... and my fate was sealed ... I felt irresistibly that we must be united," and Airy proposed two days later. Richarda's father, the Revd Richard Smith, felt that Airy lacked the financial resources to marry his daughter. Only in 1830, with Airy established in his Cambridge position, was permission for the marriage granted.[17][28][29]
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The Airys had nine children, the first three of whom died young.
Airy wasknighted on 17 June 1872.[34]
Airy retired in 1881, living with his two unmarried daughters atCroom's Hill near Greenwich. In 1891, he suffered a fall and an internal injury. He survived the consequential surgery only a few days. His wealth at death was £27,713, equivalent to £3,746,548.49 in 2021.[35] Airy and his wife and three pre-deceased children are buried at St. Mary's Church inPlayford, Suffolk.[17] A cottage owned by Airy still stands, adjacent to the church and now in private hands.[36]
SirPatrick Moore claimed in his autobiography that the ghost of Airy has been seen haunting the Royal Greenwich Observatory after nightfall.[37]
For a list of works by George Biddell Airy (with digital copies) see Wikisource.
A complete list of Airy's 518 printed papers is in Airy (1896). Among the most important are:
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This article incorporates text from a publication now in thepublic domain: Rambaut, Arthur Alcock (1911). "Airy, Sir George Biddell". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 1 (11th ed.). Cambridge University Press. pp. 445–447.
Works by or aboutGeorge Biddell Airy atWikisource
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| Preceded by | 31stPresident of the Royal Society 1871–1878 | Succeeded by |
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