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Early years
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 no less than three important memoirs to the Philosophical Transactions of the Royal Society, and eight to the Cambridge Philosophical Society. At the Cambridge observatory Airy soon showed his power of organization. The only telescope in the establishment when he took charge was the transit 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 a mural circle was installed, and regular observations were instituted with it in 1833. In the same year the Duke of Northumberland presented the Cambridge observatory with a fine object-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 to Greenwich in 1835. 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 paper On the Diffraction of an Object-Glass with Circular Aperture, and his enunciation of the complete theory of the rainbow. In 1831 the Copley 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 of Jupiter, his report to the British Association on the progress of astronomy during the 19th century, and his work On an Inequality of Long Period in the Motions of the Earth and Venus. 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. Major achievements
In June 1835 Airy was appointed Astronomer Royal in succession to John 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 that Lord Auckland, the first Lord 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 reorganize the whole management. He remodelled the volumes of observations, put the library on a proper footing, mounted the new (Sheepshanks) equatorial and organized a new magnetic observatory. In 1847 an altazimuth was erected, designed by Airy to enable observations of the moon to be made not only on the meridian, but whenever it might be visible. In 1848 Airy invented the reflex zenith tube to replace the zenith sector previously employed. At the end of 1850 the great transit circle of 8 inch (203 mm) aperture and 11 ft 6 inch (3.5 m) focal length was erected, and is still the principal instrument of its class at the observatory. The mounting in 1859 of an equatorial of 13 inch (330 mm) 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 of spectroscopic work in 1868 and of the photographic registration of sunspots in 1873. 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, of James 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 less 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. For this work Airy received in 1848 a testimonial from the Royal Astronomical Society, and it at once led to the discovery by P.A. 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 the king 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 famous Tables de la Lune were dedicated to La Haute Amirauté de sa Majesté la Reine de la Grande Bretagne et d'Irlande. In 1851 Airy established a new Prime Meridian at Greenwich. This line, the fourth "Greenwich Meridian," became the definitive internationally recognised line in 1884. One of the most remarkable of Airy's researches was his determination of the mean density of the earth. In 1826 the idea occurred to him of attacking this problem by means of pendulum experiments at the top and bottom of a deep mine. His first attempt, made in the same year, at the Dolcoath mine in Cornwall, failed in consequence of an accident to one of the pendulums; 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 the Harton pit near South Shields 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 1,256 ft (383 m) From this he was led to the final value of 6.566 for the mean density of the earth as compared with that of water (Phil. Trans. cxlvi. 342). 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." In 1862 Airy presented a new technique to determine the strain and stress field within a beam (Phil. Trans. 1863, Vol.153, pp. 49-80). This technique, sometimes called Airy stress function method, can be used to find solutions to many two-dimensional problems in solid mechanics; e.g. it was used to determined the stress and strain field around a crack tip and thereby this method contributed to the development of fracture mechanics. In 1872 Airy conceived the idea of treating the lunar 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 the Monthly Notices of the Royal Astronomical Society, vol. xxxiv, No. 3. It consisted essentially in the adoption of Delaunay's final numerical expressions for longitude, latitude, and parallax, with a symbolic term attached to each number, the value of which was to be determined by substitution in the equations of motion. 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 mere computer". (Note that at the time that this was written, the term "computer" referred to a human being who performed calculating work, either manually or with mechanical aids). 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." In 1881 Airy resigned the office of Astronomer Royal and resided at the White House, Greenwich, not far from the Royal Observatory, until his death. Airy and his wife are buried at St. Mary's Church in Playford (Suffolk, England). A cottage owned by Airy still stands, adjacent to the church and now in private hands. A complete list of Airy's printed papers, numbering no less than 518, will be found in his Autobiography, edited in 1896 by his son, Wilfrid Airy, B.A., M.Inst.C.E. Amongst the most important of his works not already mentioned may be named the following:
The crater Airy on Mars is named in honor of Airy. Within that crater lies another smaller crater called Airy-0 whose location defines the prime meridian of that planet, as does the location of Airy's 1850 telescope for Earth. There is also an Airy crater on the Moon named in his honor. References
See also
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