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Edmond Halley
Discovery of Radioactivity
 
 
 
Dr Subodh Mahanti

The period during which he (Halley) held the post of Astronomer Royal, compared with those of his predecessor Flamsteed, and his successor Bradley , is hardly entitled, if we look at its effect upon the progress of science, to be called more than strong twilight night between two bright summer days.

Augustus De Morgan (1806-71), the first President of the London Mathematical Society.


Under this marble, together with his beloved wife, rest Edmond Halley LL.D. unquestionably the greatest astronomer of his age.

The inscription on the Halley's tomstone at the churchyard of St. Margaret, Lee near London.

 

The above two assessments about Edmond Halley's contributions to science are two extremes. (Though Halley's first name is often given as Edmund, he always wrote it as Edmond.) A large number of people perhaps know Halley because of the Halley's comet (the best-known periodic comet, returning to perihelion at average intervals of 76 years_the time between returns ranges from 74 to 79 years) and for his role in the publication of Newton's Philosophiae Naturalis Principia Mathematica (popularly known as principia).

Halley was fortunate to live through a period of scientific revolution that strengthened the foundation of modern science. When the England's monarchy was restored under Charles II (1630-85) in 1660 Halley was four year old. Two years later Charles II granted a charter to the informal organisation of natural philosophers under the aegis of 'invisible college' which later became the Royal Society of London with the motto, Nullis in verba - take nobody's word; see for yourself _ set the stage for the centuries to come. Halley's scientific work and his life covered a vast range. He made enormous contribution in almost every branches of physics and astronomy. Halley published his first scientific paper when he was 20. It was on the theory of planetary orbits and was published in Philosophical Transaction of the Royal Society. In 1679 Halley published his Catalogue of the Southern Stars (Catalogus stellarum australium). It was the first catalogue of telescopically determined star positions in the southern hemisphere. It not only established Halley's reputation as astronomer but was also partly responsible for his being awarded a Master of Arts degree by the Oxford University without going through the usual examinations. Halley's catalog of stars was useful for navigation at sea.

Halley's masterpiece on comets, A Synopsis of the astronomy of comets (Astronomiae cometicae) published in 1705 laid the foundation of modern cometary study. Halley predicted with considerable accuracy the path of totality of the solar eclipse visible over England in 1715. after a long and careful study he was the first to realise in 1693 that the Moon's mean motion had a secular acceleration. An apparent gradual acceleration of the Moon's motion in its orbit, as measured relative to mean solar time. He was the first to predict extraterrestrial nature of the precursors of meteors. He was the first to suggest that observations of transits of Venus could be used to measure the distance of the Sun. However, this was first done long after his death. He offered the first proof of motions of stars by showing that they had moved since Ptolemy's time. In 1721 he raised the problem of what has come to be called Olbers' paradox_the apparent contradiction between the simple observation that the night sky is dark and the theoretical expectation that an infinite, static Universe, consisting of stars and galaxies more or less uniformly distributed, should be as bright as a star.

It was Halley who published the first meteorological chart in 1686. He extensively studied the distribution of the prevailing winds, magnetic variations and tides over the oceans. In map making Halley was the first to use an isometrically representation. (Isometric projection is a method of drawing figures and maps so that these dimensions are shown not in perspective but foreshortened equally). The sea voyages undertaken by Halley are considered as the first sea voyages undertaken for purely scientific purposes. It can be said that Halley practically founded the sciences of geomagnetism and physical oceanography. He is considered the founder of geophysics, especially for his paper on trade winds and his work on tides. In 1686 he formulated the mathematical law concerning barometric heights and pressure above the sea levels. He also made many advances in barometric designs. By studying extensively the evaporation and salinity of lakes during the period 1687-94 he drew conclusions about the age of the Earth. He was constantly concerned with the magnetism of the Earth and developed a general theory about this. He was also concerned with weather and published on the relation of barometric pressure to the weather. He improved the design of the diving bells. In 1693 he published the mortality tables for the city of Breslau. This was the first attempt to relate the mortality and age in a population. It influenced the future development of actuarial tables in life insurance.

Halley published innumerable articles on natural history and classical studies. Halley published important editions of Apollonius of Myndus (fl. 4th century BC) and of other ancient astronomers. He also published papers in pure mathematics. Many would like to cell Halley an 'idea man'. His intellect was so lively for him to concentrate on a single problem for long. However, by any standard he made remarkable contributions.

One may be really bewildered to know the type of appointments Halley held. During 1696-98 Halley was deputy controller of the mint at Chester. Between 1698-1700 Halley was commissioned as a naval captain and he actually commanded a Royal Navy man-of-war, the Paramour, making prolonged and eventful ocean voyages. At the instance of the Queen Anne he made two diplomatic missions (1702 & 1703) to Vienna (Austria). As he completed his first Austrian mission, the Holy Roman Emperor presented him with a valuable diamond ring. His first mission to Vienna was to advise on the fortification of a port on the Adriatic and on his second mission he oversaw the actual building of the fortifications. In 1703 Halley was elected to the Savillian Chair of Geometry at Oxford. In 1720 he succeeded Flamsteed as astronomer royal at Greenwitch. (Following the realisation that knowledge of the stars and their position was the key to navigation the British government created the post of astronomer royal. Flansteed was the first incumbent). Halley served as the first corresponding secretary to the Royal Society of London and published the scientific works of its members.

Halley was born on November 8, 1656. His father, also Edmond Halley, was a wealthy London merchant, a soapmaker and salter. After studying at St. Paul's School at London he entered the Queen's College, Oxford, which he left in 1676 without a degree. While still a student Halley published a little book on Kepler's laws. John Flamsteed (1646-1719) who became Britain's first astronomer royal took note of this book and he was impressed by Halley's work. Flamsteed encouraged Halley to take up the study of astronomy seriously. On the lines of Flamsteed, who compiled an accurate catalog of northern stars, Halley wanted to prepare a catalogue of the stars of the southern hemisphere. With this view he sailed in a ship of the East India Company in November 1676 for the Island of St. Helena, the southernmost territory under British rule in the South Atlantic. This was possible for the financial assistance from his father and a letter of introduction from the king. Halley spent one and a half year (1676-78) at this bleak island. The weather of the Island was harsh and inhospitable and particularly it was extremely hostile for astronomical observation. Without being frustrated by this adverse condition. Halley spent hour after hour gazing at the sky with his telescope. He was successful in listing the positions of no fewer than 341 previously uncharted stars. He became an immediate celebrity among scientific elite. Flamsteed heralded him as the "The Southern Tycho", thereby he linked Halley's name to the great Danish astronomer, Tycho Brahe (1546-1601). Halley was made a fellow of the Royal Society. Suddenly he found himself in the company of great intellects like Newton, Flamsteed Robert Hooke (1635-1702), the inventor and microscopist, and Christopher Wren (1632-1723), the famous architect.

During his long voyage to st. Helena, Halley had noticed that unlike the commonly held belief the ship's compass did not point exactly to the north pole. Though the differences was not very significant but it showed that the magnetic pole and the north pole were not the same. In any case this observation was of commercial significance as the worldwide explosion of commercial trade in the second half of the 17th century had opened up may new ocean routs. There was a fierce competition to take advantage of the situation. So alongwith the maps of the skies there was demand for better marine charts for efficient navigation. In 1698 Halley undertook a voyage which lasted for two years. 'Under his command was the world's first ship ever commissioned solely for the scientific purpose. Halley thus measured magnetic declinations around the world. He prepared new navigation charts. He also tried to determine the correct attitudes and longitudes for the major ports.

What is the distance of the Sun from the Earth Astronomers asked this question from almost the beginning of time. Today every high school student knows the answer. But at the beginning this question was one of the most difficult questions to answer. There was no direct method for measurement. In any case Aristarchus and Hipparchus of Samos (fl.3rd century BC) the two ancient Greek astronomers, attempted to answer it. Aristarchus placed the distance between 18 and 20 times more distant than the Moon. This was exceedingly far from the reality as the actual distance is more like 340 times as far. Though Hipparchus fared little better than Aristarchus but he was also still very far off. Better method was developed only after Johannes Kepler (1571-1630) made some key discoveries about planetary orbits Kepler found that the planets orbit the Sun in ellipses. Further according to Kepler the average distance of a planet from the sun and the time it takes to complete an orbit are mathematically related. This implies that from the distance of a planet from the Earth and how long it takes to orbit the sun the distance between the Sun and the Earth can be determined. So finally there was a method for accurately determining the distance between the Sun and the Earth as following the trigonometrical method known as triangulation it was theoretically possible to measure the distance between the Earth and a nearby planet. Such an attempt was made by Giovanni Domenico Cassini (1625-1712) in 1672. He used Mars for the calculation. It may be noted here that Cassini was brought to France by Louis XIV (1638-1715), whose reign encompassed a flourishing french culture. Though he came up with a much closer figure than ever before_86 million miles against the actual distance of 93 million miles but the problem remained far from being solved. It was Halley who pointed out that instead of Mars one should try to use Venus as the latter approaches closer to Earth than the former. But then it is rarely possible to see Venus during its closest approach to the Earth. This is because Venus appears to be too close to the Sun. The Venus in its closest approach to the Earth can be observed only on those rare occasions when its crosses to the Sun's disk. Such a period of crossing is called transit in in astronomy.

In 1691 Halley suggested that such a transit of Venus would be ideal situation to make measurements from all locations of the Earth. Halley in his life time did not witness a transit of Venus because they only occur in pairs separated by eight years at intervals of more than 100 years. After the suggestion made by Halley the nearest transit was to occur in 1761. Though Halley did not live to see the transit but in 1716 he had presented a paper to the Royal Society of London calling for coordinated world wide preparation to utilise the forthcoming rare opportunity. He also devised a method for observing transits of Venus across the disk of the sun for correctly determining the distance of the sun from the Earth by solar parallax.

Halley's appeal to the scientific community did not go unheeded. Perhaps the transit of Venus in 1761 was the first great international scientific event. Scientists from all over the world joined together to make use of the opportunity. The planet was sighted by 122 observers from 62 different locations, including Newfoundland, Siberia, Bejing (then Peking), calcutta, Rome, the Indian ocean and St. Helena. The popular press recorded the enthusiasm generated. However, the results were not conclusive. So a greater effort was mounted again in 1969. This time planet was sighted by 151 observers from 77 sites. Captain James Cook (1728-79) sailed to then newly founded island of Tahiti in the South pacific to observe the planet. After analysing the measurements (which took nearly 60 years to complete) made in 1769 the average value achieved turned out to be 96 million miles. This measurement expanded the solar system nearly 100 times the size that ptolemy estimated the entire universe to be. It may be noted that no transit of Venus took place in 20th century. The only transits of 21st century will occur in 2004 and 2012.

The Synopsis of the Astronomy of Comets, Halley's celebrated treatise, which laid the foundation of modern cometary science was rather brief. It was first published in Latin in 1705 as a six page folio pamphlet. An English version was also brought out in the same year. A longer and slightly modified Latin version also appeared in the Philosophical Transaction of the Royal Society. In this treatise Halley presented the orbital features of 24 comets seen from 1337 through 1698. This information was presented in tabular form. Though Halley noted in this treatise that all the 24 comets had parabolic paths he himself believed that the true paths of comets were very eccentric ellipses. The most important observation put forward in this treatise was that the comets observed in 1531, 1607 and 1682 were the same object. Halley noted that their orbital features were identical except that the historic periods between their perihelion (the point nearest the Sun in the orbit of a planet, comet or man-made satellite) passages were different over 76 years between 1531 and 1607 and just under 75 years between 1607 and 1682.

Halley played a very important role in the publication of Principia, one of the greatest master pieces of science. He persuaded Newton to work for it. when the Royal Society could not afford to finance its publication it decided that "Mr. Halley undertake the business of looking after it, and printing it at his own charges." Halley provided the necessary funds from his own pocket. He edited the text, corrected the proofs, and saw it through the press in 1687. He even contributed some laudatory Latin verses in honour of the author.

Comet Helley as it appeared on March 8, 1986

Halley's interest was not confined to pure science alone. He was equally interested in technological pursuits. In 1731 he published a method to measure longitude via lunar position. He drew up a map of magnetic declination as a possible means for longitude measurement. He produced a world map with trade winds shown. Besides mapping magnetic declination, his expedition in the Atlantic was also concerned with determining the exact location of islands and ports. He was deeply involved in instrumentation. He developed a thermometer, a device for measuring the speed of ship through the water, and an improved version of the back staff for measuring the height of the Sun and his much talked about diving bell. He also helped Harrison to build his clock.