Info

In 1782, George III granted Herschel a pension of £200 per annum (the Astronomer Royal only received £300) and as a result he could devote all his time to astronomy. The king later helped fund Herschel's telescope-making, and also provided £50 per annum so that Herschel's sister Caroline could become his assistant. She later became well known as a successful comet-hunter. Herschel was knighted in 1816 by George III's son, the Prince Regent. Herschel's astronomical career is described in Ronan (1967).

A list of twenty-two pre-discovery sightings of Uranus (the last being found in 1864) is given in Alexander (1965), p. 90. A twenty-third was unearthed more recently (Rawlins (1968)). The notoriously disorganized Pierre Charles Lemonnier observed Uranus six times in 8 days in 1769 without realizing that it was not a star, and his reputation was tarnished severely as a result. However, in his defence, it is worth noting that Lemonnier's telescope was too small to resolve the disc of Uranus, and so the only way for him to recognize that what he was observing was not a star would have been to note its motion. Lemonnier was unfortunate in that while he was observing Uranus it was near its stationary point and so its motion was much reduced from its mean value (see Baum and Sheehan (1997), pp. 55 and 79). Many more have been found subsequently: two in 1851, one in 1948 and, following the visit of the spacecraft Voyager 2 in 1986, numerous smaller satellites.

... if the various satellites of a planet move in a plane greatly inclined to that of its orbit, it can be inferred that they are kept in that plane by the action of the planet's equator, and that therefore the planet rotates around an axis nearly perpendicular to the plane of the orbits of its satellites. It may therefore be affirmed that the planet Uranus, all of whose satellites move in a single plane almost perpendicular to the ecliptic, itself turns on an axis very little inclined to the ecliptic.

The addition of a seventh planet to the Solar System was a hugely significant event, for many reasons. In the first place, it changed completely Herschel's financial circumstances and allowed him to dedicate his life to astronomy. His agenda was markedly different from that of his contemporaries, concerned as he was with grand questions concerning the physical nature and ultimate fate of the Universe. Herschel constructed large telescopes that could peer deep into outer space and built a speculative cosmology based on what he observed. He transformed the starry heavens from a static backdrop against which to measure planetary positions, into a vast dynamic region in which stars evolved from clouds of nebulous material. In so doing he became the pioneer of modern sidereal astronomy.16

The size of the known Solar System was increased greatly by the discovery of Uranus but, more importantly, its extent became less certain. Psychologically, the case for the orbit of Saturn representing the outermost limit of the influence of the Sun had been compelling prior to 1781, since no new planets had ever been discovered, even following 150 years of telescopic observation. But if there were more planets than those visible with the naked eye, why only one? The possibility that there were more planets waiting to be discovered had to be taken seriously - perhaps the Titius-Bode law would indicate where to look.

The discovery of a new planet at just over 19 AU, compared to the prediction of 19.6 AU from the Titius-Bode law, was enough to convince many of the validity of the law, despite there being no physical basis for it. If the law did govern planetary distances, then what was to be made of the absence of any planet at a distance of 2.8 AU from the Sun? Many people, including Bode, became convinced of the existence of a planet between Mars and Jupiter, and the search began.

15 Laplace Celestial Mechanics, Volume II, Book 5, Chapter 3. Quoted from Alexander (1965),

The effect of the discovery of Uranus on Herschel is discussed in Schaffer (1981). Schaffer

It is claimed often that the philosopher Hegel logically 'proved' the impossibility of anything between Mars and Jupiter, only to look rather foolish when such an object was discovered. However, this is not true (Pinkard (2000), pp. 107-8). In August 1801, Hegel defended his habilitation thesis (a requirement in German universities before the right to lecture was granted) in which he highlighted the fact that people were searching for a planet on the basis of the Titius-Bode law and then went on to describe certain numerological speculations of Plato (from the Timaeus (see, for example, Knorr (1990)) in which no planet was missing. He did not, however, endorse Plato's view.

The search for a planet between Mars and Jupiter

Baron Franz von Zach, the Hungarian-born director of the Duke of Saxe-Gotha's observatory at Seeberg, began a methodical search for the 'missing' planet in 1787, but after 13 unsuccessful years he decided to enlist some help. In 1800, he formed a'planet-hunting' club-von Zach called them jokingly the 'celestial police' - and they tried to enlist others to join. However, they were all beaten to the discovery of an object orbiting between Mars and Jupiter by one of those whose help they intended to request - Giuseppe Piazzi - based in Palermo, Sicily. On 1 January 1801, he observed a faint object in the constellation Taurus that over successive nights moved with respect to the stars. Piazzi thought he had discovered a new planet, but was cautious. He wrote to Bode and Lalande claiming to have observed a comet, though one without a tail, but to his friend Barnaba Oriani from the Brera Observatory in Milan he said:

I have announced this star as a comet, but since it is not accompanied by any nebulosity and, further, since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet. But I have been careful not to advance this supposition in public.19

Before anybody could confirm his observations, the object approached too near the Sun and became lost from view.

Men like von Zach wanted to believe that the long search was over. Even though no one except Piazzi had observed the new object, he published an article in his monthly journal entitled On a Long Supposed, Now Probably Discovered, New Primary Planet of our Solar System Between Mars and Jupiter. Piazzi's data were extremely limited, and the determination of an orbit proved difficult.21 Parabolic, circular, and elliptical orbits were all tried, but nobody could reconcile theory satisfactorily with the available observations. It was predicted that the new object would emerge from the glare of the Sun around September 1801, but despite considerable effort, astronomers failed to relocate it.

By then the object had a name. Piazzi wanted to call it Ceres Ferdinandea -Ceres was the patron goddess of Sicily and King Ferdinand of Naples and Sicily was Piazzi's patron. Piazzi's desire to honour a royal patron by attaching his name to a new celestial body was as unsuccessful as Herschel's was, and Ceres became the accepted name. But where was Ceres? The answer was provided

0 0

Post a comment