Info

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203° .8

19° .6

heliocentric longitude 0 July 1914

84° .0

262°.6

• It indicates for the unknown amass between Neptune's and the earth's; a visibility of the 12-13 magnitude according to albedo; and a disc of more that 1" in diameter.

• From the analogy of other members of the solar family, in which eccentricity and inclination are usually correlated, the inclination of its orbit to the plane of the ecliptic should be about 10°. This renders it more difficult to find.

Lowell co-ordinated three unsuccessful searches for a planet beyond Neptune, the third during the last 3 years of his life. His will specified that the search should be continued at the Flagstaff Observatory until the planet was found.

Pickering continued with his predictions. Analysis of the motion of Neptune, which had begun to deviate from its tabular longitude, led him in 1919to refine his elements for Planet O, which he now placed at 55.1 AU. Further searches were made, but the results were once again negative. By March 1929, Pickering had become disheartened:

... it has been suggested in some newspaper articles that all the large observatories would be hunting for planet O when it comes into opposition this February, I wish to state here that I myself have the gravest doubts on that point. Indeed if anyone is hunting for it I shall be much gratified.

As it happened, the fourth systematic search for Lowell's Planet X began in April 1929 under the direction of Vesto Melvin Slipher, and the man charged with comparing the 14 x 17 in. photographic plates (which covered an area of sky about 12 by 14°) to detect moving bodies was a young assistant at the

80 The first was carried out during 1905-7 and the second in 1911.

Quoted from Grosser (1964).

observatory, Clyde Tombaugh.82 A special blink comparator was used (in which the viewer sees two images alternately in the eyepiece - stars remain fixed but other objects appear to jump about), but even then the task was daunting, with some of the plates showing over half a million stars! On 18 February 1930, Tombaugh found two faint images of an unknown planet on photographic plates produced in January, and subsequent photographs provided confirmation that the object was orbiting beyond Neptune. The announcement of the discovery was withheld until 13 March, the anniversary of both the discovery of Uranus and the birth of Lowell, by which time those working on the project were convinced that Planet X had at last been found.

A telegram was dispatched to the Harvard College Observatory on 12 March which the next day was used to inform the rest of the world:

Systematic search begun years ago supplementing Lowell's investigation for Trans-Neptunian planet has revealed object which since seven weeks has in rate of motion and path consistently conformed to Trans-Neptunian body at approximate distance he assigned. Fifteenth magnitude. Position March twelve days three hours GMT was seven seconds of time West from Delta Geminorum, agreeing with Lowell's predicted longitude.

Astronomers everywhere turned their attention to this new member of the solar family. Numerous suggestions were made for a name, but astronomers settled finally on Pluto, the brother of Jupiter and Neptune. Preliminary orbits allowed other observatories to locate Pluto onprevious photographic images; it appeared on plates produced at the Konigstuhl Observatory and at Harvard in 1914, at the Mount Wilson Observatory in 1919 and 1925, at the Yerkes Observatory in 1921 and 1927, and at Belgium's Royal Observatory in Ucce in 1927. It also transpired that Pluto had been photographed from Flagstaff twice during the first month of Tombaugh's search. These prediscovery sightings then allowed a much more accurate set of elements to be produced; these are compared with the predictions of Lowell and Pickering in Table 10.1.

Just like Adams' and Leverrier's predictions nearly 100 years previously, the longitudes of Planets X and O were quite close to that of the newly discovered planet. Most of the other elements were also in the right ball park; indeed, the agreement was rather better than Adams and Leverrier had achieved, though the mean distance of Planet O was substantially in error. Following the discovery, opinions were divided as to the nature of the new planet. Some thought that Pluto was Lowell's Planet X, though Pickering defended his own prediction. However, the majority were of the opinion that the discovery of Pluto was

82 For Tombaugh's own account of the search, see Tombaugh and Moore (1980).

Table 10.1. A comparison of the elements of Pluto with predictions of Lowell and Pickering.

Table 10.1. A comparison of the elements of Pluto with predictions of Lowell and Pickering.

Element

X (Lowell)

O (Pickering)

Pluto (1930)

a

0 0

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