www.Christian-Astronomers.org

Discovery of New Planets

Posted on July 23rd, 2007 by Admin.
Categories: Uncategorized.

It would be very interesting, but quite impossible in these pages, to
discuss all the exquisite researches of the mathematical astronomers,
and to inspire a reverence for the names connected with these
researches, which for two hundred years have been establishing the
universality of Newton’s law. The lunar and planetary theories, the
beautiful theory of Jupiter’s satellites, the figure of the earth, and
the tides, were mathematically treated by Maclaurin, D’Alembert,
Legendre, Clairaut, Euler, Lagrange, Laplace, Walmsley, Bailly,
Lalande, Delambre, Mayer, Hansen, Burchardt, Binet, Damoiseau, Plana,
Poisson, Gauss, Bessel, Bouvard, Airy, Ivory, Delaunay, Le Verrier,
Adams, and others of later date.

By passing over these important developments it is possible to trace
some of the steps in the crowning triumph of the Newtonian theory, by
which the planet Neptune was added to the known members of the solar
system by the independent researches of Professor J.C. Adams and of
M. Le Verrier, in 1846.

It will be best to introduce this subject by relating how the
eighteenth century increased the number of known planets, which was
then only six, including the earth.

On March 13th, 1781, Sir William Herschel was, as usual, engaged on
examining some small stars, and, noticing that one of them appeared to
be larger than the fixed stars, suspected that it might be a comet.
To test this he increased his magnifying power from 227 to 460 and
932, finding that, unlike the fixed stars near it, its definition was
impaired and its size increased. This convinced him that the object
was a comet, and he was not surprised to find on succeeding nights
that the position was changed, the motion being in the ecliptic. He
gave the observations of five weeks to the Royal Society without a
suspicion that the object was a new planet.

For a long time people could not compute a satisfactory orbit for the
supposed comet, because it seemed to be near the perihelion, and no
comet had ever been observed with a perihelion distance from the sun
greater than four times the earth’s distance. Lexell was the first to
suspect that this was a new planet eighteen times as far from the sun
as the earth is. In January, 1783, Laplace published the elliptic
elements. The discoverer of a planet has a right to name it, so
Herschel called it Georgium Sidus, after the king. But Lalande urged
the adoption of the name Herschel. Bode suggested Uranus, and this
was adopted. The new planet was found to rank in size next to Jupiter
and Saturn, being 4.3 times the diameter of the earth.

In 1787 Herschel discovered two satellites, both revolving in nearly
the same plane, inclined 80 degrees to the ecliptic, and the motion of both
was retrograde.

In 1772, before Herschel’s discovery, Bode[1] had discovered a curious
arbitrary law of planetary distances. Opposite each planet’s name
write the figure 4; and, in succession, add the numbers 0, 3, 6, 12,
24, 48, 96, etc., to the 4, always doubling the last numbers. You
then get the planetary distances.

  Mercury, dist.– 4  4 +   0 =   4
  Venus      “     7  4 +   3 =   7
  Earth      “    10  4 +   6 =  10
  Mars       “    15  4 +  12 =  16
   –                 4 +  24 =  28
  Jupiter  dist.  52  4 +  48 =  52
  Saturn     “    95  4 +  96 = 100
  (Uranus)   “   192  4 + 192 = 196
   –                 4 + 384 = 388

All the five planets, and the earth, fitted this rule, except that
there was a blank between Mars and Jupiter. When Uranus was
discovered, also fitting the rule, the conclusion was irresistible
that there is probably a planet between Mars and Jupiter. An
association of twenty-four astronomers was now formed in Germany to
search for the planet. Almost immediately afterwards the planet was
discovered, not by any member of the association, but by Piazzi, when
engaged upon his great catalogue of stars. On January 1st, 1801, he
observed a star which had changed its place the next night. Its motion
was retrograde till January 11th, direct after the 13th. Piazzi fell
ill before he had enough observations for computing the orbit with
certainty, and the planet disappeared in the sun’s rays. Gauss
published an approximate ephemeris of probable positions when the
planet should emerge from the sun’s light. There was an exciting hunt,
and on December 31st (the day before its birthday) De Zach captured
the truant, and Piazzi christened it Ceres.

The mean distance from the sun was found to be 2.767, agreeing with
the 2.8 given by Bode’s law. Its orbit was found to be inclined over
10 degrees to the ecliptic, and its diameter was only 161 miles.

On March 28th, 1802, Olbers discovered a new seventh magnitude star,
which turned out to be a planet resembling Ceres. It was called
Pallas. Gauss found its orbit to be inclined 35 degrees to the
ecliptic, and to cut the orbit of Ceres; whence Olbers considered that
these might be fragments of a broken-up planet. He then commenced a
search for other fragments. In 1804 Harding discovered Juno, and in
1807 Olbers found Vesta. The next one was not discovered until 1845,
from which date asteroids, or minor planets (as these small planets
are called), have been found almost every year. They now number about
700.

It is impossible to give any idea of the interest with which the first
additions since prehistoric times to the planetary system were
received. All of those who showered congratulations upon the
discoverers regarded these discoveries in the light of rewards for
patient and continuous labours, the very highest rewards that could be
desired. And yet there remained still the most brilliant triumph of
all, the addition of another planet like Uranus, before it had ever
been seen, when the analysis of Adams and Le Verrier gave a final
proof of the powers of Newton’s great law to explain any planetary
irregularity.

After Sir William Herschel discovered Uranus, in 1781, it was found
that astronomers had observed it on many previous occasions, mistaking
it for a fixed star of the sixth or seventh magnitude. Altogether,
nineteen observations of Uranus’s position, from the time of
Flamsteed, in 1690, had been recorded.

In 1790 Delambre, using all these observations, prepared tables for
computing its position. These worked well enough for a time, but at
last the differences between the calculated and observed longitudes of
the planet became serious. In 1821 Bouvard undertook a revision of the
tables, but found it impossible to reconcile all the observations of
130 years (the period of revolution of Uranus is eighty-four years).
So he deliberately rejected the old ones, expressing the opinion that
the discrepancies might depend upon “some foreign and unperceived
cause which may have been acting upon the planet.” In a few years the
errors even of these tables became intolerable. In 1835 the error of
longitude was 30″; in 1838, 50″; in 1841, 70″; and, by comparing the
errors derived from observations made before and after opposition, a
serious error of the distance (radius vector) became apparent.

In 1843 John Couch Adams came out Senior Wrangler at Cambridge, and
was free to undertake the research which as an undergraduate he had
set himself–to see whether the disturbances of Uranus could be
explained by assuming a certain orbit, and position in that orbit, of
a hypothetical planet even more distant than Uranus. Such an
explanation had been suggested, but until 1843 no one had the boldness
to attack the problem. Bessel had intended to try, but a fatal
illness overtook him.

Adams first recalculated all known causes of disturbance, using the
latest determinations of the planetary masses. Still the errors were
nearly as great as ever. He could now, however, use these errors as
being actually due to the perturbations produced by the unknown
planet.

In 1844, assuming a circular orbit, and a mean distance agreeing with
Bode’s law, he obtained a first approximation to the position of the
supposed planet. He then asked Professor Challis, of Cambridge, to
procure the latest observations of Uranus from Greenwich, which Airy
immediately supplied. Then the whole work was recalculated from the
beginning, with more exactness, and assuming a smaller mean distance.

In September, 1845, he handed to Challis the elements of the
hypothetical planet, its mass, and its apparent position for September
30th, 1845. On September 22nd Challis wrote to Airy explaining the
matter, and declaring his belief in Adams’s capabilities. When Adams
called on him Airy was away from home, but at the end of October,
1845, he called again, and left a paper with full particulars of his
results, which had, for the most part, reduced the discrepancies to
about 1″. As a matter of fact, it has since been found that the
heliocentric place of the new planet then given was correct within
about 2 degrees.

Airy wrote expressing his interest, and asked for particulars about
the radius vector. Adams did not then reply, as the answer to this
question could be seen to be satisfactory by looking at the data
already supplied. He was a most unassuming man, and would not push
himself forward. He may have felt, after all the work he had done,
that Airy’s very natural inquiry showed no proportionate desire to
search for the planet. Anyway, the matter lay in embryo for nine
months.

Meanwhile, one of the ablest French astronomers, Le Verrier,
experienced in computing perturbations, was independently at work,
knowing nothing about Adams. He applied to his calculations every
possible refinement, and, considering the novelty of the problem, his
calculation was one of the most brilliant in the records of
astronomy. In criticism it has been said that these were exhibitions
of skill rather than helps to a solution of the particular problem,
and that, in claiming to find the elements of the orbit within certain
limits, he was claiming what was, under the circumstances, impossible,
as the result proved.

In June, 1846, Le Verrier announced, in the _Comptes Rendus de
l’Academie des Sciences_, that the longitude of the disturbing planet,
for January 1st, 1847, was 325, and that the probable error did not
exceed 10 degrees.

This result agreed so well with Adams’s (within 1 degrees) that Airy urged
Challis to apply the splendid Northumberland equatoreal, at Cambridge,
to the search. Challis, however, had already prepared an exhaustive
plan of attack which must in time settle the point. His first work
was to observe, and make a catalogue, or chart, of all stars near
Adams’s position.

On August 31st, 1846, Le Verrier published the concluding
part of his labours.

On September 18th, 1846, Le Verrier communicated his results to the
Astronomers at Berlin, and asked them to assist in searching for the
planet. By good luck Dr. Bremiker had just completed a star-chart of
the very part of the heavens including Le Verrier’s position; thus
eliminating all of Challis’s preliminary work. The letter was received
in Berlin on September 23rd; and the same evening Galle found the new
planet, of the eighth magnitude, the size of its disc agreeing with Le
Verrier’s prediction, and the heliocentric longitude agreeing within
57′. By this time Challis had recorded, without reduction, the
observations of 3,150 stars, as a commencement for his search. On
reducing these, he found a star, observed on August 12th, which was
not in the same place on July 30th. This was the planet, and he had
also observed it on August 4th.

The feeling of wonder, admiration, and enthusiasm aroused by this
intellectual triumph was overwhelming. In the world of astronomy
reminders are met every day of the terrible limitations of human
reasoning powers; and every success that enables the mind’s eye to see
a little more clearly the meaning of things has always been heartily
welcomed by those who have themselves been engaged in like
researches. But, since the publication of the _Principia_, in 1687,
there is probably no analytical success which has raised among
astronomers such a feeling of admiration and gratitude as when Adams
and Le Verrier showed the inequalities in Uranus’s motion to mean that
an unknown planet was in a certain place in the heavens, where it was
found.

At the time there was an unpleasant display of international jealousy.
The British people thought that the earlier date of Adams’s work, and
of the observation by Challis, entitled him to at least an equal share
of credit with Le Verrier. The French, on the other hand, who, on the
announcement of the discovery by Galle, glowed with pride in the new
proof of the great powers of their astronomer, Le Verrier, whose life
had a long record of successes in calculation, were incredulous on
being told that it had all been already done by a young man whom they
had never heard of.

These displays of jealousy have long since passed away, and there is
now universally an _entente cordiale_ that to each of these great men
belongs equally the merit of having so thoroughly calculated this
inverse problem of perturbations as to lead to the immediate discovery
of the unknown planet, since called Neptune.

It was soon found that the planet had been observed, and its position
recorded as a fixed star by Lalande, on May 8th and 10th, 1795.

Mr. Lassel, in the same year, 1846, with his two-feet reflector,
discovered a satellite, with retrograde motion, which gave the mass of
the planet about a twentieth of that of Jupiter.

FOOTNOTES:

[1] Bode’s law, or something like it, had already been fore-shadowed
by Kepler and others, especially Titius (see _Monatliche
Correspondenz_, vol. vii., p. 72).