e
story
scienc
he
he
in
beh d t
Accounting for Anomaly
The Discovery of Neptune
T
On September 25, 1846, two German astronomers
worked through the night at the Berlin Observatory. They
were searching a small square of sky for a hazy dot. The
astronomer at the telescope, Johann Gottfried Galle, had
been driven to his telescope after receiving a short letter
from the French mathematician Urbain Le Verrier. This
letter contained Le Verrier's prediction for the location of
an undiscovered planet beyond the orbit of Uranus. Le
Verrier had been unable to convince his fellow French
astronomers to search for a planet predicted by his
mathematical calculations. The two Germans took a
chance on Le Verrier and knew well the challenge ahead of
them. Galle sought an eighth magnitude fuzzy dot against
a background of thousands of brighter stars. All he had
was Le Verrier's prediction, one night of observation time
on the observatory's smallest telescope, and the young
graduate student Heinrich d'Arrest as his only help.
Little did they know that across the English Channel
another young astronomer, John Couch Adams, had
compiled five years of work into a prediction almost
matching Le Verrier's. The head of Cambridge
Observatory, James Challis, respected his good friend
Adams' work. However, his duties were teaching and
finishing the observatory's usual workload calculating
comet trajectories. But every few nights Challis would
spend time watching for the predicted planet's movement.
Early that September morning at the Berlin Observatory,
the Germans tired of watching stars pass through their
viewfinder. Their telescope time running out, they decided
to hunt and peck. Grabbing the observatory's most recent
star map, Galle shouted positions as d'Arrest checked
each position with the telescope. After less than ten stars
he shouted, “That star is not on the map!” They awoke the
observatory's director for confirmation.
We now know the object that inspired a multinational hunt
in 1846 as the planet Neptune. Following the discovery of
Uranus in 1781, astronomers learned that it 'wobbled'
along its orbit. The precise reason why this occurred,
however, was a mystery. Some thought the law of gravity
was different at the distance of Uranus. Others thought
maybe a comet had collided with the planet. But a few
astronomers suggested the irregularities in Uranus' orbit,
called “perturbations,” were caused by another distant and
as yet unseen planet.
Accounting for Anomaly: The Discovery of Neptune
www.storybehindthescience.org
!
All astronomers acknowledged that Uranus'
orbit departs from that predicted by Newton's
gravitational law. Note, however, that they
differed in the way they accounted for that
accepted anomaly.
The discovery of Neptune well illustrates important
aspects of how science works. Foremost, it exemplifies
the power of previously well-established knowledge and
how scientists react to anomalous data. In the midnineteenth century, astronomy primarily dealt with
observations and had very little use for hypothetical work.
The men who made the predictions of Neptune, Adams
and Le Verrier, were actually not actually astronomersthey
were mathematicians with little experience behind a
telescope. They had to convince the nation's best
stargazers that their work mattered and could be
observationally tested. Secondly, it shows the interplay
between technology and science. One of many reasons
why Adams and Le Verrier encountered troubles
convincing astronomers of their work's testability lay in the
astronomer's view that existing telescopes were
inadequate for such a search. Lastly, this story illustrates
how science involves efforts and collaboration around the
world.
!
Many people dislike the thought of a science
career, seeing it as a solitary undertaking. As you
read this story, consider how it illustrates that
science is a social endeavor.
The story of Neptune's discovery began with William
Herschel's discovery of Uranus. While working on his star
catalog in mid-March 1781, Herschel came across a
peculiar hazy star. It moved across the sky, so at first he
thought it to be a comet. Even with his precise telescope it
appeared fuzzy. Curious to get a closer look he upped the
magnification power of his telescope from its standard 227
to 460. The object got bigger and sharper. Herschel knew
he found something important. Stars do not get bigger with
magnification because they are so far away, and comets
have a noticeably fuzzy tail. Only planets get bigger and
1
sharper when magnified. An international search to verify
the object as a planet commenced, and in late spring 1781
Russian astronomers confirmed the discovery of a new
planet - Uranus.
Herschel's discovery energized the astronomical
communitymaybe more planets existed in our solar
system! Johann Bode thought the orbital radius of planets
could be predicted. In 1772, about ten years before the
discovery of Uranus, he published a mathematical series
based on the older work of Johann Titius. Titius argued that
if the distance of the earth to the sun was considered to be
1 unit, then the mean radial distance to the other planets
could be estimated with the equation 0.4+0.3(2n). Here are
his results (Table 1), updated with modern measurements
for clarity:
TABLE 1
Bode-Titius Series (modernized)
Planet
Mercury
Venus
Earth
Mars
Asteroid Belt
Jupiter
Saturn
Uranus
Neptune
Pluto
Observed
AU Radius
0.38
0.72
1.0
1.52
2.8
5.2
9.55
19.2
30.06
38.4
Titius’
Prediction
0.4
0.7
1.0
1.6
2.8
5.2
10.0
19.6
38.8
77.2
The series came to be associated with Bode. Because of
the accuracy it gave to Uranus' orbit, it drove astronomers
to search for another missing planet between Mars and
Jupiter.
1. The Bode-Titius Series resides in a hazy realm
of science. On one hand, it predicts quite
accurately the distance to planets (with the
exception of Neptune and Pluto, the latter which
is no longer considered a planet). On the other
hand, it gives no hint of explanation for why the
planets should be organized as such, nor does it
account for objects other than planets (like
comets). Moreover, the Series does not apply to
newly discovered solar systems around other
sun-like stars. What value should scientists
assign to such predictive methods that have
such limitations and no underlying explanation?
In the autumn of 1800, the Baron von Zach got the itch to
search for the predicted planet between Mars and Jupiter.
He gathered twenty-four astronomers of all nationalities to
partake in the most coordinated search of the night sky
2
ever. By New Years Day of 1801 the Italian astronomer
Giuseppe Piazzi found a moving star in his catalogue of
6,000, but nobody else could find it. Hearing of the
problem, the young Carl Friedrich Gauss, at the age of 23
already called the 'prince of mathematics,' calculated an
orbit from established data. On December 1, 1801, von
Zach located his 'planet' between Mars and Jupiter. Soon
thereafter another moving object appeared nearby.
Puzzled, William Herschel calculated their sizes. He
related the disappointing news to the astronomical
communitythe new planets were too small to be planets.
Astronomers realized they were small 'planetoids.' Many
relished the discovery of the string of rocks orbiting
between Mars and Jupiter, known today as the asteroid
belt. But others continued to wonder if a full planet
remained undiscovered.
Meanwhile, astronomers realized unusual motions in
Uranus' orbit. Looking at past star maps, Johann Bode
realized William Herschel was not the first to observe
Uranus. In 1690 John Flamsteed made the first recorded
observation of Uranus, but thought it to be a star. Using this
century old data, in 1820 the French astronomer Alexis
Bouvard calculated Uranus' orbit by combining it with his
own data. No matter how he worked the numbers, he
couldn't match Uranus to its current orbit. Bouvard couldn't
answer why Flamsteed's observations were so far off. His
words hinted that a deeper mystery might lie beneath the
data: “I leave to the future the task of discovering whether
the difficulty of reconciling the two systems results from the
inaccuracy of the ancient observations, or whether it
depends on some extraneous and unknown influence
which may have acted on the planet.”
Two possible explanations surfaced. The first proposed
that Isaac Newton's law of gravity might not hold at
distances as great as Uranus. While astronomers
occasionally fudged the inverse square law to
compensate for their technological inadequacies, it
certainly had never been to the point of entirely altering the
laws of physics. Newtonian mechanics explained so many
aspects of the world that few scientists were willing to give
it up.
2. Note that the orbit of Uranus appears not to be
behaving as Newtonian physics says it should.
Scientists knew of this for decades, but few
questioned Newtonian mechanics. Many nonscientists think that scientific ideas should be
abandoned when disconfirming evidence exists.
Why would quickly giving up on well-established
scientific knowledge not be appropriate?
Most astronomers favored the other possibility: that
another planet lay beyond Uranus. However, some
observational astronomers feared their telescopes to be
Accounting for Anomaly: The Discovery of Neptune
www.storybehindthescience.org
far too weak for the task. The search for the planet beyond
Uranus revolved mostly around two people, then: the
British John Couch Adams and the French Urbain Le
Verrier. Near opposites in character and unaware of each
other's work until 1846, both used the universal law of
gravity to calculate the exact position of Uranus' neighbor.
If successful, it would be the first application of an already
existing conceptual framework to precisely locate an
unknown astronomical body.
!
While many people think that scientific
knowledge is simply an end result of research,
note how that knowledge may also be used to
make predictions and direct future research. This
is the case in all fields of science.
John Couch Adams was born in 1819 to a family of tenant
farmers. A quiet and reserved man, he earned his way into
Cambridge through diligent hard work. Earning the highest
honor of Senior Wrangler on the notorious mathematics
examination, his excellence garnered him a teaching
position after graduation. He first became aware of the
problem with Uranus' orbit in 1841, when he chanced upon
an astronomical report in a bookstore. He had just enough
money to live on following graduation, but a contest put on
by a German university in 1844 incited him to work day and
night on the Uranus problem.
Urbain Le Verrier was born in France in
1811. His father worked in the State
Property Administration and could
afford to send his son to the prestigious
École Polytechnique. Le Verrier
managed to land a stable and wellpaying job at the Administration of
Tobacco as an experimental chemist.
Excelling in mathematics, he turned to
astronomy and quickly earned a name
for his prowess with numbers. Little
could be said for his demeanor, though.
In the words of a colleague, “I do not
know whether M. Le Verrier is actually
the most detestable man in France, but I
am quite certain that he is the most
detested.”
orbit, Adams took Bouvard's data and assumed two
features: first, that the planet had a circular orbit, and
second, that its distance agreed with Bode's prediction of
38.8 astronomical units. An offer by Cambridge head
astronomer James Challis helped significantly. If Adams
helped Challis with a comet observation, Challis would
give Adams access to all modern observation data on
Uranus. Adams agreed.
By 1845 Adams concluded that two reasons accounted for
the perturbations in Uranus' orbit. The first reason was an
unseen planet that definitely did exist. The second reason
was that the calculations by Alexis Bouvard were wrong.
After fixing Bouvard's calculations, Adams concluded the
missing planet must have an elliptical orbit and be located
at double the distance of Uranus. He fashioned a page
long equation to describe the orbit of Uranus and the
missing planet. He then applied it to twenty-one
measurements between 1780 and 1840. Many pages of
calculations later, Adams presented a new model of
Uranus' measurement. He concluded that Flamsteed's
1690 measurement had been right, but in 1820 the
missing planet interfered with Bouvard's data.
Adams decided to personally take his work to George Airy,
Astronomer Royal and the Head of the Royal Greenwich
Observatory. The reserved and timid Adams forgot to
make plans, however. In September 1845, holding an
introductory letter from James Challis that
would get him a personal meeting with Airy,
Adams arrived in Greenwich and learned
Airy had left the country. A month later,
Adams again visited without announcement
and arrived at Airy's dinnertime. The maid
never told Airy of Adams' arrival.
Disenchanted, Adams left behind his paper
of equations and sulked home. After
hearing that the young astronomer had
twice made the trek, on November 5, 1845,
Airy wrote a letter of thanks to Adams,
though he offered him no help. Adams
worked furiously on refining the data and
didn't try to contact Airy until a year later.
While Adams cloistered himself, the French
mathematician Urbain Le Verrier publicly
announced his search for the missing
planet. Encouraged by the director of the
Paris Observatory, Le Verrier started his search from
scratch, unaware of Adams' work. In December 1845 Le
Verrier published a short manuscript on the orbit of the
missing planet and pointed out the many errors of Alexis
Bouvard. Six months later Le Verrier published a more
complete treatise on the orbit of Uranus, concluding the
planet must be somewhere between two and three times
the orbital distance of Uranus. He would not realize he had
competition in England until June 1846.
John Couch Adams
Adams started his work on Uranus just before Le Verrier.
Adams began with the premise that another planet beyond
the orbit of Uranus explained the perturbations better than
a variable law of gravity. He knew the gravitational pull of
Jupiter and Saturn could be calculated and their effects
subtracted from the total perturbations of Uranus. The
remaining anomalies could then be attributed entirely to
the missing planet. But he still didn't know where in the
solar system it might be. To estimate the mystery planet's
Accounting for Anomaly: The Discovery of Neptune
www.storybehindthescience.org
3
For an unknown reason, Airy took a much better liking to
Le Verrier's work, exclaiming about the paper, “I cannot
sufficiently express the feeling of delight and
satisfaction which I received from it.” One
possible explanation could be that Le Verrier
came from high class and had already
garnered an international reputation.
Nevertheless, Airy announced to the Board
of Visitors at Greenwich Observatory that a
planet should be discovered very soon,
although he stopped short of calling British
observatories into action. He convinced
James Challis of Cambridge Observatory to
head up the only British search. Challis went
about the work slowly, already inundated
with calculating comet trajectories and
disgruntled by Airy's rebuff of his friend
Adams.
In Britain, Challis made sweeps on June 29, June 30,
August 4, and August 12. In comparing the August 12 data
to that of June 30, the forty-ninth star in
the August data was missing from the
June data. Challis marked it and
continued his comet calculations.
Meanwhile, Adams sent another
refinement of his calculations to Airy.
Another miscommunication, Airy was
on vacation when the paper arrived in
Greenwich.
Le Verrier's letter to Galle asked “to find
a persistent observer, who would be
willing to devote some time to an
examination of a part of the sky in which
there may be a planet to discover.” The
young Galle had sent Le Verrier his
dissertation a few years earlier and
The British telescopes would 'sweep the sky'
worked
a subordinate job at the Berlin
Urbain Le Verrier
by a method known as transiting. To do this,
Observatory. The observatory's senior
the telescope remained mostly stationary as
astronomer, Johann Encke, felt slighted
stars passed the field of view. The telescope operator
by Le Verrier's appeal to the young Galle. Encke allowed
called out the positions of each star as it passed through
Galle one night's use of the observatory's smallest
the crosshairs while an assistant marked positions on a
telescope. By chance Heinrich d'Arrest, a graduate
star map. Every night the process repeated, searching for
student, walked by and offered help. The night sky
the appearance of a new star. Historian Martin Grosser
awaited.
remarked, “It was like searching for a particular bright
pebble on a beach by removing, one by one, thousands of
Hours passed on September 25, 1846, as Galle and
other pebbles from a large area around a point where (one
d'Arrest searched for a fuzzy disk. They spotted Neptune
had been told) the desired object was lying.” The
in the early morning, and after waking Encke, they kept
disgruntled Challis went weeks
watch on the speck until 2 AM. Le Verrier's
between checking his nightly data and
prediction was almost perfect. Galle proudly
instead calculated comet trajectories.
wrote the next day to Le Verrier, “The planet
whose position you have pointed out actually
Meanwhile, Le Verrier continued
exists.”
refining his work, conscious that the
!
British had started searching.
Announcing his plotted orbit in August
Note here the context of “discovery.” It
1846, he had calculated over 10,000
applies to the initial idea that an
pages of equations and used 279
unknown planet might account for the
measurements of Uranuswhereas
perturbations in Uranus' orbit, to the
Challis had used only twenty-one
mathematical work that predicted the
measurements. Le Verrier predicted
location of the planet, and to a lesser
the planet would appear at 326
extent to the actual observation of the
degrees longitude on January 1,
planet. This exemplifies why giving
1847. Le Verrier also claimed that the
sole credit for scientific discovery to
planet would be easily visible by most
one individual distorts how science
telescopes, and that “the planet's disk
really works.
Johann Gottfried Galle
has a large enough diameter to
preclude its being confused with the
stars.” He was clearly trying to convince astronomers to
It took until October 1 for word to reach England. Challis
begin the search, but nobody in France wanted to chase
rechecked his work and realized he had located the
Le Verrier's planet until his predicted date in January.
missing planet over a month earlier. Disheartened, he
Impatient after all his work, Le Verrier wrote Johann
wrote, “It is useless now to regret my having missed the
Gottfried Galle at the Berlin Observatory.
planet when it was so possible to detect it. All that remains
to do is to make the best of the observations that I have
4
Accounting for Anomaly: The Discovery of Neptune
www.storybehindthescience.org
succeeded in getting.” The dispirited Adams followed his
advice and began calculating an orbit for the new planet.
After the more accurate orbit of Neptune was calculated in
1847, astronomers realized how lucky they had been. The
first surprise recalled the incident of William Herschel and
John FlamsteedNeptune had been unwittingly observed in
1795 by the Frenchman Michel Lalande, but thought at the
time to be a faint star. The crucial second stroke of luck
came in Bouvard's timely examination of Uranus. For most
of the century Neptune's orbit strayed far away from
Uranus, but in 1822 Neptune passed by at its closest point
and accelerated Uranus along its orbit. Bouvard had
mistaken the fast Uranus for its standard orbit, thus
explaining the perturbations. Despite his own
inaccuracies, had Bouvard not discovered the
perturbations in Uranus' orbit, it might have been a long
time until Neptune's discovery.
Neptune was the first planet to be predicted by first
applying previous knowledge, in this case Newtonian
mechanics, and then confirming it through observation.
The discovery of Neptune illustrates that anomalous data
do not necessarily result in the rejection of well-accepted
scientific knowledge. Instead of rewriting the law of gravity,
astronomers accounted for the discrepancies of data and
fashioned a testable explanation for the perturbations in
Uranus' orbit. Their predicted result, a planet, was
established through observational evidence with
remarkable precision. Bolstered by the discovery of
Neptune, theoretical astronomy became more popular
and worked alongside observational astronomy to set the
stage for twentieth century astronomy.
4. Science textbooks often attribute the
development of scientific knowledge to a
particular person on a specific date. How does
this story show that portrayal to be wrong?
3. How does this story illustrate that science is a
social endeavor?
in
beh d t
scienc
e
he
he
story
Accounting for Anomaly: The Discovery of Neptune written by
Blair Williams, Michael P. Clough, Matthew Stanley, & Charles Kerton
Partial support for this work was provided by the National Science Foundation's
Course, Curriculum, and Laboratory Improvement (CCLI) program under
Award No. 0618446. Project Principal Investigator: Michael P. Clough. Any
opinions, findings, and conclusions or recommendations expressed in this
material are those of the authors and do not necessarily reflect the views of the
National Science Foundation.
T
Accounting for Anomaly: The Discovery of Neptune
www.storybehindthescience.org
5